Cnidarians

Leon is snorkelling, and he spotted something red, and moving, like an underwater flower. Don’t touch it, Leon! It’s a sea anemone, and it can sting, and hurt you! Even though they look more like plants, sea anemones are actually animals! Just like other animals, they can’t produce their own food.

To get their food, sea anemones hunt other animals. Although they might not look like vicious hunters, they have a secret weapon! When the sea anemone touches something, like a fish swimming by, this activates specialised cells on its tentacles, that send out long, thin, barbed threads. Each thread then injects venom into the animal. The sea anemone that Leon saw, can’t tell that Leon is too big to be eaten.

If he touched it, it would inject him with venom too, which could be quite painful. But there are quite a few other types of animals that have these special body parts too: other corals, jellyfish, and hydras. We classify all these animals into a larger group, or phylum, CNIDARIANS. But barbed threads are only one defining characteristic of this phylum. All cnidarians have no backbone, no spinal column: they are invertebrates.

They also have an empty space at the centre of their body, a body cavity, where they digest food and absorb nutrients. Their digestive system is very simple and only has one opening, through which cnidarians take in food and discard waste. This opening is surrounded by tentacles, which capture and move the food into the cavity. Cnidarians have two basic types of body. One type is shaped like a cup or a tube, with mouth and tentacles facing upwards, and the bottom attached to the surface underneath: like sea anemones!

This body type is called a POLYP. The other type looks more like a bell or an umbrella, with the mouth and tentacles facing downwards; like jellyfish! This type of body is called MEDUSA, and is able to move freely. Some cnidarians are both medusae and polyps during different stages of their development. But other species only exist as one of these types throughout their lives.

Cnidarians are a really important part of marine ecosystems. Many species are food for other animals. For example, jellyfish are one of the main sources of food for turtles. Colonies of corals, coral reefs, provide a perfect environment for thousands of other organisms to live in. But corals are very vulnerable to pollution and environmental changes, especially increases in temperature.

If the temperature of the water increases too much, organisms that live on coral reefs will go elsewhere. We call this coral bleaching. Coral bleaching can cause entire coral reefs to collapse, which affects thousands of organisms that live there. Some species of jellyfish do actually thrive in warming oceans with less oxygen, and they reproduce more than usual. This can have a negative impact on other marine organisms.

Cnidarians are really interesting, but if you want to look at them Leon, it might be better to do that from a distance rather than from up close. Or from inside a wetsuit.