In sea anemones, jellyfish, and other similar aquatic

In this lesson, you will learn about the animals of the phylum Cnidaria, what a respiratory system is, and how the respiratory system of a cnidarian works. Be sure to complete the quiz after the lesson.


Breathing underwater is impossible for us, so how do underwater creatures do it? Let’s look at how the cnidarians take in oxygen even in water.

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Cnidarians are some of the most beautiful creatures on Earth. Members of the phylum Cnidaria include coral, sea anemones, jellyfish, and other similar aquatic creatures like sea fans and hydras. Cnidarians are grouped based on the presence of a special type of stinging cell called cnidocytes, which are used for defense and sometimes for capturing food. While almost all live in saltwater environments, there are a few species that can be found in freshwater.

Respiratory System

Both humans and cnidarians have respiratory systems.

A respiratory system is the body system responsible for taking in needed gases and expelling waste gases. In humans, the needed gas is oxygen, and the waste gas is carbon dioxide. Our lungs and our blood are the primary components of our respiratory system, but how does respiration work in cnidarians?

Respiration Without Organs

Have you ever looked at a jellyfish and wondered where its lungs were? Given their bodies are virtually transparent, you would think that if jellyfish had lungs, we should be able to see them! As it turns out, jellyfish and other cnidarians don’t have lungs or any other type of respiratory organ, so it makes sense then that we aren’t able to see any obvious lungs when we look at them.Instead, all of the cells in a cnidarians body are capable of absorbing oxygen from the water around them, and expelling carbon dioxide back into the water through diffusion. Some metabolic wastes can also be passed back into the water through these same cells. Depending on the species, water enters through a mouth or through surface cells, and exits through either location.In terms of respiration, one of the biggest problems cnidarians can face is ending up in an area where the water is stagnant, or not circulating much.

In these conditions, many species will die as it is hard for them to obtain the amount of oxygen they need when the surrounding water is not constantly refreshing itself.

Coral Respiration Issues

Unlike other cnidarians, many species of coral often have symbiotic algae that live within their body structures. These symbionts share food with the coral, in exchange for the coral providing them with shelter. In the case of these types of corals, the algae often produce large amounts of oxygen, and, like in humans, too high of a concentration of oxygen can be a bad thing for cells.

To combat the high oxygen concentrations, the coral produce antioxidant chemicals to help negate some of the oxygen around them.

Lesson Summary

Cnidarians are aquatic animals that contain stinging cells called cnidocytes. Respiratory systems are the body system responsible for taking in needed gases and removing waste gases. While cnidarians do not have lungs or other respiratory organs, they do use body cells to take in oxygen and expel waste gases. This can be a problem in areas with stagnant water, as the lack of circulation decreases the available oxygen.

Coral can sometimes experience an excess of oxygen around them due to oxygen production in their symbiotic algae. Coral are able to produce antioxidants to combat the high oxygen levels, which can become poisonous to them.


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