The Starfish Nervous System

Starfish are unusual among animals for a variety of reasons. One important reason is their radial symmetry — they don't have a "front" or "back" end. Probably for this reason, they haven't concentrated the tissues of their nervous system into any kind of central body — that is, they have no brain.

The central nervous system of a starfish consists of a radial nerve running the length of each ray and a circumoral ("around the mouth") nerve ring that connects the radial nerves. Contrary to what you might expect, the nerve ring doesn't seem equipped to do any kind of processing of information. Instead, all the sensory information must go to the radial nerves, any memories must be stored in the radial nerves, and any decisions about what to do must be made in the radial nerves. And somehow, the five different radial nerves must coordinate those decisions if the starfish is going to get anywhere.

The best theory is that some sensory information is shared between the different rays (it is unclear how much, or how far it goes), and that the rays can inhibit each other — that is, one ray can take charge of the whole starfish for a time. When moving, this seems likely to be the arm in front of the starfish. When trying to locate an odor, the ray sensing the odor most intensely seems to be the one which takes charge, directing movement in its own direction.

Despite this odd arrangement, starfish seem to get along fine. Not only can they locate food and mates, they also can learn to associate particular textures of substrate (gravel vs. sand, for example) or levels of illumination (light or dark) with the presence of food. (Feeding behaviors are the easiest to train and observe, so most learning experiments have concentrated on these.) They also can distinguish different odors and learn to ignore those which are not associated with food. (In these experiments, the researchers gave clay tablets the smell of the starfish's normal prey and found that the starfish quickly learned to ignore them.) Their ability to thrive with such an odd nervous system is a reminder that our way of thinking may not be the only way.

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Last updated May 10, 2000.