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In the spring of 2013, biologist Michael Abrams had cut two arms off a young jellyfish when he witnessed something he'd never seen before.

"He started yelling... 'You won't believe this, you've got to come here and see what's happening,'" recalls his Ph.D. advisor Lea Goentoro, a biologist at Caltech in Pasadena.

The team expected that Abrams' moon jellyfish ( Aurelia aurita) had regrown its limbs, since many other marine invertebrates-including the polyp stage of moon jellyfish-regenerate themselves this way. (See "Pictures: 5 Animals That Regrow Body Parts.")

Instead, the moon jelly had rearranged its six remaining arms until they were evenly placed around the body. Muscles in the jellyfish's body had pushed and pulled on the remaining arms until they were once again evenly spaced. Being symmetrical is crucial for moon jellyfish movement.

Latest Weird & Wild Stories

The scientists had stumbled upon a phenomenon completely new to science, which they call "symmetrization." Because jellyfish often suffer from injuries-sometimes inflicted by unsuccessful predators-symmetrization is an important method to heal themselves.

"This is an amazing study and a fantastic piece of detective work," says Sara Lindsay, a marine biologist at the University of Maine, who was not involved with the study.

Goentoro and colleagues could have easily missed their discovery: They had originally set out to study limb repair in Turritopsis dohrnii, also known as the immortal jellyfish. While they waited for specimens to arrive, the scientists practiced their experimental techniques on the much more common moon jelly.

After observing the jellyfish putting themselves back together, "I repeated the experiments several times because I thought I might have made a mistake," Abrams says. (See jellyfish pictures from the National Geographic archives.)
Watch a video of jellyfish in their natural habitat.
Once he had reassured himself that the jellyfish really was rearranging its arms to restore symmetry-a process which takes between 12 hours to 4 days-Abrams and his team had to figure out how the animal manages such a feat.

After several false starts, the researchers focused on the jellyfish's musculature. Giving the moon jellies a muscle relaxant made them unable to complete the symmetrization process. On the other hand, when the team increased the muscular pulses of the juvenile jellyfish, the process occurred faster than usual.

This suggests that the muscles push against each other to get the arms as widely spaced as possible.

"It's like squeezing one side of a stress ball and you get a protrusion on the other side," says Goentoro, whose study appeared June 15 in the journal Proceedings of the National Academy of Sciences.

He started yelling... 'You won't believe this, you've got to come here and see what's happening.'

Restoring symmetry is crucial to the marine animals: Asymmetrical jellyfish have difficulties propelling themselves through the water and feeding properly. A jellyfish can lose a limb due to predators.

It's like a car: Remove one wheel and you won't be able to drive it. But if you shift the remaining wheel from the side of the vehicle into the center, the car will once again be able to move about, the researchers say.

"This isn't replacing lost parts, it's replacing their function. That's pretty cool," Lindsay says.

Goentoro adds that understanding the newly discovered phenomenon could provide new insight for scientists studying regenerative medicine. (Related: " Will We Ever Regenerate Limbs?")

"We hope it will inspire new technology for biomaterials-not by regenerating what was lost, but by restoring function," she says.

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The Surprising Way Jellyfish Put Themselves Back Together

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In the spring of 2013, biologist Michael Abrams had cut two arms off a young jellyfish when he witnessed something he'd never seen before. "He started yelling... 'You won't believe this, you've got to come here and see what's happening,'" recalls his Ph.D. advisor Lea Goentoro , a biologist at Caltech in Pasadena.

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This is the anatomy of cnidarians.

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Article that talks about jellies losing their tentacles.

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When a juvenile moon jellyfish loses tentacles, it rapidly reorganizes its remaining limbs to maintain symmetry, a new study says.

In the lab, researchers removed the tentacles from the translucent jellyfish. "Each time, they would start reshaping and reorganizing their bodies," said Michael Abrams, a biologist at California Institute of Technology and an author of the new study.

The process was quick, beginning 12 hours to four days after researchers removed the tentacles. Mr. Abrams and his colleagues describe their findings in the current issue of Proceedings of the National Academy of Sciences.

The process of realignment, they found, is started by the repeated contraction of muscles that the jellyfish use continually. "It is the same mechanical force they use to swim and feed and draw food to the mouth," Mr. Abrams said.

The only way the researchers were able to prevent the jellyfish from returning to a state of symmetry was by stopping the contractions with anesthesia.

Symmetry may be important for the moon jellies to swim and feed successfully, Mr. Abrams said.

A version of this article appears in print on , on Page D4 of the New York edition with the headline: Marine Life: A Jellyfish That Quickly Puts Itself Back Together. Order Reprints | Today's Paper | Subscribe

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A Jellyfish That Quickly Puts Itself Back Together

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When a juvenile moon jellyfish loses tentacles, it rapidly reorganizes its remaining limbs to maintain symmetry, a new study says. In the lab, researchers removed the tentacles from the translucent jellyfish. "Each time, they would start reshaping and reorganizing their bodies," said Michael Abrams, a biologist at California Institute of Technology and an author of the new study.

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This has some good info about cnidarians such as what they eat.

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Cnidarians - Jellyfish, Corals and Sea Anemones - Animalia

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Cnidarians are one of the simplest forms of multicellular organism. They include the coral animals that build reefs, jellyfish and sea anemones as well as some small fresh water forms. 95% of the mass of a jellyfish is water, combined they make up 40% of the biomass of the oceans.

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characteristics of the Cnidaria phylum

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!n 1887 Leuckart coined the name Coelenterata. His Coelenterata included sponges and Ctenophores. In 1888 Coelenterata is divided into 3 phyla.

1) Spongiaria 2) Cnidaria and 3)Ctenophora.

Therefore, Cnidaria is the most suitable name for his phylum. Coelenterata was divided into Cnidaria (for coelenterates proper) and Acnidaria (for the ctenophores). Hyman regarded ctenophora as a separate phylum.

1)Huxley (1856) proposed the name Hydrozoa.

2)Hackle in 1891 proposed the name Scyphozoa.

3)Ehrenberg in 1833 proposed the name Anthozoa.

The coelenterates are radial symmetrical, tentacle bearing aquatic organisms.

1) All are aquatic, mostly marine but a few are fresh water forms.

Ex : Hydra.

2) They may be solitary or colonial.

3) They may be sedentary or free-swimming.

4) Head and segmentation is absent.

5) Tentacles encircle the mouth in one or more whorls. They are used for food capture, and defense.

6) These are all diploblastic animals. They show ectoderm and endoderm. In between jelly like mesoglea is present.

7) The cnidoblasts are important defensive and offensive cells. They are useful for food capture.

8) Un differentiated free interestitial cells are found among the epithelial cells.

9)Mouth is present. Anus is absent.

10) Mouth leads into a central cavity called 'Coelenteron. Hence the name Coelenterate'.

11) Coelenterates are acoelomate. Because there is no true body cavity, or coelom.

12) Respiratory, circulatory and excretory systems are absent.

13) These are radial symmetrical animals.

14) Nervous system is diffused type.

15) Polyp is a nutritive zooid and fixed zooid. The medusa is a free swimming zooid and sexual zooid.

16) Nutrition is intercellular and intracellular.

17) Locomotion in medusa is by muscles.

18) IN Coelenterates Polymorphism tendency is seen in some examples.

19) Asexual reproduction is by budding.

20) Sexual reproduction takes place by the development of gonad and sex cells. Planula larva is seen.

COELENTERATE CLASSIFICATION (CNIDARIA):

Phylum coelenterata is divided into two subphyla.

Subphylum 1) Cnidaria, Subphylum 2) Acnidaria. Now-a-days these two subphyla are regarded as two independent phyla

Phylum: cnidaria and phylum: ctenophora.

According to Parkar and Haswel! Phylum Cnidaria is divided into three classes.

1) HYDROZOA

2) SCYPHOZOA

3) ANTHOZOA.

1) Hydrozoa animals are multicellular, diploblastic animals.

2) They show mouth opening, and anus is absent.

3) They show both polyp and medusa forms. Medusa is a reproductive zooid. Polyp is a fixed stage.

4) In medusa the gasto-vascular-system is transversed by canals. In medusa definite sense organs like statocyst, nervous system, and muscular system are well developed.

5) Polymorphic tendency is well developed.

6) Gonads are seen.

7) Alternation of generations is seen in the life history of these animals.

8) Velum is present on the medusa (Craspedote)

Order 1 : Hydroidea : Solitary or colonial forms. Polyp well developed. Sense organs or medusa are statocysts.

Sub-Order 1 : Anthomedusae. Ex : Hydra, Bougainvillea.

Sub-Order 2. Leptomedusae. Ex : Obelia.

Order2:Trachylina

Fixed stage is absent. They are all mobile medusae. Marginal sense organs are modified tentacles.

Sub-Order I : Trachymedusae Ex : Petasus.

Sub-Order II : Narcomedusae. Ex : Polycolpa.

Order 3: Hydrocorallina : It includes coral like hydrozoans. CaC03 skeleton is secreted by coenosarc. Polyps are dimorphic.

1)Millipora (Hydrozoans coral)

2)Stylaster (Hydrozoan corals)

Order 4 : Chondrophora : It includes organisms with big floats. Ex:l)Velella 2)Porpita.

Order 5: Pteromedusae: Pelagic hydrozoans. Ex : Tetraplatia.

Order 6: Siphonophora:

They show highest polymorphic tendency. Ex : 1) Physalia (Poutugese-man-of-war) 2) Halistemma.

1) Represented by medusoid forms.

2) Sense organs are tentaculocysts.

3) Gastrovascular system shows stomach and 4 gastric pouches. In the gastric pouches gastric filaments are present.

4) Velarium is present with endodermal canal (Acraspedote).

5) Gonads are endodermal in origin

6) Medusa arises by strobilisation.

Order 1 : Stauromedusae : (Lucernarida) Sense organs absent. Medusa is pyramidal shaped. Sedentary.

Ex : 1) Lucernaria 2) Haliclystus.

Order 2 : Coronatae : The umbrella shows coronary grooves. 4 to 16 tentaclocysts are present.Ex: 1) Periphylla. 2) Nausithoe (It lives inside Porifera animals (sponges).

Order 3 : Cubotnedusae : Medusa is cubical 4 perradial tentacalocysts are present. Free swimming.

Ex : 1) Choropsaimum (free medusa) 2)Chatybdaea.

Order 4 : Semeastomeae (or) Discomedusae : Most common medusae. Medusa is disc shaped, 4 perradial and 4 interradial tentaculocysts are present.

Ex : 1) Aurelia -Jelly fish 2)'Rhopilema3)Pelagia (Luminescent Jelly fish)

Order 5 : Rhyzostomeae : Free swimming medusa. The oral arms are branched. Tentacles absent. 8 or more tentaculocysts are present.

Ex : 1) Pilema, 2) Rhizostoma.

CLASS III: ANTHOZOA:

1) They exhibit only polyp forms.

2) Medusa stage is absent.

3) Mesentries are present, they bear nematocyst.

4) Gonads are endodermal.

5) Corals coral reefs are common.

This class is divided into 2 sub classes.

i) Sub class: Hexacorallia (or) Zoantharia.

ii) Sub class: Octocorallia (or) Alcyonaria.

Sub class: Hexacorallia : It contains tentacles and mesentries in multiples of six. It includes seven orders.

Order 1 : Actinaria : (Sea anemones) Skeleton is absent.

Ex: 1) Edwardsia 2) Adamsia

Order 2: Madreporaria : (True corals) Stony corals are present Polyps are small. Siphonoglyphs absent.

Ex: 1) Meandrina (brain coral) 2) Fungia.

Order 3 : Zoanthidea : Solitaryor colonial organisms. Polyps are united by basal stolons. Only ventral siphonoglyphis present.

Ex: 1) Zoanthus.

Order 4 : Antipatharia : Includes black corals. Two siphonoglyphs are present.

Ex: Antipathes (Black coral)

Order 5: Ceriantharia : Solitary structure. Tentacles many, arranged into two whorls. Only single siphonoglyph occurs.

Ex .- Cerianthus.

Order 6: Corallimorpharia : Solitary or aggregate, anemone like polyps. Ex : Corynactis.

Order 7: Ptychodactaria : Includes animals which are anemone like polyps. Ex: Ptychodactis.

Sub class : Octocoralia (Alcyonaria) : In these Anthozoan members the tentacles and mesentries are in multiples of eight. On the stomodium never more than one siphonoglyph will be present. It is ventral in position.

Order 1: Stolonifera : Polyps are connected by creeping stolon.

Ex: Tubipora (orange pipe coral).

Order 2 : Telestacea :

The colonies contain simple or branched stem which bears lateral polyps.

Ex: Telesto.

Order3: Alcyonacea : These are soft corals. Polyps may be dimorphic.

Ex: 1) Alcyonium (dead man's fingers).

Order 4 : Coenothecalia : It includes a single genus.

Ex: Heliopora (Blue coral)

Order 5: Gorgonacea : It is a compound tree like coral.

Ex: 1) Gorgonia (seafan) 2) Corallium (red coral)

Order 6: Pennatulacea: These are elongated members. Emended in the mud, and sea bottom.Ex : Pennatula (Sea pen)

According to Parker and Haswell this sub phylum is treated as PHYLUM CTENOPHORA.

1} These are biradially symmetrical organisms.

2) They do not show nimatocysts.

3) They show ciliary plates in 8 rows (comb plates in 8 rows).

4)The mesoglea is traversed by muscle fibres.

5) They may bear two tentacles. On tentacle Lasso cells are present.

6) On the aboral pole an aboral sense organ is present, which is a modified lithocyst.

7) There is no alternation of generations, This phylum is divided into two classes.

Class I: Tentaculata: Ctenophores without tentacles. This class is divided into four orders.

Order 1 : Cydlppidea : Two retractile tentacles. Ex : Hormiphora

Order 2 : Cestidea : The bases of the two principal tentacles are present. They are enclosed in sheaths. Ex : Cestus

Order 3 : Lobata : Many lateral non-retractile tentacles are present. The bases of the two principal tentacles are present. Ex : Deiopea

Order 4 : Platyctenea : They are creeping or sessile forms. They show a pair of retractile tentacles.

Ex : Ctenoplana

Class II: Nuda : Ctenophores without tentacles. This class includes only one order.

Order 1 : Beroldea : Mouth is wide. Gullet occupies greater part of the body. Ex : Beroe

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n 1887 Leuckart coined the name Coelenterata. His Coelenterata included sponges and Ctenophores. In 1888 Coelenterata is divided into 3 phyla. 1) Spongiaria 2) Cnidaria and 3)Ctenophora. Therefore, Cnidaria is the most suitable name for his phylum. Coelenterata was divided into Cnidaria (for coelenterates proper) and Acnidaria (for the cteno­phores).

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It's a neat picture. Japan wrote it so it must be good.

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Octocorallia and hexacorallia Octocorallia (precious coral) Most live at a depth of 100 meters (328 feet) in the sea off Japan. Some even live at a depth of more than 1,000 meters (3,280 feet). Hexacorallia (reef-building coral) Most live at a depth of approximately 30 meters (98 feet).

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talks about jellyfish stinging humans

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Jellyfish are producing at alarming rates and humans may have something to do with it. ➡ Subscribe: http://bit.ly/NatGeoSubscribe ➡ Watch all clips of Explorer here: http://bit.ly/WatchExplorer ➡ Get More Explorer: http://bit.ly/MoreExplorer ➡ EXPLORER AIRS MONDAYS 10/9c.

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Find out why the notorious box jellyfish deserves its loathsome reputation. The animal's toxins are among the strongest found in nature.

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This is a cool video, check it out!

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Still in the Tannin-stained waters of the Sunken Valley the team explore in virtual darkness looking for the mysterious sea pens. Time-lapse photography reveals the behaviour of these extraordinary creatures which anchor themselves to the seabed. Amazing clip taken from the BBC's Ocean series.

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explains everything about anthozoa which I didn't even know they were a thing

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This page is the overview, but leads to highly detailed information.

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Cnidarians - Cnidaria - Overview - Encyclopedia of Life

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Cnidarians are a diverse group of aquatic animals. More than 9,000 species are part of the Phylum Cnidaria, and all species are aquatic. Cnidarians are widespread in marine habitats and less common in fresh water. This interesting group of invertebrates includes many charismatic organisms such as hydras, sea fans, jellyfishes, sea anemones, corals, and the Portuguese man-of-war.

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A beautiful anemone playing host to a colorful Clownfish, is a popular icon of the reef aquarium!

Sea anemones have become very popular inhabitants for both the reef tank and the saltwater aquarium. Watching an anemone living in a cooperative relationship with a colorful Clownfish is a rewarding experience.

There are 10 types of anemones that will host clown fish as well as many other types available for the aquarium. Many species are very hardy, and with careful selection all levels of marine aquarists can enjoy keeping sea anemones.

Many anemone species are reasonably available, but each species varies in its aquarium hardiness and difficulty of care. When selecting a sea anemone it is important to choose not only a hardy specimen, but also a healthy specimen. It is also important to choose an anemone that is suitable for your aquarium type. Most types of anemones do well in a reef tank, but there are a few that are better kept in a saltwater aquarium rather than a reef environment.

Many types of anemones are known to be quite durable once they are established, but some anemones simply do not fare well in captivity, or are not the right choice for the aquarium. There are also some anemones to be cautious of, or to obtain only once you are well prepared. These include the Rock Anemones like Aiptasia, and the Stinging Sea Anemones from the Actinodendronidae family. Be sure to learn about the type of anemone you want so you can determine its needs and its suitability for your aquarium.

Types of Sea Anemones

The Sea Anemone
Order: Actinaria
Sea Anemones are in the order Actinaria, and are often called the "true anemones".
They have an adhesive pedal disc or foot used to hold them in place, a hollow cylindrical or column shaped body, and an oral disc, or mouth, at the top which is surrounded by a circle of tentacles containing stinging nettle-cells or nematocysts. These stinging cells are used to capture prey and push it into its mouth. With only a slight touch, the tentacles shoot harpoon-like filaments into passing prey, injecting it with a paralyzing neurotoxin, and then guiding it into the mouth.

The Tube Anemone
Order: Ceriantharia
Tube Anemones are in the order Ceriantharia. They are also known as Tube-dwelling Anemones or Burrowing Sea Anemones. They are solitary animals, living and withdrawing into tubes that are buried in soft sediments.
Tube Anemones look very similar to sea anemones, but they have elongated bodies adapted for burrowing and they lack the pedal disc or foot. The cylindrical shaped body is covered by a tube of secreted mucous and is usually hidden in the muddy substrate. The mouth is on a central disk, surrounded by short tentacles in the center and longer tentacles on the margins. Usually only the tentacles are visible above the ground.

Sea Anemone Stings

Some anemones with strong stings are the Carpet anemones in the Cryptodendrum genus (previously Stoichactis) and the Heteractis genus, Condy Anemones in the Condylactis genus, Tube Anemones in the Pachycerianthus genus. Those with very dangerous stings are the 'stinging sea anemones' belonging to the family Actinodendronidae.

Anemone Stings : Most anemones, when you touch them feel slightly sticky. They can cause a stinging sensation or even a rash. The area affected can start as a rash that spreads quickly. For most people this is just an irritation, but some people can be allergic to the poison and may have a more severe reaction. In rare cases it can cause anaphylactic shock which is a complete shutdown fo the respiratory system.

Treatment For Anemone Stings : Stings from an anemone or from a venomous fish can be painful. To relieve painful stings, provide treatment right away. This treatment can also be used to relieve the pain from the stings of venomous fish.

First rinse the area with vinegar or alcohol.

Then make a paste, half and half of sodium bicarbonate and water, and cover the wound.

Finally, after the paste dries, dusting the wound with talcum powder or papain (ingredient sold as a meat tenderizer) can help neutralise the venom.

Hardy Aquarium Anemones

Some anemones are easier to keep than others. The best anemone survival rates that we have found are among the smaller anemones and also one of the carpet anemones, the saddle anemone. Another anemone that is very common is the Bubble Tip Anemone Entacmaea quadricolor, also called the purple base or pink tip anemone. This is fortunate since many different kinds of clownfish will accept the purple base anemone as a host.

Clownfish host anemones that are easier to keep include:

Other hardy anemones include:

Some of the more difficult anemones to keep include:

We have found that anemones are often challenging. We have purchased almost every anemone available and have read surveys taken in an attempt to determine the survival rate of anemones once they enter into the aquarium industry. In both cases the survival rate is very low, particularly among the larger carpet anemones. This is unfortunate as the larger carpet anemones are natural hosts for a number of anemone fish, but fortunately clown fish will take other kinds of anemones as hosts.

One of the best books we have found on anemones and their care, and the main reference for information presented here, is the book seen to the right above. It is the "Field Guide to Anemonefishes and their Host Sea Anemones" by Daphne G. Fautin & Gerald R. Allen. Both of these authors believe that carpet anemones should not be removed from the oceans for the aquarium trade.

Clown Fish Anemone

Sea anemones have special relationships with other animals and organisms, where each animal helps the other in some way. Perhaps the most well known relationship is that of some sea anemone species providing a home and protection for clownfish. In return the clown fish help the anemone by cleaning its tentacles of detritus, and possibly running off potential predators.

Though there are over 1100 types of sea anemones, only 10 species are known to host Clownfish:

Buy Sea Anemone

Successfully keeping an anemone is often dependent upon choosing a healthy specimen when you buy sea anemones. This checklist provides some things to avoid when choosing an anemone:

Loose or open mouths

Tentacles that are short when they should be long, or skinny when they should be fat

There should not be any tears on its foot

An anemone should be a little bit sticky.

Coloring - if an anemone is white, it may have released its zooxanthellae that it depends on for much of its food. This can spell disaster over time.

Anemones that are not attached to anything may have trouble attaching.

Anemones contract when they feel threatened, make sure the anemone you get reacts when it is picked up

There are many naturally colored anemones, but occasionally artificially colored anemones are offered for sale. Primarily these are Sebae or Ritteri Anemones colored with vegetable dyes or other dyes to increase their salability. This coloring doesn't add to the anemones vitality, nor does it last.

Sea Anemone Pictures

Rock Anemones - Aiptasia Anemones and Relatives
Family: Aiptasiidae

Genus:
Aiptasia

Aiptasia anemones are so hardy and invasive they are considered as Aiptasia pest anemones in mini-reef aquariums. Another pest anemone is the Manjano Anemone or Majano. Many articles have been written on how to get rid of pest anemones, from injecting them with kalkwasser, to using aquastick (an underwater epoxy) to seal them into a hole in the rock! Apparently if they are smashed or removed by hand, small pieces of the anemone will re-grow to adult Aiptasia. Aiptasia are not used by clownfish probably because they are too small.

Curlicue Anemone
Bartholomea annulata

The Curlique Anemone is a sea anemone in the Aiptasiidae family that is not as much of a pest. It has beautiful, delicate tentacles. It prefers medium to high light conditions but will survive in low light as well. This is not a clownfish host.

Clown Fish Anemone - Host Anemones

Bubble Anemone- with two Sebae Clowns
Entamaea quadricolor
Photos © Animal-World

The Bubble Tip Anemone is a hardy anemone that likes medium to strong light and does well in aquariums. Many different kinds of clownfish will accept it as a host. We had a pair of Cinnamon clowns breed in a bubble anemone repeatedly for over a year. Pictured with the anemone above are Sebae clowns.

Saddle Anemone
Stichodactyla haddonni Photos © Animal-World

The Saddle Anemone or Carpet Anemone has a lot of very short tentacles. They are very adhesive to human skin. If you touch one it will stick fast, sometimes to the point of pulling the tentacles out of the anemone when you detach it. The one pictured above is a rarer form of the Saddle anemone which has lighter stripes running from the mouth (center) to the outside edge. The Saddle anemone is by far the hardiest of the carpet anemones. It likes strong light levels.

Long Tentacle Anemone
Macrodactyla doreensis

The Long Tentacle Anemone will get fairly large (we have one that is the size of a dinner plate) and is accepted by many different kinds of clownfish. We have personally seen Tomato, Sebae, Percula, Skunk, and Maroon clowns accept it as a host. They are usually hardy and easy to keep as long as you have medium to strong light.

Tube Anemone - Burrowing Sea Anemone
Genus including: Pachycerianthus, Ceriantheopsis. Cerianthus

This is a variety of tube anemone but we're not sure what species

Tube anemones are some of the few sea anemones that do well in low light levels. The name ' tube dwelling anemone' is derived from their ability to build a tube to live in. It was previously believed that they created their tube by releasing a mucus, which then become covered with sand. Today however, it is known that they create the tube by releasing threads of a special type of nematocysts called 'ptychocyst'. The result is a woven fibrous structured of stinging cells that help protect it from attack. Though its tentacles are non-retractable, if it is frightened or disturbed it can instantly withdraw its whole body into the tube and become hidden

They are nocturnal and should be fed finely chopped fish and shrimp every other day or so. Tube anemones are found in all the warm water oceans of the world. Their long tentacles have a powerful sting and therefore should be kept away from other anemones and corals. They are not used as hosts by clownfish.

Sea Anemones - Other Types of Anemones

Color Variations

The Condy Anemones are inexpensive anemone which come from the Caribbean, thus the name Caribbean Anemone. They are usually very hardy and come in several colors. The most attractive have purple tipped tentacles. Condylactus like high light levels but will do okay in low to medium light. These anemones are the most inexpensive and are very hardy. They can do well in any marine aquarium, not just reefs.

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Sea anemones have become very popular inhabitants for both the reef tank and the saltwater aquarium. Watching an anemone living in a cooperative relationship with a colorful Clownfish is a rewarding experience. There are 10 types of anemones that will host clown fish as well as many other types available for the aquarium.

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background of the Cnidaria ph

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The Tealia anemone is a good example of an Anthozoan.

These flower-like animals resemble plants, but they have a mouth
at the center of their tentacles, and a primitive digestive system.

© Jonathan Bird/ORG
For educational use only.

The phylum Cnidaria (pronounced nid-AIR-ee-ah) contains approximately 9000 living species worldwide. They are among the simplest of the so-called "higher" organisms, but are also among the most beautiful. The creatures in this phylum are radially symmetrical. This means that the parts of the body extend outward from the center like the spokes on a bicycle wheel. A common example of radial symmetry is the sea star (a member of the Echinoderm phylum) or the anemone, a Cnidarian (seen below).

The Cnidarians include the hydroids, jellyfish, anemones, and corals. All Cnidarians have tentacles with stinging cells in their tips which are used to capture and subdue prey. In fact, the phylum name "Cnidarian" literally means "stinging creature." The stinging cells are called cnidocytes and contain a structure called a nematocyst. The nematocyst is a coiled thread-like stinger. When the nematocyst is called upon to fire, the thread is uncoiled, and springs straight. The harpoon-like thread punctures through the cnidocyte wall and into the prey. Most Cnidarians also have a toxin in their stinger which helps to disable the prey. The nematocyst is fired either by the tentacle touching something, or in some cases by a nerve impulse from the animal telling it to fire.

Most Cnidarian's nematocysts are not harmful to humans, as the stinger cannot penetrate sufficiently into human skin to inflict any harm. There are some jellyfish, however, which can deliver extremely painful, and in a few cases, even fatal, stings to humans.

The Cnidarian can have one of two basic body types, polypoid or medusoid. The polypoid (POL-ip-oyd) is the configuration of corals and anemones, with the tentacles and mouth generally facing up, and the other side affixed to a substrate or connected to a colony of other creatures of the same species. In the Medusoid, the organism is essentially upside-down, with the mouth and tentacles generally pointed down. These types of Cnidarians are usually free-swimmers, like jellyfish. This is why the anemone is sometimes called an upside-down jellyfish; that's just about what it is!

The Phylum is divided up into 3 Classes, called the Hydrozoans (meaning "water-animals"), the Anthozoans (meaning "Flower-animals") and Scyphozoans (meaning "bowl-animals").

HYDROZOANS
The Hydrozoans (hydroids) and Anthozoans (anemones, corals) are mostly all bottom dwelling animals, in a polypoid shape. This means that they live attached to the bottom with their tentacles and mouth pointing up. The northern Red Anemone is a good example of an Anthozoan. Some are solitary, meaning that they live alone, but some are colonial, meaning that they live in groups, connected together with living tissue, and share food resources.

SCYPHOZOANS
The Scyphozoans (SKY-foe-zo-ans) are the animals we call jellyfishes, although since they aren't real fishes, a better word is "jellies." The jellies drift along in the water generally with the mouth and tentacles pointing down. We call this a medusoid body form. Some jellies are extremely venomous to humans. Some can even kill people with enough contact of tentacle on bare skin, but most are not that dangerous, producing only a mild rash similar to that caused by poison ivy.

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JONATHAN BIRD'S BLUE WORLD

The two different forms of a Cnidarian body.

A coral colony consists of hundreds or thousands of tiny polyps. Each polyp is an individual animal (basically a small anemone) but they live together as a group.

The Lion's Mane Jelly is a venomous Scyphozoan which can sting people with its long tentacles. Since the tentacles can hang so far down, the jelly can use these tentacles to kill fish which swim through them without ever seeing the jelly itself way up above! This is an example of a cnidarian with a medusoid shape.

The are Pink Hearted hydroids, members of the class Hydrozoa. They look like delicate plants but they are animals that sting and capture food.

This is fire coral. It has a potent sting that leaves an itchy rash on human skin. It is not technically a coral, but a kind of hydroid that encrusts other objects (including other corals).

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The Tealia anemone is a good example of an Anthozoan. These flower-like animals resemble plants, but they have a mouth at the center of their tentacles, and a primitive digestive system. © Jonathan Bird/ORG For educational use only. The phylum Cnidaria (pronounced nid-AIR-ee-ah) contains approximately 9000 living species worldwide.

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Introduction to Sea Anemone

Sea Anemone are predatory and as you can tell from their name, they live in bodies of water. They are related to both and corral. It is a type of polyp that is attached to a surface in the water. They are very small, often less than one inch in diameter. The largest ones are only about 1 ½ inches wide. However, they can be up to 6 feet tall.

Sea Anemone Description

Sea Anemone offer hundreds of that allow them to identify their surroundings. They can also sting things around them and release venom through those tentacles. They can be very colorful and often look like flowers. They are often various shades of red and pink. Others are white in color and they can have a variety of overall appearance features.

The body consists of a foot that is shaped like a pedal. That is what allows them to secure and stay attached to their location in the water. They have a body that looks like a long cylinder. The tentacles are all around the mouth that is very hard to see. There are over 1,000 species of Sea Anemone and that is why their appearances can be so varied.

Sea Anemone Distribution

You will find them living in all of the oceans around the world. They can live in a variety of water temperatures and at a variety of different depths. They are mainly found though in the coastal waters that are considered to be tropical. They tend to live in areas that offer plenty of green algae. Due to their venom there are very few types of aquatic life that will consume them.

Sea Anemone Behavior

Experts believe that the relationship between the Sea Anemone and green algae is very spectacular. It ensures that both are able to survive. This entity ensures that the green algae has a safe place to thrive in the water. It also gives them access to sunlight so that they can thrive.

In exchange though the green algae offers plenty to the Sea Anemone too. This includes both oxygen and glucose. Those are both byproducts of the photosynthesis process of the green algae. They have a similar type of relationship with Clown Fish.

They have been known to sting humans such as scuba divers. Most of the time though the toxins they deliver from the venom aren't powerful enough to really affect a human. For those that are sensitive to them though it could be a factor.

Sea Anemone Feeding

Due to the large polyps that the Sea Anemone display they are able to consume prey that is much larger than they are. They mainly consume fish as they approach them. Since they blend in so well with the surroundings the fish and other prey come along and don't even know they are in danger.

The tentacles from the Sea Anemone inject them with venom which immobilizes them. In addition to fish they consume zooplankton, crustaceans, larvae, and worms. They are opportunistic and they will consume food any time it come along. If there is no food they can't survive as they aren't able to mobile their bodies.

Sea Anemone Reproduction

For the Sea Anemone a complex process called lateral fusion is what takes place for reproduction. This involves the side of the entity opening up and then an identical part being created that is a second living one.

They will release eggs and sperm into the water and then the sperm finds the eggs to fertilize them. This is how they take part in reproduction as well. However, only a very small number of their eggs will survive to the age of maturity. They are able to live attached to rocks or the bottom of the sea for up to 50 years.

Sea Anemone Related Articles

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Sea Anemone are predatory and as you can tell from their name, they live in bodies of water. They are related to both and corral. It is a type of polyp that is attached to a surface in the water. They are very small, often less than one inch in diameter.

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National Geographic's latest stories about oceans

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Credit:

K. Raskoff, Monterey Peninsula College, Arctic Exploration 2002, NOAA

Jellyfish and comb jellies are gelatinous animals that drift through the ocean's water column around the world. They are both beautiful-the jellyfish with their pulsating bells and long, trailing tentacles, and the comb jellies with their paddling combs generating rainbow-like colors. Yet though they look similar in some ways, jellyfish and comb jellies are not very close relatives (being in different phyla-Cnidaria and Ctenophora, respectively) and have very different life histories.

Both groups are ancient animals, having roamed the seas for at least 500 million years. And, in the modern age, they are having similar effects on ecosystems. As seawater temperature rises, predators of jellies are removed by fishing, more structures are built in seawater, and more nutrients flow into the ocean, some types of jellyfish and comb jellies may be finding it easier to grow and survive. Whatever the reason, huge explosions in jelly numbers (a jelly bloom) can disrupt fisheries, make for unpleasant swimming, or foul up the works of power plants that use seawater for cooling. Invasive jellies have also wreaked havoc in some parts of the world.

While jellyfish and comb jellies have several anatomical differences, the basics are the same. Both have two major cell layers: the external epidermis and the internal gastrodermis. (Ctenophores also have musculature in their in-between layer, the mesoderm, but it likely evolved separately from the mesoderm found in bilaterians like people.)

The gastrodermis lines the all-purpose gut and an opening where food enters and reproductive cells are released and taken in. Jellies have no need for a stomach, intestine, or lungs: nutrients and oxygen slip in and out of their cell walls through the gastrodermis or even their bodies' outer cells. The outer cells that make up the epidermis contain a loose network of nerves called the "nerve net." This is the most basic nervous system known in a multicellular animal. (See Brains of Jelly? for more.)

Between these layers is a gelatinous material called mesoglea, which makes up most of their bodies. (Although some small species have very thin mesoglea.) Jellyfish and comb jellies are 95 percent water and so, rightly, mesoglea is mostly water! It also contains some structural proteins, muscle cells, and nerve cells, forming a kind of internal skeleton.

Comb jellies are named for their unique feature: plates of giant fused cilia, known as combs, which run in eight rows up and down their bodies. The combs act like tiny oars, propelling the comb jelly through the water. Many microscopic organisms, such as bacteria, also use cilia to swim-but comb jellies are the largest known animals to do so. The comb-rows often produce a rainbow effect. This is not bioluminescence, but occurs when light is scattered in different directions by the moving cilia.

Until 2015 scientists believed that comb jellies removed their waste via their "mouth," or what was believed to be the one hole in their body plan. A new study showed that comb jellies in fact release indigestible particles through pores on the rear end of the animal. This discovery adds another piece to the evolutionary puzzle of when animals evolved to have anuses.

Many comb jellies have a single pair of tentacles (often each tentacle is branched, giving the illusion of many tentacles) that they use like fishing lines to catch prey. They are armed with sticky cells (colloblasts) and unlike jellyfish, the tentacles of comb jellies don't sting. (See The Stings: Nematocysts and Colloblasts for more.)

Jellyfish transition between two different body forms throughout their lives. The familiar body plan that looks like an upside down bell with tentacles hanging down from the inside is called the medusa. The polyp, the other cnidarian body plan, is the opposite, with the mouth and tentacles above, like a sea anemone. (See more in Reproduction & Lifecycle.)

Jellyfish also have a stinging adaptation that is unique to them and their close relatives (including sea anemones and hydras): nematocysts, or stinging cells. (See The Stings: Nematocysts and Colloblasts for more.)

Jellyfish and comb jellies vary greatly in size depending on the species. Most jellies range from less than half an inch (1 cm) wide to about 16 inches (40 cm), though the smallest are just one millimeter wide! The largest jellies are the Lion's Mane Jellyfish (Cyanea capillata), which can be almost 6 feet wide (1.8 m) with tentacles over 49 feet (15 m) long. Larger individuals have been seen, but they are not typical. Venus's girdle (Cestum veneris), a belt shaped comb jelly, can be 40 inches (1 meter) long.

The Nervous System

Credit:
Bastian Bentlage

Jellies don't have brains as we typically think of them: rather, they have a network of neurons ("nerve net") that allows jellies to sense their environments, such as changes in water chemistry indicating food or the touch of another animal. The nerve net has some specialized structures such as statocysts, which are balance sensors that help jellies know whether they are facing up or down, and light-sensing organs called ocelli, which can sense the presence and absence of light.

Additionally, some jellyfish have sensory structures called rhopalia, which contain receptors to detect light, chemicals and movement. One group of jellyfish, the cubozoan jellyfish, have complex eyes with lenses, corneas and retinas in their rhopalia. Although they respond to visual stimuli, scientists don't know how the jellyfish interpret the images created by their eyes since they don't have a brain with which to process them. Their nerve ring, a ring-shaped concentration of nerves found in jellyfish, seems to be involved, however.

A 2017 study of the upside-down jellyfish, Cassiopea, found that a brain is not required to experience sleep. At night Cassiopea enters a sleep-like state where it pulses less frequently than during the day and is slow to respond to disturbances. When kept awake throughout the night, the next day the jellyfish appear to be tired-their pulsing was noticeably slower than if they had a solid night of sleep. It is the first time an animal without a brain was observed sleeping. The discovery suggests sleep among all animals is an ancient characteristic with a shared evolutionary beginning, considering the neural network of jellyfish evolved before centralized nervous systems like a brain.

Diversity

All jellyfish are Cnidaria, an animal phylum that contains jellies, sea anemones, and corals, among others. There are more than 10,000 species of Cnidaria, and less than 4,000 of these are Medusazoa-those animals we think of as jellyfish. Those 4,000 jellyfish can be divided into four different groups.

SCYPHOZOA are the most familiar jellyfish, including most of the bigger and more colorful jellies that interact with humans, and are sometimes called "true jellyfish" for this reason. Scyphozoa spend most of their lives in the medusa body form, and there are at least 200 species.

HYDROZOA are jellyfish look-alikes but not in the same group as the "true jellyfish." The swimming medusa stages of this group are often small and inconspicuous, whereas the bottom-dwelling polyps, or hydroids, usually take the form of large colonies. (See Reproduction & Lifecycle.) In the water column, the colonial siphonophores may be quite spectacular. These include the notorious Portuguese Man-o-Wars and many deep-sea forms, some of which stretch out up to 50 meters in length like giant fishing nets. Colonial siphonophores are composed of many specialized individuals called zooids that are genetically identical because they all come from a single fertilized egg. In 2016, researchers discovered what they believe to be a new hydrozoan species of Crossota, 12,140 feet (3,700 meters) deep within the Mariana Trench. Floating in the water column like a glowing spaceship, this Crossota jellyfish is an exception to most hydrozoans and will spend the majority of its life as a large medusa. There are around 3,700 species of Hydrozoa.

CUBOZOA are the box jellyfish, named for their box-like bells. Some cubozoans, such as the sea wasp (Chironex fleckeri), produce some of the most potent venom known. Cubozoan jellyfish also have a more developed nervous system than other jellyfish, including complex eyes with lenses, corneas and retinas. Some even engage in elaborate (for a jellyfish) courtship behavior! There are at least 36 species. In 2011, Allen Collins, a jellyfish expert at the Smithsonian, discovered a new species, which was named Tamoya ohboya in a public naming contest. (Listen to a podcast about box jellies.)

STAUROZOA are the stalked jellyfishes, which don't float through the water like other jellies, but rather live attached to rocks or seaweed. They are trumpet-shaped, and mostly live in cold water. There are around 50 staurozoan species, many notable for their unique combination of beauty and camouflage.

Jellies are found in oceans worldwide, in shallow and deep water, and a few can even be found living in freshwater.

Compared to jellyfish, there are far fewer species of ctenophores: only 100-150 species have been found, but quite a few are out there yet to be discovered and fully documented. The best-known comb jellies are those found close to shore because, there, they are most likely to run into people. Those can be roughly divided into three groups.

CYDIPPIDS all have rounded bodies-some spherical, some oval-with branched tentacles. (This means that their tentacles are fringed with smaller tentacles.) These tentacles can be withdrawn into the jelly's body into special sheaths or pouches on either side of their mouths.

LOBATES are defined by two flattened lobes that extend from the typical rounded ctenophore body down below their mouths. They also have short tentacles and tend to grow larger than cydippids.

BEROIDS (also known as "nuda") are sack-shaped and have no tentacles at all-but they do have a very large mouth, which they can zip shut very tightly.

Open ocean ctenophores are much less known. They tend to be very fragile because they don't have to endure rough coastal waves; many of them are so fragile that they cannot be collected by submersibles and are known only by photographs. They come in a great diversity of forms. Some are shaped like belts (Cestida), while others don't float in the water column at all, but live on the seafloor! (These are known as benthic ctenophores.)

Comb jellies live throughout the world's ocean, although most species prefer warmer water.

Evolution

Jellyfish and comb jellies are in different phyla, but scientists have long argued over whether they have an especially close relationship apart from the rest of the animal kingdom. To distinguish them, all Cnidaria and Ctenophora were once described as Coelenterata-but that term is no longer commonly used.

To this day, some researchers believe they are sister groups, while others think they are not closely related. Either way, there are still plenty of other questions to argue about, such as how long ago the two groups diverged, and even whether ctenophores might be the most ancient group of animals, diverging even earlier than sponges in the animal tree of life. These arguments continue because, as some of the simplest animals alive today, understanding their place in the tree of life helps people understand how all other animals-including people-evolved.

Whichever came first, comb jellies and jellyfish (and other Cnidarians) made an important step in evolutionary history: they are the earliest known animals to have organized tissues-their epidermis and gastrodermis-and a nervous system. They're also the first animals known to swim using muscles instead of drifting with the whims of the waves.

The oldest ancestors of modern day jellies lived at least 500 million years ago, and maybe as long as 700 million years ago. That makes jellyfish three-times as old as the first dinosaurs!

Because jellies have no bones or other hard parts, finding jellyfish fossils is rare. But in 2007, a group of scientists including Allen Collins from the Smithsonian National Museum of Natural History, discovered some beautifully-preserved jellyfish fossils buried in Utah from 505 million years ago. From around the same period, scientists have also found well-preserved comb jelly fossils in the Burgess Shale.

Credit:
Mary Elizabeth Miller, Dauphin Island Sea Lab

Jellyfish and ctenophores are carnivorous, and will eat just about anything they run into! Most jellies primarily eat plankton, tiny organisms that drift along in the water, although larger ones may also eat crustaceans, fish and even other jellyfish and comb jellies. Some jellyfish sit upside down on the bottom and have symbiotic algae (zooxanthellae) in their tissues, which photosynthesize, and so get much of their energy the way plants do.

While their nematocysts and colloblasts do help them defend themselves, plenty of animals manage to catch and eat jellies: more than 150 animal species are known to eat jellies, including fish, sea turtles, crustaceans, and even other jellyfish. Jellies are the favorite food of the ocean sunfish ( Mola mola) and endangered leatherback turtle ( Dermochelys coriacea), which will migrate thousands of miles for the gelatinous delicacy. Young jellyfish are small enough to be part of the general zooplankton population and are eaten by many animals.

Humans also eat jellyfish: people have fished for jellies for at least 1700 years off the coast of China. Some 425,000 tons (more than 900 million pounds) of jellyfish are caught each year by fisheries in 15 countries, and most are consumed in Southeast Asia. Eating jellyfish may become more common around the world as we overfish more preferable fish species.

Feeding Adaptations

Jellyfish and ctenophores both have tentacles with specialized cells to capture prey: nematocysts and colloblasts, respectively. Jellyfishes' nematocysts are organelles within special cells (cnidocytes) that contain venom-bearing harpoons. The cell is activated upon touch or chemical cue, causing the harpoon to shoot out of the cell and spear the prey or enemy, releasing toxin-a process that takes only 700 nanoseconds. A small number of jellyfish are very toxic to humans, such as the box jellyfish ( Chironex fleckeri) and Irukandji jellyfish ( Carukia barnesi), which can cause severe reactions and even death in some people.

Many comb jellies have colloblasts lining their tentacles, which work like nematocysts but release glue instead of venom. Upon touch, a spiral filament automatically bursts out of colloblast cells that releases the sticky glue. Once an item is stuck, the comb jelly reels in its tentacle and brings the food into its mouth. One species of ctenophore ( Haeckelia rubra) recycles nematocysts from hydrozoan jellyfish it consumes and uses these to stun and kill prey.

Comb jellies come in many shapes and sizes, and so within the group there are many ways to feed. The rounded and tentacled cydippids have branched tentacles lined with colloblasts that they use, in the traditional jelly style, like a fishing line to trap food and bring it to their mouths.

The lobate ctenophores have two flattened lobes that reach below their mouths. Special cilia waving between the lobes generate a current to pull planktonic food between the lobes and into the jelly's mouth, allowing them to feed on plankton continuously. They also use colloblast-lined tentacles to catch food.

The tentacle-less beroids depend on their large mouths. Instead of catching food with colloblasts, they swallow their prey (often other ctenophores!) whole and then clamp their mouths shut, giving them no escape route. Inside their mouths they have small cilia that act as teeth, pulling food apart, which also direct the food into the comb jelly's gut.

Defense Adaptations

Many jellyfish and comb jellies are able to produce light-an ability known as bioluminescence. They have proteins in some tissues that undergo a chemical reaction to produce blue or green light in response to stimuli such as touch. No one's quite sure why jellies bioluminesce, but it seems to be mainly a defense tactic. A bright enough flash could be enough to startle a predator-or to attract an even bigger predator to make the jelly's predator into prey.

Jellies have also adapted their body color to camouflage in the darkness. Most are nearly colorless and transparent, so they can be difficult for predators to see. However, some deep sea jellyfish and comb jellies are a bright red or orange color. Why would they be red instead of black to blend in with the dark water? Red cannot be seen in dark water (deeper than 200 meters), so there's no greater protection from black than red. But red is preferred to black because pigment is easier for animals to produce. Some deep sea jellies just have dark red guts, possibly serving to mask luminescent prey from other larger predators with eyes.

Jellyfish have a complex life cycle: a single jellyfish reproduces both sexually and asexually during its lifetime, and takes on two different body forms.

An adult jellyfish is called a medusa, which is the familiar umbrella-shaped form that we see in the water. Medusa jellyfish reproduce sexually by spawning-the mass release of eggs and sperm into the open ocean-with entire populations sometimes spawning all together. Male and female jellyfish (there aren't many hermaphrodites) release the sperm and eggs from their mouths. In most species, fertilization takes place in the water; in others, the sperm swim up into the female's mouth and fertilize the eggs within.

The fertilized eggs then develop into planulae (singular: planula), which are ciliated free-swimming larvae shaped a bit like a miniature flattened pear. After several days of development, the planulae attach to a firm surface and transform into flower-like polyps. The polyps have a mouth and tentacles that are used to feed on zooplankton.

Polyps reproduce asexually by budding-when a polyp divides roughly in half to produce a new genetically identical polyp-or they can produce or transform into medusae, depending on the type of jellyfish. Hydrozoan polyps bud medusae from their sides; cubozoan polyps each transform into a medusa.

In schyphozoans, a process called strobilation takes place (shown in video and in diagram). During strobilation, a polyp splits into 10-15 plate-like segments stacked atop one another in a tower called a strobila. After a segment separates from the strobila, it is called an ephyra, a juvenile jellyfish. Ephyrae mature into the medusa form.

Most jellyfish are short lived. Medusa or adult jellyfish typically live for a few months, depending on the species, although some species can live for 2-3 years in captivity. Polyps can live and reproduce asexually for several years, or even decades.

One jellyfish species is almost immortal. Turritopsis nutricula, a small hydrozoan, can revert back to the polyp stage after reaching adult medusa stage through a process called transdifferentiation. This is the only animal known to do so.

In comparison to the jellyfish, comb jellies have a very simple lifecycle. Most species are hermaphroditic and able to release both eggs and sperm into the water, which drift with the waves until they find other gametes. Because most species have both male and female gametes, it's thought that they can self-fertilize as well.

This method may not seem very efficient, since it's likely that most of the gametes never find a match. But ctenophores make up for this by releasing them every day. If they run out of food while producing so many eggs and sperm, they can shrink and hunker down until they run into more food and can start reproducing again.

Once eggs and sperm find each other, the embryo develops into a larva that looks just like a small adult ctenophore-and, from there, all it has to do is grow up.

One species ( Mertensia ovum) can reproduce even when it is still larva, and scientists think other species are also able to reproduce at a young age. This means that comb jelly populations can grow very fast under certain conditions.

Around the world, vast aggregations of jellyfish and comb jellies seem to be more common. These aggregations are known as "jellyfish blooms" or "jellyfish outbreaks," which can cause a wide array of problems. Too many jellies in the water can be a danger to swimmers, forcing towns to close their beaches. Jellies have clogged up machinery at coastal power plants, causing power outages. They can interfere with fisheries by eating fish larvae, and fisherman catch jellies instead of the fish they want. Where they occur, blooms of jellyfish even change seawater chemistry. Scientists hope to address this problem through the discovery of a practical application for jellyfish, like substituting jellyfish for the fish used in aquaculture feed. Jellyfish mucus, which has been shown to bind to microplastics, may even one day be used in water treatment facilities to help combat the world's growing plastic problem.

Why are jellies becoming more common around the world? It seems likely that their spread is human-caused, although some scientists have argued that the blooms are part of a natural cycle. If the blooms are human-caused, there are several probable culprits.

OVERFISHING Over the past two decades, between 100 and 120 million tons of marine life have been removed from the ocean by fisheries each year on average. A lot of these marine species, including fish and invertebrates such as squid, eat some of the same food that jellies do: mainly, zooplankton. As these other predators of plankton are fished from the sea, jellies have less competition for food, and are able to grow and reproduce with fewer limits.

NUTRIENTS When fertilizers runoff from the rivers to the seas, they can create dead zones: areas of ocean where little life survives. The nitrogen and phosphorus in fertilizer helps phytoplankton grow very quickly, and there can be so many of these single-celled plant-like animals that they deplete oxygen from the water. Most animals can't survive in these conditions, but many jellies can better tolerate low-oxygen environments.

CLIMATE CHANGE The ocean is warming, and this might give some jellies a boost. The warmer water could help jelly embryos and larvae develop more quickly, allowing their populations to grow more quickly. And jellies that prefer warmer water will have more area to live in. However, this could also hurt some species as cold-water jelly species see their habitat shrink.

SUBMARINE SPRAWL Many industries, such as shipping, drilling and aquaculture, build docks, oil platforms and other structures in the water-sometimes referred to as "ocean sprawl"-which can serve as nurseries for jellyfish. To undergo their polyp stage, jellyfish need solid surfaces to settle upon. It's much easier for jellyfish polyps to attach to man-made structures made of wood, brick and concrete than sand. Ocean sprawl provides more and better habitat for jellyfish to reproduce and complete their lifecycles.

Jellies are very good at surviving: they have broad diets, reproduce quickly, can shrink down if food runs out and then revive, and tolerate low-oxygen water. So, as you can imagine, they are also very good at thriving in new ecosystems once they arrive.

In the 1980s, the sea walnut (Mnemiopsis leidyi), a type of comb jelly, was brought to the Black Sea in ship ballast water. It reproduced and spread quickly, gobbling up zooplankton and leaving little behind for the larvae of commercial fish species, including anchovy, scad and sprat. Within a decade, the comb jellies took over the Black Sea and many of the fish populations collapsed, bringing local fisheries down with them. In a stroke of accidental luck, a different species of comb jelly ( Beroe ovum)-a predator of the sea walnut-was brought over in a ship, and it's helping to bring down the population. A similar story of fishery collapse coinciding with jellyfish blooms is playing out off the coast of Japan.

However, the collapse of a fishery doesn't always end in jellyfish. A crash in the pollock and walleye fishery in the Bering Sea left an opening for jellyfish but, after reigning for a few years, the jellies gave up their crown as the fish returned. And when the Peruvian anchovy fishery collapsed in the 1970s, no jellyfish swarmed in to take their place.

While jellyfish and comb jellies have several anatomical differences, the basics are the same. Both have two major cell layers: the external epidermis and the internal gastrodermis. (Ctenophores also have musculature in their in-between layer, the mesoderm, but it likely evolved separately from the mesoderm found in bilaterians like people.)

The gastrodermis lines the all-purpose gut and an opening where food enters and reproductive cells are released and taken in. Jellies have no need for a stomach, intestine, or lungs: nutrients and oxygen slip in and out of their cell walls through the gastrodermis or even their bodies' outer cells. The outer cells that make up the epidermis contain a loose network of nerves called the "nerve net." This is the most basic nervous system known in a multicellular animal. (See Brains of Jelly? for more.)

Between these layers is a gelatinous material called mesoglea, which makes up most of their bodies. (Although some small species have very thin mesoglea.) Jellyfish and comb jellies are 95 percent water and so, rightly, mesoglea is mostly water! It also contains some structural proteins, muscle cells, and nerve cells, forming a kind of internal skeleton.

Comb jellies are named for their unique feature: plates of giant fused cilia, known as combs, which run in eight rows up and down their bodies. The combs act like tiny oars, propelling the comb jelly through the water. Many microscopic organisms, such as bacteria, also use cilia to swim-but comb jellies are the largest known animals to do so. The comb-rows often produce a rainbow effect. This is not bioluminescence, but occurs when light is scattered in different directions by the moving cilia.

Until 2015 scientists believed that comb jellies removed their waste via their "mouth," or what was believed to be the one hole in their body plan. A new study showed that comb jellies in fact release indigestible particles through pores on the rear end of the animal. This discovery adds another piece to the evolutionary puzzle of when animals evolved to have anuses.

Many comb jellies have a single pair of tentacles (often each tentacle is branched, giving the illusion of many tentacles) that they use like fishing lines to catch prey. They are armed with sticky cells (colloblasts) and unlike jellyfish, the tentacles of comb jellies don't sting. (See The Stings: Nematocysts and Colloblasts for more.)

Jellyfish transition between two different body forms throughout their lives. The familiar body plan that looks like an upside down bell with tentacles hanging down from the inside is called the medusa. The polyp, the other cnidarian body plan, is the opposite, with the mouth and tentacles above, like a sea anemone. (See more in Reproduction & Lifecycle.)

Jellyfish also have a stinging adaptation that is unique to them and their close relatives (including sea anemones and hydras): nematocysts, or stinging cells. (See The Stings: Nematocysts and Colloblasts for more.)

Jellyfish and comb jellies vary greatly in size depending on the species. Most jellies range from less than half an inch (1 cm) wide to about 16 inches (40 cm), though the smallest are just one millimeter wide! The largest jellies are the Lion's Mane Jellyfish (Cyanea capillata), which can be almost 6 feet wide (1.8 m) with tentacles over 49 feet (15 m) long. Larger individuals have been seen, but they are not typical. Venus's girdle (Cestum veneris), a belt shaped comb jelly, can be 40 inches (1 meter) long.

The Nervous System

Credit:
Bastian Bentlage

Jellies don't have brains as we typically think of them: rather, they have a network of neurons ("nerve net") that allows jellies to sense their environments, such as changes in water chemistry indicating food or the touch of another animal. The nerve net has some specialized structures such as statocysts, which are balance sensors that help jellies know whether they are facing up or down, and light-sensing organs called ocelli, which can sense the presence and absence of light.

Additionally, some jellyfish have sensory structures called rhopalia, which contain receptors to detect light, chemicals and movement. One group of jellyfish, the cubozoan jellyfish, have complex eyes with lenses, corneas and retinas in their rhopalia. Although they respond to visual stimuli, scientists don't know how the jellyfish interpret the images created by their eyes since they don't have a brain with which to process them. Their nerve ring, a ring-shaped concentration of nerves found in jellyfish, seems to be involved, however.

A 2017 study of the upside-down jellyfish, Cassiopea, found that a brain is not required to experience sleep. At night Cassiopea enters a sleep-like state where it pulses less frequently than during the day and is slow to respond to disturbances. When kept awake throughout the night, the next day the jellyfish appear to be tired-their pulsing was noticeably slower than if they had a solid night of sleep. It is the first time an animal without a brain was observed sleeping. The discovery suggests sleep among all animals is an ancient characteristic with a shared evolutionary beginning, considering the neural network of jellyfish evolved before centralized nervous systems like a brain.

Diversity

All jellyfish are Cnidaria, an animal phylum that contains jellies, sea anemones, and corals, among others. There are more than 10,000 species of Cnidaria, and less than 4,000 of these are Medusazoa-those animals we think of as jellyfish. Those 4,000 jellyfish can be divided into four different groups.

SCYPHOZOA are the most familiar jellyfish, including most of the bigger and more colorful jellies that interact with humans, and are sometimes called "true jellyfish" for this reason. Scyphozoa spend most of their lives in the medusa body form, and there are at least 200 species.

HYDROZOA are jellyfish look-alikes but not in the same group as the "true jellyfish." The swimming medusa stages of this group are often small and inconspicuous, whereas the bottom-dwelling polyps, or hydroids, usually take the form of large colonies. (See Reproduction & Lifecycle.) In the water column, the colonial siphonophores may be quite spectacular. These include the notorious Portuguese Man-o-Wars and many deep-sea forms, some of which stretch out up to 50 meters in length like giant fishing nets. Colonial siphonophores are composed of many specialized individuals called zooids that are genetically identical because they all come from a single fertilized egg. In 2016, researchers discovered what they believe to be a new hydrozoan species of Crossota, 12,140 feet (3,700 meters) deep within the Mariana Trench. Floating in the water column like a glowing spaceship, this Crossota jellyfish is an exception to most hydrozoans and will spend the majority of its life as a large medusa. There are around 3,700 species of Hydrozoa.

CUBOZOA are the box jellyfish, named for their box-like bells. Some cubozoans, such as the sea wasp (Chironex fleckeri), produce some of the most potent venom known. Cubozoan jellyfish also have a more developed nervous system than other jellyfish, including complex eyes with lenses, corneas and retinas. Some even engage in elaborate (for a jellyfish) courtship behavior! There are at least 36 species. In 2011, Allen Collins, a jellyfish expert at the Smithsonian, discovered a new species, which was named Tamoya ohboya in a public naming contest. (Listen to a podcast about box jellies.)

STAUROZOA are the stalked jellyfishes, which don't float through the water like other jellies, but rather live attached to rocks or seaweed. They are trumpet-shaped, and mostly live in cold water. There are around 50 staurozoan species, many notable for their unique combination of beauty and camouflage.

Jellies are found in oceans worldwide, in shallow and deep water, and a few can even be found living in freshwater.

Compared to jellyfish, there are far fewer species of ctenophores: only 100-150 species have been found, but quite a few are out there yet to be discovered and fully documented. The best-known comb jellies are those found close to shore because, there, they are most likely to run into people. Those can be roughly divided into three groups.

CYDIPPIDS all have rounded bodies-some spherical, some oval-with branched tentacles. (This means that their tentacles are fringed with smaller tentacles.) These tentacles can be withdrawn into the jelly's body into special sheaths or pouches on either side of their mouths.

LOBATES are defined by two flattened lobes that extend from the typical rounded ctenophore body down below their mouths. They also have short tentacles and tend to grow larger than cydippids.

BEROIDS (also known as "nuda") are sack-shaped and have no tentacles at all-but they do have a very large mouth, which they can zip shut very tightly.

Open ocean ctenophores are much less known. They tend to be very fragile because they don't have to endure rough coastal waves; many of them are so fragile that they cannot be collected by submersibles and are known only by photographs. They come in a great diversity of forms. Some are shaped like belts (Cestida), while others don't float in the water column at all, but live on the seafloor! (These are known as benthic ctenophores.)

Comb jellies live throughout the world's ocean, although most species prefer warmer water.

Evolution

Jellyfish and comb jellies are in different phyla, but scientists have long argued over whether they have an especially close relationship apart from the rest of the animal kingdom. To distinguish them, all Cnidaria and Ctenophora were once described as Coelenterata-but that term is no longer commonly used.

To this day, some researchers believe they are sister groups, while others think they are not closely related. Either way, there are still plenty of other questions to argue about, such as how long ago the two groups diverged, and even whether ctenophores might be the most ancient group of animals, diverging even earlier than sponges in the animal tree of life. These arguments continue because, as some of the simplest animals alive today, understanding their place in the tree of life helps people understand how all other animals-including people-evolved.

Whichever came first, comb jellies and jellyfish (and other Cnidarians) made an important step in evolutionary history: they are the earliest known animals to have organized tissues-their epidermis and gastrodermis-and a nervous system. They're also the first animals known to swim using muscles instead of drifting with the whims of the waves.

The oldest ancestors of modern day jellies lived at least 500 million years ago, and maybe as long as 700 million years ago. That makes jellyfish three-times as old as the first dinosaurs!

Because jellies have no bones or other hard parts, finding jellyfish fossils is rare. But in 2007, a group of scientists including Allen Collins from the Smithsonian National Museum of Natural History, discovered some beautifully-preserved jellyfish fossils buried in Utah from 505 million years ago. From around the same period, scientists have also found well-preserved comb jelly fossils in the Burgess Shale.

Credit:
Mary Elizabeth Miller, Dauphin Island Sea Lab

Jellyfish and ctenophores are carnivorous, and will eat just about anything they run into! Most jellies primarily eat plankton, tiny organisms that drift along in the water, although larger ones may also eat crustaceans, fish and even other jellyfish and comb jellies. Some jellyfish sit upside down on the bottom and have symbiotic algae (zooxanthellae) in their tissues, which photosynthesize, and so get much of their energy the way plants do.

While their nematocysts and colloblasts do help them defend themselves, plenty of animals manage to catch and eat jellies: more than 150 animal species are known to eat jellies, including fish, sea turtles, crustaceans, and even other jellyfish. Jellies are the favorite food of the ocean sunfish ( Mola mola) and endangered leatherback turtle ( Dermochelys coriacea), which will migrate thousands of miles for the gelatinous delicacy. Young jellyfish are small enough to be part of the general zooplankton population and are eaten by many animals.

Humans also eat jellyfish: people have fished for jellies for at least 1700 years off the coast of China. Some 425,000 tons (more than 900 million pounds) of jellyfish are caught each year by fisheries in 15 countries, and most are consumed in Southeast Asia. Eating jellyfish may become more common around the world as we overfish more preferable fish species.

Feeding Adaptations

Jellyfish and ctenophores both have tentacles with specialized cells to capture prey: nematocysts and colloblasts, respectively. Jellyfishes' nematocysts are organelles within special cells (cnidocytes) that contain venom-bearing harpoons. The cell is activated upon touch or chemical cue, causing the harpoon to shoot out of the cell and spear the prey or enemy, releasing toxin-a process that takes only 700 nanoseconds. A small number of jellyfish are very toxic to humans, such as the box jellyfish ( Chironex fleckeri) and Irukandji jellyfish ( Carukia barnesi), which can cause severe reactions and even death in some people.

Many comb jellies have colloblasts lining their tentacles, which work like nematocysts but release glue instead of venom. Upon touch, a spiral filament automatically bursts out of colloblast cells that releases the sticky glue. Once an item is stuck, the comb jelly reels in its tentacle and brings the food into its mouth. One species of ctenophore ( Haeckelia rubra) recycles nematocysts from hydrozoan jellyfish it consumes and uses these to stun and kill prey.

Comb jellies come in many shapes and sizes, and so within the group there are many ways to feed. The rounded and tentacled cydippids have branched tentacles lined with colloblasts that they use, in the traditional jelly style, like a fishing line to trap food and bring it to their mouths.

The lobate ctenophores have two flattened lobes that reach below their mouths. Special cilia waving between the lobes generate a current to pull planktonic food between the lobes and into the jelly's mouth, allowing them to feed on plankton continuously. They also use colloblast-lined tentacles to catch food.

The tentacle-less beroids depend on their large mouths. Instead of catching food with colloblasts, they swallow their prey (often other ctenophores!) whole and then clamp their mouths shut, giving them no escape route. Inside their mouths they have small cilia that act as teeth, pulling food apart, which also direct the food into the comb jelly's gut.

Defense Adaptations

Many jellyfish and comb jellies are able to produce light-an ability known as bioluminescence. They have proteins in some tissues that undergo a chemical reaction to produce blue or green light in response to stimuli such as touch. No one's quite sure why jellies bioluminesce, but it seems to be mainly a defense tactic. A bright enough flash could be enough to startle a predator-or to attract an even bigger predator to make the jelly's predator into prey.

Jellies have also adapted their body color to camouflage in the darkness. Most are nearly colorless and transparent, so they can be difficult for predators to see. However, some deep sea jellyfish and comb jellies are a bright red or orange color. Why would they be red instead of black to blend in with the dark water? Red cannot be seen in dark water (deeper than 200 meters), so there's no greater protection from black than red. But red is preferred to black because pigment is easier for animals to produce. Some deep sea jellies just have dark red guts, possibly serving to mask luminescent prey from other larger predators with eyes.

Jellyfish have a complex life cycle: a single jellyfish reproduces both sexually and asexually during its lifetime, and takes on two different body forms.

An adult jellyfish is called a medusa, which is the familiar umbrella-shaped form that we see in the water. Medusa jellyfish reproduce sexually by spawning-the mass release of eggs and sperm into the open ocean-with entire populations sometimes spawning all together. Male and female jellyfish (there aren't many hermaphrodites) release the sperm and eggs from their mouths. In most species, fertilization takes place in the water; in others, the sperm swim up into the female's mouth and fertilize the eggs within.

The fertilized eggs then develop into planulae (singular: planula), which are ciliated free-swimming larvae shaped a bit like a miniature flattened pear. After several days of development, the planulae attach to a firm surface and transform into flower-like polyps. The polyps have a mouth and tentacles that are used to feed on zooplankton.

Polyps reproduce asexually by budding-when a polyp divides roughly in half to produce a new genetically identical polyp-or they can produce or transform into medusae, depending on the type of jellyfish. Hydrozoan polyps bud medusae from their sides; cubozoan polyps each transform into a medusa.

In schyphozoans, a process called strobilation takes place (shown in video and in diagram). During strobilation, a polyp splits into 10-15 plate-like segments stacked atop one another in a tower called a strobila. After a segment separates from the strobila, it is called an ephyra, a juvenile jellyfish. Ephyrae mature into the medusa form.

Most jellyfish are short lived. Medusa or adult jellyfish typically live for a few months, depending on the species, although some species can live for 2-3 years in captivity. Polyps can live and reproduce asexually for several years, or even decades.

One jellyfish species is almost immortal. Turritopsis nutricula, a small hydrozoan, can revert back to the polyp stage after reaching adult medusa stage through a process called transdifferentiation. This is the only animal known to do so.

In comparison to the jellyfish, comb jellies have a very simple lifecycle. Most species are hermaphroditic and able to release both eggs and sperm into the water, which drift with the waves until they find other gametes. Because most species have both male and female gametes, it's thought that they can self-fertilize as well.

This method may not seem very efficient, since it's likely that most of the gametes never find a match. But ctenophores make up for this by releasing them every day. If they run out of food while producing so many eggs and sperm, they can shrink and hunker down until they run into more food and can start reproducing again.

Once eggs and sperm find each other, the embryo develops into a larva that looks just like a small adult ctenophore-and, from there, all it has to do is grow up.

One species ( Mertensia ovum) can reproduce even when it is still larva, and scientists think other species are also able to reproduce at a young age. This means that comb jelly populations can grow very fast under certain conditions.

Around the world, vast aggregations of jellyfish and comb jellies seem to be more common. These aggregations are known as "jellyfish blooms" or "jellyfish outbreaks," which can cause a wide array of problems. Too many jellies in the water can be a danger to swimmers, forcing towns to close their beaches. Jellies have clogged up machinery at coastal power plants, causing power outages. They can interfere with fisheries by eating fish larvae, and fisherman catch jellies instead of the fish they want. Where they occur, blooms of jellyfish even change seawater chemistry. Scientists hope to address this problem through the discovery of a practical application for jellyfish, like substituting jellyfish for the fish used in aquaculture feed. Jellyfish mucus, which has been shown to bind to microplastics, may even one day be used in water treatment facilities to help combat the world's growing plastic problem.

Why are jellies becoming more common around the world? It seems likely that their spread is human-caused, although some scientists have argued that the blooms are part of a natural cycle. If the blooms are human-caused, there are several probable culprits.

OVERFISHING Over the past two decades, between 100 and 120 million tons of marine life have been removed from the ocean by fisheries each year on average. A lot of these marine species, including fish and invertebrates such as squid, eat some of the same food that jellies do: mainly, zooplankton. As these other predators of plankton are fished from the sea, jellies have less competition for food, and are able to grow and reproduce with fewer limits.

NUTRIENTS When fertilizers runoff from the rivers to the seas, they can create dead zones: areas of ocean where little life survives. The nitrogen and phosphorus in fertilizer helps phytoplankton grow very quickly, and there can be so many of these single-celled plant-like animals that they deplete oxygen from the water. Most animals can't survive in these conditions, but many jellies can better tolerate low-oxygen environments.

CLIMATE CHANGE The ocean is warming, and this might give some jellies a boost. The warmer water could help jelly embryos and larvae develop more quickly, allowing their populations to grow more quickly. And jellies that prefer warmer water will have more area to live in. However, this could also hurt some species as cold-water jelly species see their habitat shrink.

SUBMARINE SPRAWL Many industries, such as shipping, drilling and aquaculture, build docks, oil platforms and other structures in the water-sometimes referred to as "ocean sprawl"-which can serve as nurseries for jellyfish. To undergo their polyp stage, jellyfish need solid surfaces to settle upon. It's much easier for jellyfish polyps to attach to man-made structures made of wood, brick and concrete than sand. Ocean sprawl provides more and better habitat for jellyfish to reproduce and complete their lifecycles.

Jellies are very good at surviving: they have broad diets, reproduce quickly, can shrink down if food runs out and then revive, and tolerate low-oxygen water. So, as you can imagine, they are also very good at thriving in new ecosystems once they arrive.

In the 1980s, the sea walnut (Mnemiopsis leidyi), a type of comb jelly, was brought to the Black Sea in ship ballast water. It reproduced and spread quickly, gobbling up zooplankton and leaving little behind for the larvae of commercial fish species, including anchovy, scad and sprat. Within a decade, the comb jellies took over the Black Sea and many of the fish populations collapsed, bringing local fisheries down with them. In a stroke of accidental luck, a different species of comb jelly ( Beroe ovum)-a predator of the sea walnut-was brought over in a ship, and it's helping to bring down the population. A similar story of fishery collapse coinciding with jellyfish blooms is playing out off the coast of Japan.

However, the collapse of a fishery doesn't always end in jellyfish. A crash in the pollock and walleye fishery in the Bering Sea left an opening for jellyfish but, after reigning for a few years, the jellies gave up their crown as the fish returned. And when the Peruvian anchovy fishery collapsed in the 1970s, no jellyfish swarmed in to take their place.

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Jellyfish and comb jellies are gelatinous animals that drift through the ocean\'s water column around the world. They are both beautiful-the jellyfish with their pulsating bells and long, trailing tentacles, and the comb jellies with their paddling combs generating rainbow-like colors.

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See photos of jellyfish in this oceans photo gallery from National Geographic.

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Shows sea anemone anatomy

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Information about Cnidaria including their biology, anatomy, behaviour, reproduction, predators, prey and ecology.