Jellyfish are among the most primitive animals alive today. Like their close relatives, sea anemones and corals, jellyfish have no head, no heart and no skeleton. But do jellyfish have a brain?
Do jellyfish have a brain?
Renaissance scholars thought jellyfish were plants. It wasn’t until the 18th century that they were recognized as animals. And, no, jellyfish don’t have brains — because their bodies are organized differently from ours. Most animals we encounter have what’s called bilateral — or two-sided — symmetry. They have a head end and a tail end. In the head end, they have a concentration of nerve cells — where these cells are complex, we call it a brain.
It makes sense, because bilateral animals tend to move head-first. A concentration of nerve cells in the head lets them react quickly to a new environment. But jellyfish don’t have a head. They’re “radially symmetric,” meaning they’re organized around a central axis. They’re just as likely to meet a new environment from any direction. Jellyfish have a loose network called a “nerve net” throughout their bodies. When any part of the nerve net is stimulated, an impulse travels across it in all directions.
This simple nervous system is primitive from an evolutionary standpoint. But it lets jellyfish swim, catch prey, and balance in the water.
Jellyfish are currently classified as cnidarians, and are closely related to sea anemones and corals, as well as to the less familiar hydroids, sea fans, siphonophores and zoanthids. Cnidarians usually have both a polyp (like a sea anemone) stage and a medusa (like a jellyfish) stage in their life histories, but there are many wild variations on this basic theme. All cnidarians have stinging or adhesive cells called cnidae, the most common cnidae being the jellyfish nematocysts. Along with not having a brain, cnidarians in general and jellyfish in particular do not have discrete respiratory, circulatory or excretory organs. They have a saclike digestive cavity with a single opening that serves as both the mouth and anus. Most of the animals we think of as jellyfish fall into the class Scyphozoa. But there are jellies in two other classes, the Hydrozoa (hydromedusae) and the Cubozoa (sea wasps and box jellyfish).
Some cubozoan, or box, jellyfish have specialized eyes tucked underneath the bell. These eyes are complete with lenses, retinas and corneas analogous to our own. Although we don’t know how well they can see, these box jellies exhibit sophisticated behavior for a brainless creature. They can chase down prey, pursue a mate, or move away from a sudden shadow. Scientists think that these box jellies likely have complex visual processing centers in the nerve ring.
Jellyfish are often considered pelagic that is, they drift with ocean currents. But most jellies can swim, and usually spend their time swimming upward in the water column, then sinking slowly back down to capture prey by chance encounter. Some cubozoans are relatively speedy swimmers, maxing out at about six meters per minute. That’s pretty fast for a jellyfish.
The nerve cells or neurons that make up the jellyfish nerve net are relatively primitive. In higher vertebrates, the axon, or tail end, of the neuron is sheathed with a myelin wrap, but in jellyfish it is naked. A naked neuron cannot conduct impulses as rapidly as a sheathed neuron can, so a jellyfish’s responses tend to be slower. Cephalopods like the giant squid compensate for this shortcoming by growing huge axons that can conduct impulses quickly, even though they are not insulated with a wrapping cell. Their axons are the largest nerve cells in the animal kingdom, almost a millimeter in diameter.