How to live without a heart or a brain - Lessons from a Jellyfish

Tuesday, November 9, 2021

Drifting eerily,
bizarre blobs with ethereal tentacles can be seen on almost every seashore on Earth. These spooky creatures are ancient animals related to corals and sea anemone—the jellyfish. Their ancestors date back almost 500 million years on the evolutionary tree. They are so ancient that jellyfish don’t have organs that more recently evolved animals, like humans, might consider indispensable. To begin with, jellyfish don’t have a heart… or a brain.

At this point,
you might be wondering HOW in the name of Darwin do these animals manage to not only survive but to thrive in sometimes uninhabitable waters? Although jellyfish might not have the classic layout of heart-pumping blood throughout the body or the brain being the seat of control, it does have structures that perform the same functions.

The brain is commonly called the master controller. It controls most of the body’s functions and without it, we’d just be a blob of disorganized cells. That being said, jellyfish are clearly NOT just a blob of cells. Although they primarily drift along with ocean currents, they do respond to the world around them, but how do they do this without a brain? Jellyfish might not have a central nervous system, but they DO have a nervous system.

The neurons
of their nervous system are spread throughout the body like a sort of net, so scientists have decided to call it a nerve net. This nerve net allows jellyfish to sense their environment and reflexively make changes. Through their nerve nets, they can sense the temperature, salinity of the water, oxygen concentration, gravity, ocean currents, and more.

Some jellyfish, like this gorgeous moon jellyfish, take things one step further. They have clusters of neurons located on the edge of their bell called rhopalia. The nerve aggregates within the rhopalia help the jellyfish sense light and keep their balance. Amazingly, some jellyfish can even SEE. Within the rhopalia are structures called eye-spots.In some jellyfish, they are simple light-detecting structures, but in other jellyfish, they are like eyes that can generate 3D images of the world around them, although it may be a little blurry.

This allows the box
jellyfish Tripedalia Cystophora, for example, to locate its favorite habitat– mangroves! In some jellyfish, all the nerve aggregates in the rhopalia are connected through a nerve ring. This nerve ring is the closest a jellyfish can have to a central nervous system. This jellyfish's nervous system “level up” allows jellyfish to exhibit behaviors like courtship that we’d normally only expect from animals with a brain! Aside from a brain, jellyfish also lacks a heart.

One of the heart’s
the main function is to circulate and distribute OXYGEN that an animal breathes in, along with circulating nutrients and other chemicals. The blood contains hemoglobin, which carries this SUPER IMPORTANT oxygen, while the heart pumps this blood to all the tissues in the body. The jellyfish, though, doesn’t need the heart to meet its oxygen needs. The jellyfish’s outer layer of cells, called the ECTODERM, is so thin that oxygen dissolved in the water can simply diffuse right through it.

What’s more,
the jellyfish can STORE this absorbed oxygen. The jellyfish body is mainly composed of MESOGLEA, a jelly-like fluid sandwiched between the outer ECTODERM and the inner digestive cavity, the ENDODERM. The mesoglea is how the jellyfish manages to maintain its bell structure in water. The jellyfish absorbs oxygen through its ectoderm and can store it in the mesoglea.

This allows jellyfish
to thrive in waters that have extremely low oxygen concentrations! Similarly, the jellyfish doesn’t have a complete digestive system, lacking intestines, a liver, a pancreas, and other critical organs. The food they eat: algae, fish eggs, larvae, small fish, and sometimes even other jellyfish, are ingested through the mouth and end up in the jellyfish stomach, called the gastrovascular cavity.

Inside this cavity,
certain cells release digestive enzymes and the food is broken down. The layer of cells, the ENDODERM, is thin, like the ectoderm, allowing nutrients to be directly absorbed into the surrounding cells. Once the nutrients are absorbed, the undigested water matter is released from the mouth! All of this might make the jellyfish seem like little more than a sack of water made of cells, but they’re actually surprisingly complex animals. The rudimentary nerve net and nerve ring seem simple when compared to our large brain, but it's a mystery how jellyfish manage behaviors like courtship without a “master controller”.

The nerve nets of jellyfish
and related species are basically nature’s “first draft” of a nervous system. Studying it reveals to us the evolutionary path towards the more complex nervous systems of vertebrates. The next time you encounter a jellyfish bobbing up and down in the water, or washed ashore, take a moment to marvel at how something so simple can be so full of mysteries.

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