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One parent or two?
Reproduction: Asexual vs. Sexual

Cell division is how organisms grow and repair themselves. It is also how many organisms produce offspring. For many single-celled organisms, reproduction is a similar process. The parent cell simply divides to form two daughter cells that are identical to the parent. In many other organisms, two parents are involved, and the offspring are not identical to the parents. In fact, each offspring is unique. Look at the family in Figure below. The children resemble their parents, but they are not identical to them. Instead, each has a unique combination of characteristics inherited from both parents.

Family Portrait: Mother, Daughter, Father, and Son. Children resemble their parents, but they are never identical to them. Do you know why this is the case?

Reproduction is the process by which organisms give rise to offspring. It is one of the defining characteristics of living things. There are two basic types of reproduction: asexual reproduction and sexual reproduction.

Asexual Reproduction

Asexual reproduction involves a single parent. It results in offspring that are genetically identical to each other and to the parent. All prokaryotes and some eukaryotes reproduce this way. There are several different methods of asexual reproduction. They include binary fission, fragmentation, and budding.

Binary fission occurs when a parent cell splits into two identical daughter cells of the same size.

Fragmentation occurs when a parent organism breaks into fragments, or pieces, and each fragment develops into a new organism. Starfish, like the one in Figure below, reproduce this way. A new starfish can develop from a single ray, or arm. Starfish, however, are also capable of sexual reproduction.

Budding occurs when a parent cell forms a bubble-like bud. The bud stays attached to the parent cell while it grows and develops. When the bud is fully developed, it breaks away from the parent cell and forms a new organism. Budding in yeast is shown in Figure below.

Binary Fission in various single-celled organisms (left). Cell division is a relatively simple process in many single-celled organisms. Eventually the parent cell will pinch apart to form two identical daughter cells. In multiple fission (right), a multinucleated cell can divide to form more than one daughter cell. Multiple fission is more often observed among protists.

Starfish reproduce by fragmentation and yeasts reproduce by budding. Both are types of asexual reproduction.

Asexual reproduction can be very rapid. This is an advantage for many organisms. It allows them to crowd out other organisms that reproduce more slowly. Bacteria, for example, may divide several times per hour. Under ideal conditions, 100 bacteria can divide to produce millions of bacterial cells in just a few hours! However, most bacteria do not live under ideal conditions. If they did, the entire surface of the planet would soon be covered with them. Instead, their reproduction is kept in check by limited resources, predators, and their own wastes. This is true of most other organisms as well.

Sexual Reproduction

Sexual reproduction involves two parents. As you can see from Figure below, in sexual reproduction, parents produce reproductive cells-called gametes-that unite to form an offspring. Gametes are haploid cells. This means they contain only half the number ofchromosomes found in other cells of the organism. Gametes are produced by a type of cell division called meiosis, which is described in detail in a subsequent concept. The process in which two gametes unite is called fertilization. The fertilized cell that results is referred to as a zygote. A zygote is diploid cell, which means that it has twice the number of chromosomesas a gamete.

Mitosis, Meiosis and Sexual Reproduction is discussed at http://www.youtube.com/watch?v=kaSIjIzAtYA.

Cycle of Sexual Reproduction. Sexual reproduction involves the production of haploid gametes by meiosis. This is followed by fertilization and the formation of a diploid zygote. The number of chromosomes in a gamete is represented by the letter n. Why does the zygote have 2n, or twice as many, chromosomes?

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One parent or two? Cell division is how organisms grow and repair themselves. It is also how many organisms produce offspring. For many single-celled organisms, reproduction is a similar process. The parent cell simply divides to form two daughter cells that are identical to the parent.

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Read about Cnemidophorus uniparens (Desert Grassland Whiptail) on the Animal Diversity Web.

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Minnesota Department of Natural Resources

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Description

Appearance

Spiny waterfleas are microscopic animals, also known as zooplankton, that live in open water. Adults range from one-quarter to five-eighths inches long, and are opaque in color. They have a single long tail with one to four spines and have one large, distinctive black eyespot. The spiny waterflea is often found on fishing line or other equipment in clumps that resemble a gelatinous blob with a texture of wet cotton.

Biology

The spiny waterflea is a predatory zooplankton that eats other zooplankton. They migrate into deeper waters during the day to hide from predators, and return to shallower water at night to find food. During the spring and summer, they reproduce by cloning. In the fall, or when conditions in the lake are cold or there is less food, they will reproduce sexually and produce tough eggs that are resistant to drying and freezing. Females carry their eggs and young on their back.

Origin and Spread

The spiny waterflea is native to Europe and Asia. The species was unintentionally introduced into the United States' Great Lakes through the discharge of contaminated cargo ship ballast water. They were first discovered in Lake Ontario in 1982, and spread to Lake Superior by 1987. Refer to EDDMapS Midwest for current distribution.

Don't be fooled by these look-alikes

Look-alikes:

Regulatory Classification

The spiny waterflea ( Bythotrephes longimanus) is a regulated invasive species in Minnesota, which means it is legal to possess, sell, buy, and transport, but it may not be introduced into a free-living state, such as being released or planted in public waters.

Threat to Minnesota Waters

Invasive species cause recreational, economic and ecological damage-changing how residents and visitors use and enjoy Minnesota waters.

Spiny waterflea impacts:

Clog eyelets of fishing rods and prevent fish from being landed.

Prey on native zooplankton, including Daphnia, which are an important food source for native fishes. In some lakes, spiny waterfleas can cause the decline or elimination of some species of native zooplankton.

They do not provide a good food source for native fishes, because their long tail and spines make them difficult to eat.

What you should do

People spread spiny waterfleas primarily through the movement of water-related equipment. The species collects in gelatinous blobs on fishing lines and downrigger cables. They can survive in water contained in bait buckets, live wells, bilge areas, ballast tanks and other water-containing devices. Adults and eggs, which may be hidden in mud and debris, can stick to anchors and ropes, as well as scuba, fishing, and hunting gear. Females can produce eggs which resist drying and freezing. Therefore, transfer of a single female could start a new population.

Whether or not a lake is listed as infested, Minnesota law requires water recreationists to:

Clean watercraft of all aquatic plants and prohibited invasive species.

Drain all water by removing drain plugs and keeping them out during transport.

Dispose of unwanted bait in the trash.

Dry docks, lifts, swim rafts and other equipment for at least 21 days before placing equipment into another water body.

Additional recommendation: Air dry for more than six hours to kill any spiny wateflea eggs.

Report new occurrences of spiny waterfleas to the DNR immediately by contacting your DNR Invasive Species Specialist or log in and submit a report through EDDMapS Midwest.

Control Methods

There is no known effective population control for spiny waterflea in natural water bodies at this time.

Resources

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Spiny waterfleas are microscopic animals, also known as zooplankton, that live in open water. Adults range from one-quarter to five-eighths inches long, and are opaque in color. They have a single long tail with one to four spines and have one large, distinctive black eyespot.

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Table of Contents

Size range and diversity of structure

Scorpions are relatively large among terrestrial arthropods, with an average size of about 6 cm (2.5 inches). Scorpions exhibit few sexual differences, although males usually are more slender and have longer tails than females. Giants among scorpions include the black emperor scorpion ( Pandinus imperator), an African species found in Guinea, which attains a body length of about 18 cm (7 inches) and a mass of 60 grams (more than 2 ounces). The longest scorpion in the world is the rock scorpion ( Hadogenes troglodytes) of South Africa; females attain a length of 21 cm (8.3 inches). The length of the smallest scorpions, the Caribbean Microtityus fundorai, is 12 mm (0.5 inch). A few precursors of modern scorpions were comparative giants. Fossils of two species ( Gigantoscorpio willsi and Brontoscorpio anglicus) measure from 35 cm (14 inches) to a metre (3.3 feet) or more, and an undescribed species is estimated to have been 90 cm (35.5 inches). Most species from deserts and other arid regions are yellowish or light brown in colour; those found in moist or mountain habitats, however, are brown or black.

Distribution and abundance

In addition to desert habitats, scorpions have adapted to temperate, subtropical, and tropical environments such as grasslands, savannas, and forests. They live on all major landmasses except Greenland and Antarctica. Their range extends from Canada and central Europe to the southern tips of South America (Tierra del Fuego) and Africa, and they have been accidently introduced into New Zealand and England. Scorpions have been found at elevations from sea level to 5,000 metres (more than 16,000 feet) in mountains of Europe and North and South America. A few species live as far north as southern Canada, southern Germany, and Russia.

Breeding is seasonal and generally occurs during the warm months, ranging from late spring through early fall. Males may travel hundreds of metres to find receptive females. It appears that males find females by localizing a pheromone that the female emits from the end of her abdomen. Mating in scorpions is preceded by a complicated and characteristic courtship initiated by the male. He first faces and grasps the female, using his pincers (pedipalps). Then the pair, directed by the male, moves sideways and backward in a dancelike motion called promenade à deux. These actions result from the efforts of the pair to find a smooth surface on which the male can extrude a glandular secretion that forms a stalk to which the spermatophore (sperm-containing structure) is attached. He then maneuvers the female so that her genital opening contacts the spermatophore. Once she is positioned over the spermatophore, physical contact with it causes spermatozoa to be ejected into the genital opening (gonopore) of the female. Males that remain near females after mating are sometimes killed and eaten.

In general, females mate multiple times. In some species, mating must occur after each clutch of offspring is produced in order to fertilize another clutch of eggs. In others, multiple clutches of offspring can be produced from the sperm stored from a single mating. There are at least two species known that can produce offspring without mating at all ( parthenogenesis).

The mother invests a great amount of time and energy in her offspring. Unlike most nonmammalian animals, scorpions are viviparous, giving birth to live young instead of laying eggs. Once fertilized, the eggs are retained in the female's body, where the embryos are nourished in utero for periods varying from several months to a year. The birth process itself may last from several hours to several days. Temperate species usually give birth in spring and summer, whereas tropical species give birth throughout the year. Litter size averages 25, with a range of 1 to more than 100.

At birth a young scorpion is white and enveloped in a membrane, or chorion. After freeing itself, the immature scorpion crawls onto the mother's back, where it remains for a period ranging from 1 to 50 days. During this time the young scorpions are defenseless and utilize food reserves in their bodies while receiving water transpired through the mother's cuticle and taken up through their own. The young molt their soft embryonic cuticle for one that is fully functional when they assume independence. This early mother-young association is obligatory for newborns. Without it, they do not molt successfully and usually die. The young generally leave the mother soon after this first molt.

As in all arthropods, growth is accompanied by molting ( ecdysis). Scorpions molt an average of five times (the range is four to nine) before reaching maturity. The number of molts in some species is variable. In some Centruroides, for example, small males mature after four molts and large males after five. There are no reports of molting's occurring after reproductive maturity has been reached.

Scorpions are opportunistic predators that eat any small animal they can capture. Common prey includes insects as well as spiders and other arachnids, including other scorpions. Less-common but regular prey includes pill bugs, snails, and small vertebrates such as lizards, snakes, and rodents. The only known specialist scorpion is the Australian spiral burrow, or spider-hunting, scorpion ( Isometroides vescus), which feeds solely on burrowing spiders.

Most scorpions are sit-and-wait predators that remain motionless until a suitable victim has moved into an ambush zone. Scorpions can sense tiny ground vibrations, and some can detect airborne vibrations of flying insects. These behaviours are sophisticated to the extent that scorpions can determine the precise distance and direction of their prey. Once the prey has been detected, the scorpion turns, runs to the prey, and seizes it. The prey is stung if it is relatively large, aggressive, or active. Otherwise it is simply held by the pedipalps as it is eaten. Many of the thick-tailed scorpions (family Buthidae), however, actively search for prey. These species usually have long, slender bodies and pincers (chelae). Many have powerful venoms to compensate for their small pincers.

Scorpions lack conventional jaws, and their feeding habits are unusual. An additional pair of pincerlike appendages (chelicerae) are toothed, and, with these tools as well as the sharp edges of adjacent jawlike structures (maxillae and coxae), the scorpion chews the prey as quantities of digestive fluids secreted from the midgut pour over it. The victim's soft parts are broken down, liquefied, and sucked into the scorpion's stomach by a pumping action. The victim is gradually reduced to a ball of indigestible material, which is cast aside. Eating is a slow process, often taking many hours.
Willis John Gertsch Gary A. Polis Joseph Culin

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Scorpion | Description, Habitat, Species, Diet, & Facts

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Scorpion, any of approximately 1,500 elongated arachnid species characterized by a segmented curved tail tipped with a venomous stinger at the rear of the body and a pair of grasping pincers at the front. Primarily nocturnal, scorpions often play the role of evildoers in fables and legends.

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This article was most recently revised and updated by

Salmonella, (genus Salmonella), group of rod-shaped, gram-negative, facultatively anaerobic bacteria in the family Enterobacteriaceae. Their principal habitat is the intestinal tract of humans and other animals. Some species exist in animals without causing disease symptoms, while others can result in any of a wide range of mild to serious infections known generally as salmonellosis. Most human infections with Salmonella result from the ingestion of contaminated food or water.

Salmonella typhi causes typhoid fever; paratyphoid fever is caused by S. paratyphi, S. schottmuelleri, and S. hirschfeldii, which are considered variants of S. enteritidis.

Refrigeration prevents bacterial reproduction but does not kill these microorganisms. As a result, many Salmonella can develop in foods, which, when ingested, can result in gastroenteritis.

S. choleraesuis, from swine, can cause severe blood poisoning in humans; S. gallinarum causes fowl typhoid; and S. arizonae has been isolated from reptiles in the southwestern United States.

The Editors of Encyclopaedia Britannica Kara Rogers.

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Salmonella | Classification, Species, & Human Infection

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Salmonella, group of rod-shaped, gram-negative, facultatively anaerobic bacteria. Their principal habitat is the intestinal tract of humans and other animals. Some species exist in animals without causing disease symptoms, while others cause any of a wide range of mild to serious infections known as salmonellosis.

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The Red Kangaroo is an iconic Australian animal of the arid zone and is the largest living marsupial in the world.

Identification

The Red Kangaroo is a large kangaroo with a body length of up to 1.4m and tail up to 1m. Males tend to be orange red in colouring while females are often blue grey. Both males and females are a lighter whitish colour underneath. Red Kangaroos can be distinguished from other species of kangaroos by the black and white patches on their cheeks and the broad white stripe that extends from the corner of the mouth to ear. Male Red Kangaroos are double the body weight of females and can weigh up to 92kg while the females can weigh up to 39kg.

Habitat

The Red Kangaroo prefers sparsely wooded or open plains. They inhabit grassland, shrubland, desert, woodland and open forest, and are most common in open savanna woodland.

Distribution

The Red Kangaroo is found throughout the semi-arid and arid regions of mainland Australia.

Feeding and diet

Red Kangaroos are herbivores, primarily grazing on grasses but also eat forbs and leaves of shrubs. The Red Kangaroo has the ability to survive when water is scarce. The herbage and foliage that the Kangaroo eats provides them with their water needs as well as their nutritional requirements. In times of drought, Red Kangaroo populations can suffer as their food supplies diminish. The abundance of food is a determining factor in the Red Kangaroo life cycle.

Breeding behaviours

Reproduction is very sensitive to environmental conditions. The Red Kangaroo breeds all year round, however spring and summer tend to be times when most young are born. The females have the unique ability to delay birth of their baby until their previous Joey has left the pouch. This is called embryonic diapause. Females have one baby at a time, which at birth is the size of a jellybean. The infant immediately climbs into its mother's pouch and the gestation period is around 33 days. Until they reach about eight months of age, threatened young kangaroos, called joeys, will quickly dive for the safety of mom's pouch. As they grow, joeys' heads and feet can often be seen hanging out of the pouch. The young Joey will permanently leave the pouch at around 235 days old, but will continue to suckle until it reaches 12 months of age.

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The Red Kangaroo is an iconic Australian animal of the arid zone and is the largest living marsupial in the world.

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The Monterey Bay Aquarium Sea Otter Program has played an important role in the continued recovery of the southern sea otter population with our research, rescue, rehabilitation, and surrogacy work.

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Leafy sea dragon | Animals | Monterey Bay Aquarium

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Close kin to seahorses, leafy sea dragons are perfectly camouflaged among seaweeds and seagrass beds.

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Grizzly Bear | National Wildlife Federation

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Learn facts about the grizzly bear’s habitat, diet, life history, and more.

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BioKIDS - Kids' Inquiry of Diverse Species,

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BioKIDS - Kids' Inquiry of Diverse Species

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March's critter is one that we know you'll welcome with open arms! This month we learn about the brittle stars and how five arms (or more) are better than one.

Charming and disarming

The brittle stars truly are in a class all their own. Class Ophiuroidea, derived from the ancient Greek words ophis meaning "snake" and ura meaning "tail," refers to the snake-like way their long, slender arms move. In fact, an alternate common name is the serpent stars.

However, they'll always be the brittle stars to us. When handling these animals, their delicate arms break off easily as they tangle with themselves or other marine sediment debris. Brittle stars are fragile in other ways as well. They are considered "stress-sensitive" so their presence in a sediment sample is one indicator of a healthy benthic community.

Off the deep end

There are more than 2,000 species of brittle stars alive today and they can be found worldwide, from shallow water to the deep sea. Some occur at depths greater than 3,500 meters - that's over 10,000 feet! In the Pacific Northwest, there are 53 brittle star species. Here are some that call Puget Sound home:

Amphiodia urtica/periercta - burrowing brittle star

Amphiodia occidentalis - snaky-armed brittle Star

Amphioplus strongyloplax

Amphipholis squamata - dwarf brittle star

Amphipholis pugetana - black & white brittle star

Ophiura luetkenii - gray brittle star

Ophiura sarsii - notched brittle star

While widely dispersed throughout Puget Sound, these brittle stars are found in higher numbers at our East Anderson Island, Commencement Bay, and outer Bellingham Bay sampling locations.

At arm's length

The brittle star body consists of a central disk containing all the organs to which the arms attach. The disk is made up of fused calcareous plates that can look a little like dragon scales that can be smooth or covered with spines. Wherever an arm meets the disc, the plates are larger, forming radial shields.

All the brittle star species found in Puget Sound have five arms but species in other areas sport up to nine. They use their arms and tubed feet with suckers at the ends to get around. Their long, flexible arms provide locomotion and consist of hundreds of small interlocking plates linked by muscles that function like joints to bend to bend the animals backwards and forwards.

Lay down your arms

They may be fragile, but fortunately, brittle stars have a trick up their spiny little sleeves: just like lizards can drop their tails if grabbed by a predator, brittle stars can drop their arms if a predator gets them in their grip! This defense mechanism proves useful to ensure that the star lives to fight another day. As long as the central disc is intact, the lost or broken arms can regenerate, or grow back.

Armed to the teeth

Puget Sound species of brittle stars are facultative detritivores, which means that they eat dead organic matter ( detritus) on the sea floor. However, they aren't picky eaters, and will switch to eating other foods if detritus is scarce. The food particles just have to be small enough fit into the brittle star's tiny mouth, located on the underside of the central disc. Encircling the mouth are five jaws with "teeth" called oral papillae. We look at the number, size,and shape of these papillae to distinguish between the different species.

Seeing the light

Have you ever wondered how eyeless animals like brittle stars see? Researchers who study these animals were curious about this is as well and observed that the stars displayed particular behaviors in response to light. This light-responsive behavior, called extraocular photoreception, allows brittle stars to sense when they are exposed to light so they can move toward shade that might represent shelter from predators. This amazing ability is only skin-deep: The skin covering a brittle star's arms is packed with light-sensitive cells which help the animal sense its surroundings.

Critter of the Month

Our benthic taxonomists, Dany and Angela, are scientists who identify and count the benthic (sediment-dwelling) organisms in our samples as part of our Marine Sediment Monitoring Program. We track the numbers and types of species we see in order to understand the health of Puget Sound and detect changes over time.

Dany and Angela share their discoveries by bringing us a Benthic Critter of the Month. These posts will give you a peek into the life of Puget Sound's least-known inhabitants. We'll share details on identification, habitat, life history, and the role each critter plays in the sediment community. Can't get enough benthos? See photos from our Eyes Under Puget Sound collection on Flickr.

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The brittle stars embody nature's fragility...and resilience

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March's critter is one that we know you'll welcome with open arms! This month we learn about the brittle stars and how five arms (or more) are better than one. The brittle stars truly are in a class all their own.

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Bluehead
Thalassoma bifasciatum
This schooling reef fish has a varied assortment of color patterns based on its life phase and behavior. It gets its name from the final phase where it has a bright blue head followed by black and white bars and then a green or blue-green body with a gold sheen. The final phase is achieved by only a small percentage of males, or females who become males in the absence of males. They are social fish that school in the hundreds or thousands in shallow reef zones, and are valued as cleaners, removing parasites and dead tissue from other fish.

Order - Perciformes
Family - Labridae
Genus -Thalassoma
Species -bifasciatum

Common Names

English language common names are bluehead, blue head, blue mah, bluehead wrasse, doctor fish, parrotfish, and tikitiki. Common names in other languages include bayan (Malay), blaukopflippfisch (German), cara de cotorra (Spanish), loro cabeza azul (Spanish), and talasoma sinoglowa (Polish).

Importance to Humans

The bluehead is of no interest to fisheries due to its very small size and poor edibility. However their beautiful coloration and hardiness make them popular in the aquarium trade. They are bold and often approach scuba divers and snorkelers.

Conservation

The bluehead is not considered as endangered or vulnerable with the World Conservation Union (IUCN). The IUCN is a global union of states, governmental agencies, and non-governmental organizations in a partnership that assesses the conservation status of species.
> Check the status of the bluehead at the IUCN website.
Geographical Distribution

The bluehead is found in the western Atlantic Ocean from Florida (U.S.) to the Bahamas, Caribbean Sea, Gulf of Mexico and south to northern South America. It also occurs in Bermuda and is most abundant throughout the West Indies.

Habitat

Although it is found almost any place where water clarity is high, choice habitat for the bluehead includes coral reefs, rocky flats, sandy bottoms, and seagrass. It swims about the reef constantly and is found at depths to over 100 feet (30.5 m). This fish is most abundant on shallow reefs, sometimes occurring in schools of thousands of individuals foraging along the bottom or feeding on plankton from the water column. Juveniles often seek shelter among the tentacles of sea anemones, deliberately avoiding direct contact. If stung by the tentacles, it will be ingested by the anemone.

Biology

Distinctive Features
The body is elongate with a depth of approximately 3.5 inches (9 cm), similar in appearance to a compressed cigar. The snout is blunt. Young males and all females have a caudal fin that is truncate to slightly rounded while supermales have a lunate caudal fin. In contrast to most fishes, the bluehead swims with its pectoral fins, appearing to drag its tail.

Coloration
This is an attractive fish with a variety of color phases. The supermale, or terminal phase (TP) male, has a scaleless bright blue head followed by two dark vertical stripes separated by a white bar. The remainder of the body is green to blue-green. Caudal fins have black lobes and the pectoral fins are edged with black. Supermales make up only 4% of the population.

Small males and females (initial phase (IP)) look similar to juveniles except the mid-lateral stripe is broken into a series of blotches ranging from dark to very faint. IP blueheads also display two other color variations. One has a yellow head and body above the white underside, with two square-like spots behind the eye and not mid-lateral stripe. The other has white bars that break up the mid-lateral stripe into a series of blotches and the back is yellowish to greenish-blue in color. IP fish are capable of changing from one color pattern to another with an intermediate coloration.

The juvenile phase has a black mid-lateral stripe. Those individuals that live among coral reefs are yellow above this stripe while inshore, non-reef associated fish are white. The area below the stripe is white in both inshore and offshore fishes.

These fish resemble the wrasse blenny ( Hemiemblemaria simulus), however it is distinguishable by a spot behind the second dorsal ray that extends to the base of tail. The wrasse blenny has a spot behind the third dorsal ray that does note extend to the base of caudal fin. The juvenile tiger grouper ( Mycteroperca tigris) mimics the initial phase bluehead. By mimicking a non-predatory, common fish, the tiger grouper is able to closely approach potential prey. Another advantage is the juvenile tiger grouper experiences reduced predation pressure as it mimics parasite cleaners that are avoided by some predators.

Size, Age, and Growth
Blueheads reach a maximum length of 9.8 inches (25 cm), with a maximum reported age of 3 years. Individuals reach sexual maturity at lengths as small as 1.5 inches (3.8 cm).

Dentition
Two canine teeth are located on the upper and lower jaws and a single row of conical teeth on the margin of both jaws that become progressively longer toward the front of the mouth.

Food Habits
Adult blueheads feed primarily on zooplankton and many small benthic organisms. They are also known to eat the eggs of other small fish including the saddle blenny ( Malacoctenus triangulatus), beaugregory damselfish ( Stegastes leucostictus), and sergeant majors ( Abudefduf saxatilis).

IP blueheads are on the most successful cleaner fish in the western Atlantic Ocean. They set up "cleaning stations", providing an important service to other reef fish by "cleaning" the ectoparasites and dead tissues off their skin and scales. The blueheads receive nutrients from these parasites and tissues, thereby creating a symbiotic relationship. The cleaning wrasse advertises these services by performing a dance to attract would-be customers. When a fish is interested in being cleaned, it also sends specific signals to the wrasse such as holding its mouth open and spreading its fins. During this cleaning, the wrasse removes parasites from the fins and gills as well as inside the mouth and between the teeth.

Reproduction
Spawning occurs throughout the entire year. At midday blueheads find each other, with the smaller yellow IP fishes releasing eggs and sperm in large groups. Large numbers of these IP fish swim a few feet above the bottom substrate, then in mass they dash upwards into the water column above. At the peak of this dash, all fish release sperm or eggs. Following such an event, a cloud of gametes is sometimes seen drifting off with the prevailing water currents. The larger blueheads spawn in pairs with a single female and a single male. The dominant TP males form harems, spawning with one female at a time. He stakes out a prominent spawning site from which he courts females. This courtship includes approaching females while displaying the dashing motion, similar to the group spawning rush behavior. If the female is receptive, she joins him in a dash towards the surface for the release of eggs and sperm. TP males may spawn with up to 100 females in a day, however 30-50 is more typical.

Blueheads sometimes display sex reversal. These fish live in large schools composed mainly of females with very few dominant males. If these males are absent, the largest female quickly changes sex, becoming a dominant male. This insures the mating success of the school. The pelagic eggs hatch between 18 and 24 hours after fertilization. The larva are 12 mm in length with one large dark spot at the front edge of the dorsal fin and a series of small black dots along the distal edges of the dorsal, anal, and caudal fins. After spending 6-8 weeks in the water column, they bury into the sand as they metamorphose into juveniles. Juvenile fish turn yellow in color and collect in groups over seagrass and coral habitats. As they mature into the IP, the fish settle in with a local population and have high site fidelity.

Parasites
Unidentified crab zoeae have been reported to infest the gills of a gray angelfish held in a trap. Predators
Trumpetfish ( Aulostomus maculatus), red hind ( Epinephelus gutatus), greater soapfish ( Rypticus saponaceus), and yellowfin grouper ( Mycteroperca venenosa) have been documented as containing blueheads in their stomach contents.

Taxonomy

The bluehead, a member of the wrasse family, was originally described as Labrus bifasciatus by German naturalist Marcus Bloch (1791). This original name was changed by later workers to the currently valid name of Thalassoma bifasciatum (Bloch, 1791). Synonyms used to refer to this fish in past scientific literature include Thalassoma nitida Gunther 1862 and Julis nitida Gunther 1862.
Prepared by: Cathleen Bester

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Bluehead Thalassoma bifasciatum This schooling reef fish has a varied assortment of color patterns based on its life phase and behavior.

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(Image credit: Getty Images )

"Amoeba" is a term that describes a simple eukaryotic organism that moves in a characteristic crawling fashion. However, a comparison of the content of the various amoebas shows that these organisms are not necessarily closely related to each other.

What does an amoeba look like?

All living organisms can be broadly divided into two groups - prokaryotes and eukaryotes - which are distinguished by the relative complexity of their cells. Eukaryotes are highly organized unicellular or multicellular organisms, such as animals and plants. Prokaryotes, on the other hand, are basic single-celled organisms, such as and archaea.
Related: ​​Strange single-celled life-form has a truly bizarre genome
Amoebas are eukaryotes. Their single cells, like those of other eukaryotes, possess certain characteristic features: Their cellular contents are enclosed within a cell membrane, and their is packaged into a central cellular compartment called the nucleus, according to a 2014 report published in the journal BMC Biology (opens in new tab). In addition, they contain specialized structures called organelles, which perform a range of cellular functions including energy production and transport.

Most of these organelles are common to all eukaryotic cells, but there are a few exceptions. For example, the parasitic amoebas Entamoeba histolytica, which cause amoebic dysentery in humans, do not have the golgi apparatus, the responsible for modifying and transporting proteins, according to a 2005 study published in The Journal of Biological Chemistry (opens in new tab). Researchers found that E. histolytica instead contain golgi-like compartments or vesicles - small fluid-filled pouches - that execute similar functions.

There are also amoebas that don't have mitochondria, the organelle responsible for generating cellular energy, because they live in environments lacking in , or "anoxic conditions," Sutherland Maciver, a reader in the department of biomedical sciences at the University of Edinburgh, told Live Science.

According to a 2014 review published in the journal (opens in new tab), these organisms without mitochondria can contain organelles called hydrogenosomes or mitosomes, which are related to mitochondria but are thought to be highly altered versions of the organelle. This is the case for E. histolytica and the free-living amoeba Mastigamoeba balamuthi, which doesn't depend on other organisms for survival.

How does an amoeba move?

Structurally, amoebas closely resemble the cells of higher organisms. "They are like our cells, and in fact, when they are moving they look very much like our white blood cells," Maciver said. (White blood cells are cells that help defend the body against disease.)

Like our white blood cells, amoebas move using pseudopodia, which translates to "false feet" in Latin. These short-lived, outward projections of the cytoplasm - the semifluid material inside the cell membrane - help amoebas to grip a surface and propel themselves forward. As the pseudopodium moves out along a surface in one direction, the back end of the amoeba contracts, Maciver said.
Related: Robert Hooke: English scientist who discovered the cell
"As it contracts, it does two things," he said. "The contraction pushes the cytoplasm forward to fill the expanding pseudopod, but the contraction also pulls up adhesions at the back end of the cell." Maciver describes these adhesions between an amoeba and the surface on which it moves as physical molecular adhesions, which are constantly formed at the front end and broken at the back. This movement, using pseudopodia, is a characteristic that unites various amoebas and distinguishes them from other - simple eukaryotic organisms like amoebas that are not plants, animals or fungi.

Amoebas can also use their pseudopodia to feed. A 1995 report, published in the journal Applied and Environmental Microbiology (opens in new tab), gives the example of the soil-dwelling amoeba Acanthamoeba castellanii, which ingests both solids and liquids using its pseudopodia. The process of ingesting solid material is called phagocytosis, and the process of engulfing drops of liquid is known as pinocytosis, also known as cell drinking, according to Dosage Form Design Considerations (opens in new tab) (Academic Press/Elsevier, 2018).

(Image credit: CDC/ Dr. Mae Melvin)

"Most of the known amoebae eat bacteria," Maciver told Live Science. He explained that amoebas have receptors on their cell surface that bind to bacteria, which are then taken into amoebas by phagocytosis, usually at the rear of the cell.

In the case of giant amoebas, such as Amoeba proteus, the process of phagocytosis is slightly different, according to Maciver. Giant amoebae engulf their prey "by the willful gathering of pseudopods around the bacteria." In both cases, as the bacteria is drawn in, the cell membrane that surrounds it pinches off to form an intra-cellular compartment called the vacuole.

How are amoebas classified?

For centuries, the various systems of classifying organisms, including amoebas, were based on similarities in observable characteristics and morphology. "There isn't actually a coherent group of organisms called the amoebae," Maciver said. "Rather, amoebae are any protozoan cells that move by crawling." (The term "protozoa" refers to a subset of protists, which again are simple eukaryotic organisms that are not plants, animals or fungi, Live Science previously reported.)

Early analyses compared the DNA sequences that encode part of the ribosome, the site of protein synthesis in a cell; specifically, scientists looked at the genes for the so-called 18S subunit of ribosomes, or "SSU rDNA." Based on the analyses of SSU rDNA and other DNA sequences, eukaryotic organisms are now organized in a manner that better represents their evolutionary relationships - the phylogenetic tree, according to the 2008 Protistology article.

Burki listed five supergroups for eukaryotic organisms: Ophiskontha, Amoebozoa, Excavata, Archaeplastida and SAR, which includes three subgroups named Stramenopiles, Alveolata and Rhizaria. Animals and fungi are in the group Ophiskontha. Amoeboid protists and some parasitic lineages that lack mitochondria are part of Amoebozoa. Together, Ophiskontha and Amoebozoa form a larger supergroup called Amorphea, according to the review in the journal Trends in Ecology & Evolution.

(Image credit: Getty Images)

"If you look at the great diversity of the protists, you can see that there are amoebae in virtually all the groups," Maciver said. "There's even an amoeboid organism within the brown algae [Labyrinthula]." Most amoebas are present within the Amoebozoa group, though, Maciver said. In addition, he noted that amoebas are also present within Rhizaria and Excavata. Nucleariids, a group of amoebae with filopodia, belong to the Opisthokonta supergroup, for example, and Labyrinthulids fit within the Stramenopiles.

Why are amoebas important?

Amoebas are known to cause a range of human diseases. Amebiasis, or amoebic dysentery, is an infection caused by E. histolytica, a human intestinal parasite, according to the Centers for Disease Control and Prevention (opens in new tab) (CDC). According to the medical database StatPearls (opens in new tab), E. histolytica can invade the colon wall and cause colitis, where the inner lining of the colon becomes inflamed, and the parasite can cause severe and dysentery.

Though E. histolytica infections can occur anywhere in the world, it is most common in tropical regions that have substandard sanitation systems and crowded conditions.

While the initial symptoms include redness, itchiness and blurred vision, if left untreated the infection can cause severe pain and lead to vision loss, according to the CDC.

Amoebas also cause different infections of the brain. , which has been dubbed "the brain-eating amoeba," causes primary amoebic meningoencephalitis (PAM). Though the disease is rare, it is almost always fatal, according to the (opens in new tab). Early symptoms include fever and vomiting, and the disease ultimately progresses to more severe symptoms such as hallucinations and coma. N. fowleri is present in warm freshwater bodies such as hot springs, lakes and rivers, or in poorly chlorinated swimming pools or contaminated, hot tap water. These amoebas enter from the nose and travel to the brain. However, the infection can't be contracted by swallowing water, according to the CDC.

Early symptoms include headaches, nausea and low-grade fever, partial paralysis, seizures and speech difficulties. B. mandrillaris is found in the soil and can enter the body through open wounds or when people breathe in contaminated dust, according to the CDC. Since the amoeba was discovered in the 1980s, about 200 cases of the infection have been reported worldwide; this includes more than 100 confirmed cases in the U.S.

Maciver cites the example of cooling towers as a place where both amoebas and these bacteria can grow. Cooling towers tend to expel water droplets, which passersby can breathe in. "What's known to happen on many occasions, is that we breathe in a droplet of water containing an amoeba that is full of these pathogens [Legionella]," he said. If bacteria enter the body of an immunocompromised individual in such a manner, they can ultimately infect macrophages, one of the immune system's many defensive cells.

"A macrophage not only looks like an amoeba, its biochemical pathways and cell biology are quite similar," Maciver said. "So the same programmed events that allow the bacteria to escape the amoeba now operate to allow Legionella to escape the macrophage."

Amoebas are also important for recycling nutrients in the soil. According to Maciver, when nutrients become available they are taken up by bacteria, which "effectively lock up all the nutrients in bacterial mass." When bacteria are consumed, nutrients are released back into the soil. "If you have a cycle whereby amoeba eat bacteria, the overall effect is to increase nutrient availability for plants," Maciver said.

Additional resources and readings

Bibliography

Acharya, P. C., Fernandes, C., Mallik, S., Mishra, B., & Tekade, R. K. (2018). Physiologic Factors Related to Drug Absorption. In Dosage form design considerations (Vol. i, pp. 117-147). chapter, Academic Press/Elsevier.
Originally published on Live Science.
Bogitsh, B. J., Carter, C. E., & Oeltmann, T. N. (2019). General Characteristics of the Euprotista (Protozoa). In Human parasitology (fifth edition) (5th ed.). chapter, Academic Press.

Editor's note: This article was last updated on Jan. 28, 2022 by Live Science staff writer Nicoletta Lanese.

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(Image credit: Getty Images ) "Amoeba" is a term that describes a simple eukaryotic organism that moves in a characteristic crawling fashion. However, a comparison of the content of the various amoebas shows that these organisms are not necessarily closely related to each other.

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Home | Online exhibits | The world's biomes

Grasslands are characterized as lands dominated by grasses rather than large shrubs or trees. In the Miocene and Pliocene Epochs, which spanned a period of about 25 million years, mountains rose in western North America and created a continental climate favorable to grasslands. Ancient forests declined and grasslands became widespread. Following the Pleistocene Ice Ages, grasslands expanded in range as hotter and drier climates prevailed worldwide. There are two main divisions of grasslands:

Savanna

Savanna is grassland with scattered individual trees. Savannas of one sort or another cover almost half the surface of Africa (about five million square miles, generally central Africa) and large areas of Australia, South America, and India. Climate is the most important factor in creating a savanna. Savannas are always found in warm or hot climates where the annual rainfall is from about 50.8 to 127 cm (20-50 inches) per year. It is crucial that the rainfall is concentrated in six or eight months of the year, followed by a long period of drought when fires can occur. If the rain were well distributed throughout the year, many such areas would become tropical forest. Savannas which result from climatic conditions are called climatic savannas. Savannas that are caused by soil conditions and that are not entirely maintained by fire are called edaphic savannas. These can occur on hills or ridges where the soil is shallow, or in valleys where clay soils become waterlogged in wet weather. A third type of savanna, known as derived savanna, is the result of people clearing forest land for cultivation. Farmers fell a tract of forest, burn the dead trees, and plant crops in the ashes for as long as the soil remains fertile. Then, the field is abandoned and, although forest trees may recolonize, grass takes over on the bare ground (succession), becoming luxuriant enough to burn within a year or so. In Africa, a heavy concentration of elephants in protected parkland have created a savanna by eating leaves and twigs and breaking off the branches, smashing the trunks and stripping the bark of trees. Elephants can convert a dense woodland into an open grassland in a short period of time. Annual fires then maintain the area as a savanna.

The soil of the savanna is porous, with rapid drainage of water. It has only a thin layer of humus (the organic portion of the soil created by partial decomposition of plant or animal matter), which provides vegetation with nutrients. Savannas are sometimes classified as forests. The predominant vegetation consists of grasses and forbs (small broad-leaved plants that grow with grasses). Different savannas support different grasses due to disparities in rainfall and soil conditions. Because the savanna supports such a large number of species competing for living space, usually only one or a few kinds of grass are more successful than the others in a particular area. For example, in drier savannas such as those on the Serengeti plains or Kenya's Laikipia plateau, the dominant grasses on well-drained soils are Rhodes grass and red oat grass; throughout the East African savannas, star grasses are dominant; the lemon grasses are common in many western Uganda savannas. Deciduous trees and shrubs are scattered across the open landscape. One type of savanna common in southwestern Kenya, Tanzania, and Uganda, known as grouped-tree grassland, has trees growing only on termite mounds — the intervening soil being too thin or poorly drained to support the growth of trees at all. Frequent fires and large grazing mammals kill seedlings, thus keeping the density of trees and shrubs low. Savannas receive an average annual rainfall of 76.2-101.6 cm (30-40 inches).

Savanna has both a dry and a rainy season. Seasonal fires play a vital role in the savanna's biodiversity. In October, a series of violent thunderstorms, followed by a strong drying wind, signals the beginning of the dry season. Fire is prevalent around January, at the height of the dry season. Fires in savannas are often caused by poachers who want to clear away dead grass to make it easier to see their prey. The fires do not devastate the community. Most of the animals killed by the fires are insects with short life spans. A fire is a feast for some animals, such as birds that come to sites of fires to eat grasshoppers, stick insects, beetles, mice, and lizards that are killed or driven out by the fire. Underground holes and crevices provide a safe refuge for small creatures. Larger animals are usually able to run fast enough to escape the fire. Although the dry stems and leaves of grasses are consumed by fire, the grasses' deep roots remain unharmed. These roots, with all their starch reserves, are ready to send up new growth when the soil becomes more moist. The scattered shrubs can also subsist on food reserves in their roots while they await the time to venture above the soil again. Unlike grasses and shrubs, trees survive a fire by retaining some moisture in all their above-ground parts throughout the dry season. Sometimes they have a corky bark or semisucculent trunk covered with smooth resinous bark, both being fire resistant. A fire leaves scorched earth covered with a fine layer of powdery black ash in its wake. During March, violent thunderstorms occur again, this time heralding the rainy season. When the rains come, savanna bunch grasses grow vigorously. Some of the larger grasses grow an inch or more in 24 hours. The savannas experiences a surge of new life at this time. For example, many antelope calves are born. With so much grass to feed on, mothers have plenty of milk. Calves die if the rains fail to come.

Other animals (which do not all occur in the same savanna) include giraffes, zebras, buffaloes, kangaroos, mice, moles, gophers, ground squirrels, snakes, worms, termites, beetles, lions, leopards, hyenas, and elephants.

There are also some environmental concerns regarding savannas such as poaching, overgrazing, and clearing of the land for crops.

Temperate grassland

Temperate grasslands are characterized as having grasses as the dominant vegetation. Trees and large shrubs are absent. Temperatures vary more from summer to winter, and the amount of rainfall is less in temperate grasslands than in savannas. The major manifestations are the veldts of South Africa, the puszta of Hungary, the pampas of Argentina and Uruguay, the steppes of the former Soviet Union, and the plains and prairies of central North America. Temperate grasslands have hot summers and cold winters. Rainfall is moderate. The amount of annual rainfall influences the height of grassland vegetation, with taller grasses in wetter regions. As in the savanna, seasonal drought and occasional fires are very important to biodiversity. However, their effects aren't as dramatic in temperate grasslands as they are in savannas. The soil of the temperate grasslands is deep and dark, with fertile upper layers. It is nutrient-rich from the growth and decay of deep, many-branched grass roots. The rotted roots hold the soil together and provide a food source for living plants. Each different species of grass grows best in a particular grassland environment (determined by temperature, rainfall, and soil conditions). The seasonal drought, occasional fires, and grazing by large mammals all prevent woody shrubs and trees from invading and becoming established. However, a few trees, such as cottonwoods, oaks, and willows grow in river valleys, and some nonwoody plants, specifically a few hundred species of flowers, grow among the grasses. The various species of grasses include purple needlegrass, blue grama, buffalo grass, and galleta. Flowers include asters, blazing stars, coneflowers, goldenrods, sunflowers, clovers, psoraleas, and wild indigos.

Precipitation in the temperate grasslands usually occurs in the late spring and early summer. The annual average is about 50.8 to 88.9 cm (20-35 inches). The temperature range is very large over the course of the year. Summer temperatures can be well over 38° C (100 degrees Fahrenheit), while winter temperatures can be as low as -40° C (-40 degrees Fahrenheit).

The fauna (which do not all occur in the same temperate grassland) include gazelles, zebras, rhinoceroses, wild horses, lions, wolves, prairie dogs, jack rabbits, deer, mice, coyotes, foxes, skunks, badgers, blackbirds, grouses, meadowlarks, quails, sparrows, hawks, owls, snakes, grasshoppers, leafhoppers, and spiders.

There are also environmental concerns regarding the temperate grasslands. Few natural prairie regions remain because most have been turned into farms or grazing land. This is because they are flat, treeless, covered with grass, and have rich soil.

Temperate grasslands can be further subdivided. Prairies are grasslands with tall grasses while steppes are grasslands with short grasses. Prairie and steppes are somewhat similar but the information given above pertains specifically to prairies — the following is a specific description of steppes.

Steppes are dry areas of grassland with hot summers and cold winters. They receive 25.4-50.8 cm (10-20 inches) of rainfall a year. Steppes occur in the interiors of North America and Europe. Plants growing in steppes are usually greater than 1 foot tall. They include blue grama and buffalo grass, cacti, sagebrush, speargrass, and small relatives of the sunflower. Steppe fauna includes badgers, hawks, owls, and snakes. Today, people use steppes to graze livestock and to grow wheat and other crops. Overgrazing, plowing, and excess salts left behind by irrigation waters have harmed some steppes. Strong winds blow loose soil from the ground after plowing, especially during droughts. This causes the dust storms of the Great Plains of the U.S.

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Home | Online exhibits | The world's biomes Grasslands are characterized as lands dominated by grasses rather than large shrubs or trees. In the Miocene and Pliocene Epochs, which spanned a period of about 25 million years, mountains rose in western North America and created a continental climate favorable to grasslands.

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Home | Online exhibits | The world’s biomes

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Marine regions cover about three-fourths of the Earth's surface and include oceans, coral reefs, and estuaries. Marine algae supply much of the world's oxygen supply and take in a huge amount of atmospheric carbon dioxide. The evaporation of the seawater provides rainwater for the land.

Oceans

The largest of all the ecosystems, oceans are very large bodies of water that dominate the Earth's surface. Like ponds and lakes, the ocean regions are separated into separate zones: intertidal, pelagic, abyssal, and benthic. All four zones have a great diversity of species. Some say that the ocean contains the richest diversity of species even though it contains fewer species than there are on land.

The intertidal zone is where the ocean meets the land — sometimes it is submerged and at other times exposed, as waves and tides come in and out. Because of this, the communities are constantly changing. On rocky coasts, the zone is stratified vertically. Where only the highest tides reach, there are only a few species of algae and mollusks. In those areas usually submerged during high tide, there is a more diverse array of algae and small animals, such as herbivorous snails, crabs, sea stars, and small fishes. At the bottom of the intertidal zone, which is only exposed during the lowest tides, many invertebrates, fishes, and seaweed can be found. The intertidal zone on sandier shores is not as stratified as in the rocky areas. Waves keep mud and sand constantly moving, thus very few algae and plants can establish themselves — the fauna include worms, clams, predatory crustaceans, crabs, and shorebirds.

The pelagic zone includes those waters further from the land, basically the open ocean. The pelagic zone is generally cold though it is hard to give a general temperature range since, just like ponds and lakes, there is thermal stratification with a constant mixing of warm and cold ocean currents. The flora in the pelagic zone include surface seaweeds. The fauna include many species of fish and some mammals, such as whales and dolphins. Many feed on the abundant plankton.

The benthic zone is the area below the pelagic zone, but does not include the very deepest parts of the ocean (see abyssal zone below). The bottom of the zone consists of sand, slit, and/or dead organisms. Here temperature decreases as depth increases toward the abyssal zone, since light cannot penetrate through the deeper water. Flora are represented primarily by seaweed while the fauna, since it is very nutrient-rich, include all sorts of bacteria, fungi, sponges, sea anemones, worms, sea stars, and fishes.

The deep ocean is the abyssal zone. The water in this region is very cold (around 3° C), highly pressured, high in oxygen content, but low in nutritional content. The abyssal zone supports many species of invertebrates and fishes. Mid-ocean ridges (spreading zones between tectonic plates), often with hydrothermal vents, are found in the abyssal zones along the ocean floors. Chemosynthetic bacteria thrive near these vents because of the large amounts of hydrogen sulfide and other minerals they emit. These bacteria are thus the start of the food web as they are eaten by invertebrates and fishes.

Coral reefs

Coral reefs are widely distributed in warm shallow waters. They can be found as barriers along continents (e.g., the Great Barrier Reef off Australia), fringing islands, and atolls. Naturally, the dominant organisms in coral reefs are corals. Corals are interesting since they consist of both algae (zooanthellae) and tissues of animal polyp. Since reef waters tend to be nutritionally poor, corals obtain nutrients through the algae via photosynthesis and also by extending tentacles to obtain plankton from the water. Besides corals, the fauna include several species of microorganisms, invertebrates, fishes, sea urchins, octopuses, and sea stars.

Estuaries

Estuaries are areas where freshwater streams or rivers merge with the ocean. This mixing of waters with such different salt concentrations creates a very interesting and unique ecosystem. Microflora like algae, and macroflora, such as seaweeds, marsh grasses, and mangrove trees (only in the tropics), can be found here. Estuaries support a diverse fauna, including a variety of worms, oysters, crabs, and waterfowl.

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Home | Online exhibits | The world's biomes Marine regions cover about three-fourths of the Earth's surface and include oceans, coral reefs, and estuaries. Marine algae supply much of the world's oxygen supply and take in a huge amount of atmospheric carbon dioxide. The evaporation of the seawater provides rainwater for the land.

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Home | Online exhibits | The world's biomes

About 420 million years ago, during the Silurian Period, ancient plants and arthropods began to occupy the land. Over the millions of years that followed, these land colonizers developed and adapted to their new habitat. The first forests were dominated by giant horsetails, club mosses, and ferns that stood up to 40 feet tall.

Life on Earth continued to evolve, and in the late Paleozoic, gymnosperms appeared. By the Triassic Period (245-208 mya), gymnosperms dominated the Earth's forests. In the Cretaceous Period (144-65m mya), the first flowering plants (angiosperms) appeared. They evolved together with insects, birds, and mammals and radiated rapidly, dominating the landscape by the end of the Period. The landscape changed again during the Pleistocene Ice Ages — the surface of the planet that had been dominated by tropical forests for millions of years changed, and temperate forests spread in the Northern Hemisphere.

Today, forests occupy approximately one-third of Earth's land area, account for over two-thirds of the leaf area of land plants, and contain about 70% of carbon present in living things. They have been held in reverence in folklore and worshipped in ancient religions. However, forests are becoming major casualties of civilization as human populations have increased over the past several thousand years, bringing deforestation, pollution, and industrial usage problems to this important biome.

Present-day forest biomes, biological communities that are dominated by trees and other woody vegetation ( Spurr and Barnes 1980), can be classified according to numerous characteristics, with seasonality being the most widely used. Distinct forest types also occur within each of these broad groups.

There are three major types of forests, classed according to latitude:

Tropical forest

Left: Ranomafana National Park, Madagascar; right: Hawaiian forest.

Tropical forests are characterized by the greatest diversity of species. They occur near the equator, within the area bounded by latitudes 23.5 degrees N and 23.5 degrees S. One of the major characteristics of tropical forests is their distinct seasonality: winter is absent, and only two seasons are present (rainy and dry). The length of daylight is 12 hours and varies little.

Temperature is on average 20-25° C and varies little throughout the year: the average temperatures of the three warmest and three coldest months do not differ by more than 5 degrees.

Precipitation is evenly distributed throughout the year, with annual rainfall exceeding 200 cm.

Soil is nutrient-poor and acidic. Decomposition is rapid and soils are subject to heavy leaching.

Canopy in tropical forests is multilayered and continuous, allowing little light penetration.

Flora is highly diverse: one square kilometer may contain as many as 100 different tree species. Trees are 25-35 m tall, with buttressed trunks and shallow roots, mostly evergreen, with large dark green leaves. Plants such as orchids, bromeliads, vines (lianas), ferns, mosses, and palms are present in tropical forests.

Fauna include numerous birds, bats, small mammals, and insects.

Further subdivisions of this group are determined by seasonal distribution of rainfall:

evergreen rainforest: no dry season.

seasonal rainforest: short dry period in a very wet tropical region (the forest exhibits definite seasonal changes as trees undergo developmental changes simultaneously, but the general character of vegetation remains the same as in evergreen rainforests).

semievergreen forest: longer dry season (the upper tree story consists of deciduous trees, while the lower story is still evergreen).

moist/dry deciduous forest (monsoon): the length of the dry season increases further as rainfall decreases (all trees are deciduous).

More than one half of tropical forests have already been destroyed.

Temperate forest

Temperate forests occur in eastern North America, northeastern Asia, and western and central Europe. Well-defined seasons with a distinct winter characterize this forest biome. Moderate climate and a growing season of 140-200 days during 4-6 frost-free months distinguish temperate forests.

Temperature varies from -30° C to 30° C.

Precipitation (75-150 cm) is distributed evenly throughout the year.

Soil is fertile, enriched with decaying litter.

Canopy is moderately dense and allows light to penetrate, resulting in well-developed and richly diversified understory vegetation and stratification of animals.

Flora is characterized by 3-4 tree species per square kilometer. Trees are distinguished by broad leaves that are lost annually and include such species as oak, hickory, beech, hemlock, maple, basswood, cottonwood, elm, willow, and spring-flowering herbs.

Fauna is represented by squirrels, rabbits, skunks, birds, deer, mountain lion, bobcat, timber wolf, fox, and black bear.

Further subdivisions of this group are determined by seasonal distribution of rainfall:

moist conifer and evergreen broad-leaved forests: wet winters and dry summers (rainfall is concentrated in the winter months and winters are relatively mild).

dry conifer forests: dominate higher elevation zones; low precipitation.

mediterranean forests: precipitation is concentrated in winter, less than 100 cm per year.

temperate coniferous: mild winters, high annual precipitation (greater than 200 cm).

temperate broad-leaved rainforests: mild, frost-free winters, high precipitation (more than 150 cm) evenly distributed throughout the year.

Only scattered remnants of original temperate forests remain.

Boreal forest (taiga)

Boreal forests, or taiga, represent the largest terrestial biome. Occuring between 50 and 60 degrees north latitudes, boreal forests can be found in the broad belt of Eurasia and North America: two-thirds in Siberia with the rest in Scandinavia, Alaska, and Canada. Seasons are divided into short, moist, and moderately warm summers and long, cold, and dry winters. The length of the growing season in boreal forests is 130 days.

Temperatures are very low.

Precipitation is primarily in the form of snow, 40-100 cm annually.

Soil is thin, nutrient-poor, and acidic.

Canopy permits low light penetration, and as a result, understory is limited.

Flora consist mostly of cold-tolerant evergreen conifers with needle-like leaves, such as pine, fir, and spruce.

Fauna include woodpeckers, hawks, moose, bear, weasel, lynx, fox, wolf, deer, hares, chipmunks, shrews, and bats.

Current extensive logging in boreal forests may soon cause their disappearance.

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Home | Online exhibits | The world's biomes About 420 million years ago, during the Silurian Period, ancient plants and arthropods began to occupy the land. Over the millions of years that followed, these land colonizers developed and adapted to their new habitat.

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Sand dunes in Death Valley National Monument, California.

Deserts cover about one fifth of the Earth's surface and occur where rainfall is less than 50 cm/year. Although most deserts, such as the Sahara of North Africa and the deserts of the southwestern U.S., Mexico, and Australia, occur at low latitudes, another kind of desert, cold deserts, occur in the basin and range area of Utah and Nevada and in parts of western Asia. Most deserts have a considerable amount of specialized vegetation, as well as specialized vertebrate and invertebrate animals. Soils often have abundant nutrients because they need only water to become very productive and have little or no organic matter. Disturbances are common in the form of occasional fires or cold weather, and sudden, infrequent, but intense rains that cause flooding.

There are relatively few large mammals in deserts because most are not capable of storing sufficient water and withstanding the heat. Deserts often provide little shelter from the sun for large animals. The dominant animals of warm deserts are nonmammalian vertebrates, such as reptiles. Mammals are usually small, like the kangaroo mice of North American deserts.

Desert biomes can be classified according to several characteristics.
There are four major types of deserts:

Hot and dry desert

The four major North American deserts of this type are the Chihuahuan, Sonoran, Mojave and Great Basin. Others outside the U.S. include the Southern Asian realm, Neotropical (South and Central America), Ethiopian (Africa) and Australian.

The seasons are generally warm throughout the year and very hot in the summer. The winters usually bring little rainfall.

From left: Baja, Mexico desert; desert in Uluru National Park, Australia; desert near the Kofa Mountains, Arizona.

Temperatures exhibit daily extremes because the atmosphere contains little humidity to block the Sun's rays. Desert surfaces receive a little more than twice the solar radiation received by humid regions and lose almost twice as much heat at night. Many mean annual temperatures range from 20-25° C. The extreme maximum ranges from 43.5-49° C. Minimum temperatures sometimes drop to -18° C.

Rainfall is usually very low and/or concentrated in short bursts between long rainless periods. Evaporation rates regularly exceed rainfall rates. Sometimes rain starts falling and evaporates before reaching the ground. Rainfall is lowest on the Atacama Desert of Chile, where it averages less than 1.5 cm. Some years are even rainless. Inland Sahara also receives less than 1.5 cm a year. Rainfall in American deserts is higher — almost 28 cm a year.

Soils are course-textured, shallow, rocky or gravely with good drainage and have no subsurface water. They are coarse because there is less chemical weathering. The finer dust and sand particles are blown elsewhere, leaving heavier pieces behind.

Canopy in most deserts is very rare. Plants are mainly ground-hugging shrubs and short woody trees. Leaves are "replete" (fully supported with nutrients) with water-conserving characteristics. They tend to be small, thick and covered with a thick cuticle (outer layer). In the cacti, the leaves are much-reduced (to spines) and photosynthetic activity is restricted to the stems. Some plants open their stomata (microscopic openings in the epidermis of leaves that allow for gas exchange) only at night when evaporation rates are lowest. These plants include: yuccas, ocotillo, turpentine bush, prickly pears, false mesquite, sotol, ephedras, agaves and brittlebush.

The animals include small nocturnal (active at night) carnivores. The dominant animals are burrowers and kangaroo rats. There are also insects, arachnids, reptiles and birds. The animals stay inactive in protected hideaways during the hot day and come out to forage at dusk, dawn or at night, when the desert is cooler.

Semiarid desert

The major deserts of this type include the sagebrush of Utah, Montana and Great Basin. They also include the Nearctic realm (North America, Newfoundland, Greenland, Russia, Europe and northern Asia).

The summers are moderately long and dry, and like hot deserts, the winters normally bring low concentrations of rainfall. Summer temperatures usually average between 21-27° C. It normally does not go above 38° C and evening temperatures are cool, at around 10° C. Cool nights help both plants and animals by reducing moisture loss from transpiration, sweating and breathing. Furthermore, condensation of dew caused by night cooling may equal or exceed the rainfall received by some deserts. As in the hot desert, rainfall is often very low and/or concentrated. The average rainfall ranges from 2-4 cm annually.

From left: Red Rock Canyon National Conservation Area, southern Nevada; sagebrush near Bridger, Montana; Castle Valley, Utah, east of Arches National Park.

The soil can range from sandy and fine-textured to loose rock fragments, gravel or sand. It has a fairly low salt concentration, compared to deserts which receive a lot of rain (acquiring higher salt concentrations as a result). In areas such as mountain slopes, the soil is shallow, rocky or gravely with good drainage. In the upper bajada (lower slopes) they are coarse-textured, rocky, well-drained and partly "laid by rock bench." In the lower bajada (bottom land) the soil is sandy and fine-textured, often with "caliche hardpan." In each case there is no subsurface water.

The spiny nature of many plants in semiarid deserts provides protection in a hazardous environment. The large numbers of spines shade the surface enough to significantly reduce transpiration. The same may be true of the hairs on the woolly desert plants. Many plants have silvery or glossy leaves, allowing them to reflect more radiant energy. These plants often have an unfavorable odor or taste. Semiarid plants include: Creosote bush, bur sage ( Franseria dumosa or F. deltoidea), white thorn, cat claw, mesquite, brittle bushes ( Encelia farinosa), lyciums, and jujube.

During the day, insects move around twigs to stay on the shady side; jack rabbits follow the moving shadow of a cactus or shrub. Naturally, many animals find protection in underground burrows where they are insulated from both heat and aridity. These animals include mammals such as the kangaroo rats, rabbits, and skunks; insects like grasshoppers and ants; reptiles are represented by lizards and snakes; and birds such as burrowing owls and the California thrasher.

Coastal desert

These deserts occur in moderately cool to warm areas such as the Nearctic and Neotropical realm. A good example is the Atacama of Chile.

The cool winters of coastal deserts are followed by moderately long, warm summers. The average summer temperature ranges from 13-24° C; winter temperatures are 5° C or below. The maximum annual temperature is about 35° C and the minimum is about -4° C. In Chile, the temperature ranges from -2 to 5° C in July and 21-25° C in January.

The average rainfall measures 8-13 cm in many areas. The maximum annual precipitation over a long period of years has been 37 cm with a minimum of 5 cm.

The soil is fine-textured with a moderate salt content. It is fairly porous with good drainage. Some plants have extensive root systems close to the surface where they can take advantage of any rain showers. All of the plants with thick and fleshy leaves or stems can take in large quantities of water when it is available and store it for future use. In some plants, the surfaces are corrugated with longitudinal ridges and grooves. When water is available, the stem swells so that the grooves are shallow and the ridges far apart. As the water is used, the stem shrinks so that the grooves are deep and ridges close together. The plants living in this type of desert include the salt bush, buckwheat bush, black bush, rice grass, little leaf horsebrush, black sage, and chrysothamnus.

Some animals have specialized adaptations for dealing with the desert heat and lack of water. Some toads seal themselves in burrows with gelatinous secretions and remain inactive for eight or nine months until a heavy rain occurs. Amphibians that pass through larval stages have accelerated life cycles, which improves their chances of reaching maturity before the waters evaporate. Some insects lay eggs that remain dormant until the environmental conditions are suitable for hatching. The fairy shrimps also lay dormant eggs. Other animals include: insects, mammals (coyote and badger), amphibians (toads), birds (great horned owl, golden eagle and the bald eagle), and reptiles (lizards and snakes).

Cold desert

These deserts are characterized by cold winters with snowfall and high overall rainfall throughout the winter and occasionally over the summer. They occur in the Antarctic, Greenland and the Nearctic realm. They have short, moist, and moderately warm summers with fairly long, cold winters. The mean winter temperature is between -2 to 4° C and the mean summer temperature is between 21-26° C.

The winters receive quite a bit of snow. The mean annual precipitation ranges from 15-26 cm. Annual precipitation has reached a maximum of 46 cm and a minimum of 9 cm. The heaviest rainfall of the spring is usually in April or May. In some areas, rainfall can be heavy in autumn. The soil is heavy, silty, and salty. It contains alluvial fans where soil is relatively porous and drainage is good so that most of the salt has been leached out.

The plants are widely scattered. In areas of shadscale, about 10 percent of the ground is covered, but in some areas of sagebush it approaches 85 percent. Plant heights vary between 15 cm and 122 cm. The main plants are deciduous, most having spiny leaves. Widely distributed animals are jack rabbits, kangaroo rats, kangaroo mice, pocket mice, grasshopper mice, and antelope ground squirrels. In areas like Utah, population density of these animals can range from 14-41 individuals per hectare. All except the jack rabbits are burrowers. The burrowing habit also applies to carnivores like the badger, kit fox, and coyote. Several lizards do some burrowing and moving of soil. Deer are found only in the winter.

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Home | Online exhibits | The world's biomes Sand dunes in Death Valley National Monument, California. Deserts cover about one fifth of the Earth's surface and occur where rainfall is less than 50 cm/year.

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Home | Online exhibits | The world's biomes

Freshwater is defined as having a low salt concentration — usually less than 1%. Plants and animals in freshwater regions are adjusted to the low salt content and would not be able to survive in areas of high salt concentration (i.e., ocean). There are different types of freshwater regions:

Ponds and lakes

These regions range in size from just a few square meters to thousands of square kilometers. Scattered throughout the earth, several are remnants from the Pleistocene glaciation. Many ponds are seasonal, lasting just a couple of months (such as sessile pools) while lakes may exist for hundreds of years or more. Ponds and lakes may have limited species diversity since they are often isolated from one another and from other water sources like rivers and oceans. Lakes and ponds are divided into three different "zones" which are usually determined by depth and distance from the shoreline.

The topmost zone near the shore of a lake or pond is the littoral zone. This zone is the warmest since it is shallow and can absorb more of the Sun's heat. It sustains a fairly diverse community, which can include several species of algae (like diatoms), rooted and floating aquatic plants, grazing snails, clams, insects, crustaceans, fishes, and amphibians. In the case of the insects, such as dragonflies and midges, only the egg and larvae stages are found in this zone. The vegetation and animals living in the littoral zone are food for other creatures such as turtles, snakes, and ducks.

The near-surface open water surrounded by the littoral zone is the limnetic zone. The limnetic zone is well-lighted (like the littoral zone) and is dominated by plankton, both phytoplankton and zooplankton. Plankton are small organisms that play a crucial role in the food chain. Without aquatic plankton, there would be few living organisms in the world, and certainly no humans. A variety of freshwater fish also occupy this zone.

Plankton have short life spans — when they die, they fall into the deep-water part of the lake/pond, the profundal zone. This zone is much colder and denser than the other two. Little light penetrates all the way through the limnetic zone into the profundal zone. The fauna are heterotrophs, meaning that they eat dead organisms and use oxygen for cellular respiration.

Temperature varies in ponds and lakes seasonally. During the summer, the temperature can range from 4° C near the bottom to 22° C at the top. During the winter, the temperature at the bottom can be 4° C while the top is 0° C (ice). In between the two layers, there is a narrow zone called the thermocline where the temperature of the water changes rapidly. During the spring and fall seasons, there is a mixing of the top and bottom layers, usually due to winds, which results in a uniform water temperature of around 4° C. This mixing also circulates oxygen throughout the lake. Of course there are many lakes and ponds that do not freeze during the winter, thus the top layer would be a little warmer.

Streams and rivers

These are bodies of flowing water moving in one direction. Streams and rivers can be found everywhere — they get their starts at headwaters, which may be springs, snowmelt or even lakes, and then travel all the way to their mouths, usually another water channel or the ocean. The characteristics of a river or stream change during the journey from the source to the mouth. The temperature is cooler at the source than it is at the mouth. The water is also clearer, has higher oxygen levels, and freshwater fish such as trout and heterotrophs can be found there. Towards the middle part of the stream/river, the width increases, as does species diversity — numerous aquatic green plants and algae can be found. Toward the mouth of the river/stream, the water becomes murky from all the sediments that it has picked up upstream, decreasing the amount of light that can penetrate through the water. Since there is less light, there is less diversity of flora, and because of the lower oxygen levels, fish that require less oxygen, such as catfish and carp, can be found.

Wetlands

Wetlands are areas of standing water that support aquatic plants. Marshes, swamps, and bogs are all considered wetlands. Plant species adapted to the very moist and humid conditions are called hydrophytes. These include pond lilies, cattails, sedges, tamarack, and black spruce. Marsh flora also include such species as cypress and gum. Wetlands have the highest species diversity of all ecosystems. Many species of amphibians, reptiles, birds (such as ducks and waders), and furbearers can be found in the wetlands. Wetlands are not considered freshwater ecosystems as there are some, such as salt marshes, that have high salt concentrations — these support different species of animals, such as shrimp, shellfish, and various grasses.

Visit our gallery of wetlands images, which illustrate the amazing diversity of wetland ecosystems.

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Home | Online exhibits | The world's biomes Freshwater is defined as having a low salt concentration - usually less than 1%. Plants and animals in freshwater regions are adjusted to the low salt content and would not be able to survive in areas of high salt concentration (i.e., ocean).