The Secrets Hiding in the Simplest Animal Brains

The Secrets Hiding in the Simplest Animal Brains

Jessica Leigh Hester

One researcher thinks mollusks called chitons are more complex than they seem.

Sumner-Rooney roots for underdogs. She’s fascinated, for instance, by Astyanax mexicanus, a cave-dwelling tetra fish that evolved without eyes, and cave salamanders, whose ghostly pallor and gangly limbs are adapted to a lightless life. Her work focuses on brain structure in poorly understood animals such as chitons or tusk shells, which use tentacle-like organs called captacula to assist in gathering food.

Animals with radial symmetry, such as jellyfish and sea urchins, don’t have a front or a rear, so there’s no place to put an “anterior [front] condensation” of neurons. But somehow they still manage to coordinate muscle movement and process sensory information coming in from across their bodies. “They pose quite a problem for very classical definitions, but they’re clearly very capable of complex tasks that we would usually attribute to a brain,” Sumner-Rooney says. Such a brain, she adds, “doesn’t quite fit with the image we have in our heads.”

But she isn’t studying consciousness, to whatever extent chitons are capable of “thinking” about the currents churning around them. Instead, she’s interested in what all those neurons are doing, what they’re capable of, and why. For example, some species of chitons demonstrate homing habits; they can roam to find food, and then find their way back. Does the nerve ring help make this possible? Maybe the neural ring has some role in supporting the hundreds and hundreds of eyes, or ocelli, that stipple many species’ shells. There’s evidence that these can perceive shadows passing overhead, and that chitons clamp down on their rocks when they do. Is it an integration of information or a response driven by reflexes—similar to a human jerking a hand away from a flame?