Cnidaria Digestive System

Instructor: Heather Pier

Heather has taught high school and college science courses, and has a master's degree in geography-climatology.

Explore the digestive system of the phylum Cnidaria. Learn about the structure and function of their digestive system and its parts, as well as what foods they are digesting.

What Is a Cnidarian?

Before we get too deep into digestive bits and parts, let's start with the answer to the very important question in the heading: A cnidarian is an animal belonging to the phylum Cnidaria. The animals in this phylum are all invertebrates and mostly saltwater aquatic species. The best-known cnidarians are jellyfish, sea anemones, and corals.

Although the species that make up the phylum may look very different, they all share one defining characteristic: their bodies contain stinging cells called nematocysts. Sometimes the nematocysts are used for protection and sometimes to help catch food.

How Do Cnidarians Eat?

It's pretty obvious how most animals (including humans!) eat-- food enters the body through the mouth, passes through a digestive system, and exits through the anus. But have you ever wondered how a jellyfish eats?

Jellyfish don't have an obvious mouth (or anus, for that matter) and it is hard to imagine where a digestive system might be within their small, transparent bodies.

Have you ever wondered where a jellyfishs mouth is?

The lack of an obvious means of eating and digesting food is true of most cnidarians. But they are animals and not plants, so they can't make their own food through photosynthesis. So there must be a way for them to eat. Most cnidarians, especially anemones and jellyfish, use their nematocysts to sting and capture food.

The nematocysts on the animal's body come in contact with a potential meal and release stinging barbs into the prey, rending the prey temporarily stunned or even killed. How severe the nematocyst's sting is depends on the species, but certain jellyfish can be particularly lethal.

Once their food has been captured in their tentacles or polyps, it can be moved toward their mouth area. Cnidarians' mouths don't look like a human mouth--there are no teeth or lips, but it functions in the same way, as the entrance point for the digestive system. The size of the mouth varies by species, but even the tiniest coral polyps have a mouth area.

What Do Cnidarians Eat?

Clearly, cnidarians aren't swimming over to the nearest drive-thru to pick up their dinners. They have to work a little harder than most animals because they aren't strongly predatory species that can easily hunt down prey. These creatures have to rely on their food to come to them, either through floating in the water column or unintentionally coming into contact with the cnidarian.

All cnidarians are carnivores, meaning that some form of meaty food is the basis of their diet. The meat consumed varies depending on the size of the species eating it, from microscopic zooplankton (animal larvae) and phytoplankton (plant larvae) to small fish.

Some coral and anemones have symbiotic, or mutually beneficial, relationships with certain species of fish. The most famous example of this is the relationship between clownfishes and their host anemones. Corals and anemones won't eat their symbiotic pals but can eat other types of fish if the opportunity presents itself.

An anemone will not eat its symbiotic clownfish but it can eat other species of fish.

The Cnidarian Digestive System

Take a moment to consider what your life would be like if your mouth and your anus performed the same job. That is what life is like every day for cnidarians.

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