All About Cnidarians

Whether it's jellyfish, corals, or sea anemones – cnidarians are among the oldest and most unusual creatures on Earth!

What Are Cnidarians?

Cnidarians are invertebrates that primarily inhabit the ocean, though they can also be found in freshwater environments. This group includes jellyfish, corals, and sea anemones. There are over 11,000 species worldwide, some of which drift freely in the water while others are anchored to the ocean floor.

Main Characteristics of Cnidarians

1. Stinging Cells for Defense

All cnidarians have specialized stinging cells. When touched, these cells release a toxin. This mechanism allows cnidarians to capture prey or defend themselves against predators.

2. Radial Body Structure

Most cnidarians feature a body that radiates outward from the center – a characteristic that is particularly evident in starfish.

3. Two “States”

Cnidarians exist in two typical forms: as free-swimming jellyfish and/or as stationary polyps (the Greek term for “many-footed”). Some jellyfish start their life as polyps on the ocean floor before eventually swimming freely in the seas. Other cnidarians, like corals, live exclusively as polyps attached to the seabed.

Cnidarian Family Tree

Amazing Facts About Cnidarians

• There are 11,000 species of cnidarians.
• They are divided into five classes: anthozoa, stalked jellyfish, box jellyfish, true jellyfish and hydrozoans.
• Anthozoa are attached to the sea bed. They include corals and sea anemones.
• The box jellyfish include the particularly poisonous sea wasp.
• Well-known true jellyfish are the moon jellyfish and the fried egg jellyfish. The Portuguese man o' war is one of the hydrozoans.
• Cnidarians are poisonous and cause painful wounds when touched.
• They live in salt water, some also in fresh water. Most are carnivores.

Species List