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Hydrozoans have many cell types, but it only forms two tissue types for their bodies, and it is the epidermus and gastrodermis.

Their body can range in sizes. It can be as small as planula larvae which is a few micrometers and can grow as big as a massive colonial coral covering huge amount of areas

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Hydrozoans life cycles are complex and diverse. In many hydrozoan species, it has a free living planula larva that can transform into a primary polyp. The primary polyp buds other polyps to produce a benthic colonial stage. Within the hydrozoans, it has extraordinary variation in the life cycle that relates to the polyp, colony and medusa morphologies.

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What do they prey on?

Crustaceans like copepods

What do they eat?

Carnivores

How much do they weigh?

0.4-0.9 lb (20-400 g)

How long are they?

0.2-2.4 in (0.5-6 cm)

What do they look like?

Varied colors

Skin Type

Smooth

What are their main threats?

Sea Anemones, Other Sea Creatures, Larger Jellyfish

Where you'll find them

Oceans, Seas

Kingdom

Animalia

Class

Hydrozoa

Scientific Name

Craspedacusta sowerbii

Genus

Craspedacusta

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Hydrozoans are small and predatory animals. They are mostly found in saltwater. Their body consists of a dome-like umbrella rigged by tentacles. It has a tube-like structure that hangs down from the centre of the umbrella and has a mouth at its tip. Some hydrozoans live on the surface by floating, with large sail like structures for locomotion. Hydrozoans have long tentacles with nematocytes in the water to catch food.

Hydrozoans have both polyp and medusa stages in their life cycle. Most hydrozoans form colonies of asexual polyps and have free-swimming sexual medusae.
Most are found in small colonies made up of tiny polyps that are connected together by hollow stems known as the hydrocauli.

Apart from all other cnidarians, hydrozoans have their gonads derived from epidermal tissue, whereas in all other cnidarians they are derived from the gastrodermal tissue.

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They are hermaphroditic, which means that male and female reproductive organs are present in an individual.

Protozoa generally display alteration generation between the medusa and polyps stages. There are some hydrozoa that its life cycle lacks any trace of a medusa form. There are some that are only represented by the medusa for and lack the polyps stage.

Eggs of a Hydra are retained in the individual, while the sperms are released. The retained eggs will be fertilized by the neighboring individuals of the same species.

Vegetative reproduction, also known as asexual reproduction, by budding is common.

After fertilization, miniature Hydras, the offsprings will eventually be released.

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Hydrozoan jellyfish are poisonous, they have venom filled nematocysts that they use to sting and kill small sea creatures.  Their sting can cause excruciating pain and sometimes even be deadly.

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Hydrozoans mainly inhabit marine environments and some have invaded the freshwater environments.

They can be found in colonies or as individuals.

There are roughly 3,700 known species

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Hydrozoans are  a taxonomic class of very small, predatory animals that mostly live in saltwater

These organisms are related to jellyfish and corals and belong to the phylum Cnidaria.

However, hydrozoan jellyfish are not considered real or pure jellyfish

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The Portuguese Man O’ War is often confused with a jellyfish but is actually a siphonophore, a colony of specialized polyps and medusoids, which is incorrect and may lead to improper treatment of stings, as the venom is different. A second sting may lead to an allergic reaction.

The Portuguese Man O’ War has an air bladder which is an internal organ that contributes to the ability of a fish to control its buoyancy. It enables the Portuguese Man O’ War to stay at the current water depth, ascend, or descend without swimming.

The air bladder is known as the ‘pneumatophore’ or ‘sail’. The Portuguese Man o’ War has no means of propulsion and is pushed by the winds and the current. If it is swept tp the surface the pneumatophore can be deflated allowing the Man O’ War to briefly submerge.

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The most widely-known freshwater hydrozoan is Hydra, a genus of simple, fresh-water animals possessing radial symmetry. Hydra can be found in most unpolluted freshwater ponds, lakes and streams in the temperate and tropical regions by gently sweeping a collecting net through weedy areas. Hydra are usually a few millimeters long and are best studied with a microscope.

It is sessile, with a pedal disc which attaches it to substrate. Like all cnidarians, Hydra uses nematocysts, stinging cells which disable its prey. Hydra eat small crustaceans (such as brine shrimp), insect larvae, and annelid worms. It may reproduce sexually, or by asexual reproduction

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Portuguese man o' war also known as blue bottle are infamous for their very painful sting.

Although rarely deadly, a Bluebottle sting can be very dangerous as it causes fever, shock and respiratory distress. It causes intense and distressing pain that lasts for an hour or more. They also leave whip-like, red welts on the skin which last for about 2-3 days.