How Do Jellyfish Function Without A Heart Or Brain?

Jellyfish do not have brains. They don’t have a heart either. But jellyfish do have a very basic set of nerves at the base of their tentacles. These nerves detect touch, temperature, salinity etc.

The Earth has no shortage of weird animals, and it shouldn’t surprise you that the vast majority of these bizarre earthlings are found underwater, considering how much of the deep blue remains unexplored. This article, however, is about a water-dweller that is actually quite common worldwide, has been widely studied, and should be a household name: the Jellyfish.

What is a Jellyfish?

A jellyfish, contrary to what its name suggests, is not actually a fish. These animals are types of plankton and distant relatives of corals and sea anemones. Most notably, their bodies are essentially made of water, having no bones whatsoever. In fact,  jellyfish are composed of about 95% water! A good way of visualizing this is: if a jellyfish washed up on the beach, after a while, due to evaporation, it would basically disappear!

Another characteristic feature of jellyfish is their tentacles, although not all species have them. The tentacles serve a dual purpose: they are used to catch food and for self-defense. Each tentacle is covered with special cells called cnidoblasts, which are capable of unleashing a painful sting.


Jellyfish (Photo credit:

What a jellyfish is actually doing when it stings you is injecting its venom into your body. It just so happens that this venom is, unfortunately, a neurotoxin (affecting the brain) and is used to incapacitate or kill jellyfish prey, but is seldom fatal to humans (phew!). If you do happen to be stung though, you will experience a set of symptoms that include pain, skin rashes, fever and muscle cramps. The best way to deal with the sting is by rubbing the affected area with alcohol, vinegar or ammonia.

What do jellyfish eat?

As far as their diet is concerned, small jellyfish eat small fish and zooplankton, whereas larger jellyfish consume crustaceans and other larger aquatic animals.

Do jellyfish have brains?

The most intriguing aspect of jellyfish is that along with having no bones, eyes or gills (like fish do), jellyfish also do not have brains! They don’t have a heart either. With that in mind, how in the world are these animals even alive, let alone living fully functional lives?

How Do Jellyfish Live Without A Brain?

The thing is, while jellyfish don’t have a brain or central nervous system, they do have a very basic set of nerves at the base of their tentacles. These nerves detect touch, temperature, salinity etc. and the jellyfish reflexively respond to these stimuli. Since they don’t have a brain, they live passively, depending entirely on their automatic reflexes.

They sense light and orient themselves, by sensing it via light-sensitive cells along the ridge of their bells. Similarly, using different sense cells, they can also sense chemicals in the water and maintain their balance. Since they don’t have a brain, their movements are also limited, and mainly depend on ocean currents to drift, so they can move about their habitats. Catching prey is also a matter of chance, as they don’t actively hunt down their food; instead, they just wait for it to hit their tentacles.

How Do Jellyfish Live Without a Heart?

First, we should understand exactly what a heart does. It basically pumps blood around the body so that the cells of an animal can get oxygen and nutrients from the blood and expel soluble wastes, like carbon dioxide. However, what is special about the jellyfish anatomy is the thickness of the outer layer, known as the Ectoderm. It is only a few cells thick, so oxygen simply diffuses into the jellyfish body.

Jellyfish also have a very rudimentary digestive system, where simple nutrients merely diffuse into the rest of the body. Both these processes of oxygen and nutrient diffusion don’t need a powerful pump like the heart for them to permeate throughout the organism’s body, due to the simplicity of jellyfish anatomy.

Jellyfish Anatomy


Jellyfish are genuinely fascinating creatures that manage to survive without many of the organs that we’ve come to think of as vital for life. Their bodies are well-suited to survive the conditions of the endless blue, drifting across the vast oceans, displaying their beauty for all to see – at a distance!

Suggested Reading


  1. National Ocean Service
  2. Ocean Portal
  3. NCBI
  4. Marine Education Society of Australasia
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About the Author:

Brendan has a Bachelors of Science degree in Biotechnology from Mumbai University (India). He likes superheroes, and swears loyalty to members of the Justice League. He likes to take part in discussions regarding the human body, and when he is not doing that, he is generally reading superhero trivia.

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