Do Jellyfish Have Brains?

Sea jellies may not have brains, bones, lungs or a heart, but we’ll tell you what they do have

Do jellyfish have brains? The short answer is no, jellyfish do not have brains. But this certainly does not mean they are simple creatures. In fact, not having a centralized nervous system is just one of sea jellies’ many incredible tools for defense against the brutal elements of the deep blue sea. Without a brain, a sea jelly is able to survive traumatic injuries to its bell and still function. While jellyfish may not have brains, a heart, lungs, bones or blood, their structure is incredibly unique—probably why so many of us are fascinated by them!

Instead of a brain, a sea jelly has a set of nerves commonly referred to as a nerve net or nerve ring. This nerve net helps them detect shifts in their environment, including temperature, gravity, water salinity, oxygen concentration, vibrations and currents. These nerves are responsible for the jellyfish’s automatic responses. 

Jellyfish also have clusters of nerve endings (almost like neurons) called rhopalia along their bells. Rhopalia allow jellyfish to sense light and maintain balance, ultimately helping jellies determine which way is right-side up. 

In addition, a sea jelly’s basic anatomy consists of six parts:

  1. The epidermis is the top layer of the jellyfish bell which absorbs oxygen, allowing it to disperse throughout the body. This allows jellyfish to survive without lungs, a heart or blood. 
  2. The mesoglea is the actual “jelly” part of the jellyfish, a thick gelatinous material made of mostly water with collagen and proteins. It’s also responsible for storing oxygen for extended periods of time. 
  3. The gastrovascular cavity is the inner part of the jellyfish that functions as the entire digestion system. There is one orifice that (to put it politely) serves both ingestion and elimination needs for the jellyfish. This multipurpose area is the access point to the gastrovascular cavity.
  4. The gastrodermis is the layer on the underside of the jellyfish bell and aids in extracellular digestion and gas exchange. 
  5. The tentacles of a jellyfish come in all shapes and sizes, depending on the type of jellyfish. Tentacles are what hold a sea jelly’s nematocysts, the tiny cells responsible for their stings. A lion’s mane jellyfish holds the record for the longest tentacle, with some measuring in at more than 100 feet long—that’s about as long as a blue whale!
Jellyfish diagram
Cross-section diagram of the hydrozoan species Olindias formosa’s jellyfish-like medusa form.

Some species, such as the box jellyfish and moon jellies, even have “eyes” or eye-like structures. While moon jelly “eyes” are simple light-detecting structures called ocelli, box jelly “eyes” include a cornea, pupil, lens and retina. Numbers of eyes vary by creature¾while the moon jelly has only two “eyes,” box jellies have up to 24 “eyes.”

Did you know jellyfish are among the oldest species of animals in the world? Fossils have been found that indicate jellyfish have been around for more than 500 million years. Even without bones, brains, lungs or a heart, sea jellies have certainly made their mark in our beautiful ocean and our hearts. 

With an estimated 2,000 species out there, jellyfish are just one of the spectacular creatures inhabiting our beautiful ocean. It’s up to all of us to help protect them so they inspire wonder and feed sea turtles and other ocean animals for another 500 million years. Please make a donation to Ocean Conservancy to celebrate these creatures—give today and make a difference for the future of our ocean!

Our work is focused on solving some of the greatest threats facing our ocean today. We bring people, science and policy together to champion innovative solutions and fight for a sustainable ocean.
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