Cnidaria facts for kids

Quick facts for kids Cnidaria
East Coast sea nettle Chrysaora quinquecirrha (?)
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
(unranked):	Radiata
Phylum:	Cnidaria Hatschek, 1888

Cnidaria is a group of over 10,000 different kinds of animals. You can find them only in water, mostly in the ocean, but some live in freshwater. A special thing about cnidarians is their unique cells called cnidocytes. They use these cells mainly to catch their food.

Their bodies are made of a jelly-like material called mesoglea. This jelly is between two thin layers of skin-like tissue. Cnidarians have two main body shapes: the swimming medusa (like a jellyfish) and the sessile polyp (like a sea anemone). Both shapes have a mouth surrounded by tentacles. These tentacles are covered with those special cnidocyte cells. They have one opening that they use for both eating and breathing.

Types of Cnidarians

There are five main groups, or classes, of cnidarians. You might know jellyfish, which are found in four of these groups.

• Anthozoa: These are the sea anemones and corals. They usually stay in one place.
• Scyphozoa: These are the 'true' jellyfish, like the ones you often see.
• Cubozoa: These are known as box jellies. Some of them can be very dangerous.
• Hydrozoa: This group includes animals like Hydra and the Portuguese man o' war.
• Staurozoa: These are called stalked jellyfish because they have a stalk that attaches them to surfaces.

Most cnidarians eat other living things, from tiny plankton to animals much bigger than themselves. Many animals, like starfish, sea slugs, fish, and turtles, eat cnidarians. While many corals live in warm, shallow ocean waters, other cnidarians can be found deep in the ocean, in very cold polar regions, and even in freshwater.

Jellyfish move by squeezing water out of their bell-shaped bodies. This is like a type of jet propulsion. Their jelly-like bodies help them spring back into shape. Because their bodies are very thin, they can't swim strongly against ocean currents. Instead, they mostly drift with the currents, using their movement to steer.

Some cnidarians, like Hydra and certain sea anemones, can move slowly. They might creep along like snails or crawl like inchworms. Some can even do somersaults! A few can swim a little bit by wiggling their bases.

Cnidarians don't have a brain or a central nervous system like humans do. However, they do have special nerve tissues that help them react to their surroundings. They don't have lungs or gills. Both layers of their body absorb oxygen from the water and release carbon dioxide. A cool fact about all cnidarians is that they can regenerate. This means they can grow back lost parts or recover from injuries.

Cnidarians and People

The dangerous "sea wasp" Chironex fleckeri

Jellyfish stings have caused many deaths over time, especially from box jellies, which are very dangerous. On the other hand, Coral reefs are very important. They provide places for fish to live, protect coastlines from waves, and attract tourists. But coral reefs are in danger from too much fishing, mining for building materials, pollution, and damage from tourism.

The "sea wasp" Chironex fleckeri is known as one of the most dangerous jellyfish in the world. It's hard to spot because it's almost clear. Most jellyfish stings only cause mild problems. However, some other box jellies can cause something called Irukandji syndrome. This syndrome takes about 30 minutes to show up and can last for hours or even weeks. People usually need to go to the hospital, and a few deaths have happened from it.

Some large jellyfish are eaten as a delicacy in East and Southeast Asia, especially in Japan, Korea, and other parts of Southeast Asia. Fishing for these jellyfish often happens during the day in calm weather, usually in two short seasons. The value of jellyfish as food depends on how well they are prepared. "Jellyfish Masters" keep their special preparation methods secret! Jellyfish meat is very low in cholesterol and sugars.

Images for kids

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  Pacific sea nettles, Chrysaora fuscescens

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  Oral end of actinodiscus polyp

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  A hydra's nematocyst, before firing.      "trigger" cilium

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  Stranded scyphozoans on a Cambrian tidal flat in Blackberry Hill, Wisconsin.

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  The fossil coral Cladocora from Pliocene rocks in Cyprus

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  Illustrated tree of cnidarians and their closest relatives

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  The dangerous Carukia barnesi, one of the known species of box jellyfish which can cause Irukandji syndrome.

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  Cerianthus filiformis (Ceriantharia)

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  Sea anemones (Actinaria, part of Hexacorallia)

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  Coral Acropora muricata (Scleractinia, part of Hexacorallia)

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  Sea fan Gorgonia ventalina (Alcyonacea, part of Octocorallia)

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  Box jellyfishCarybdea branchi (Cubozoa)

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  Siphonophore Physalia physalis (Hydrozoa)

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  Myxobolus cerebralis (Myxozoa)

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  Polypodium hydriforme (Polypodiozoa)

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  Jellyfish Phyllorhiza punctata (Scyphozoa)

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  Stalked jelly Haliclystus antarcticus (Staurozoa)

See also

In Spanish: Cnidaria para niños