Learning without a brain: classical conditioning in the ophiuroid Ophiocoma echinata

Abstract

Brittle stars (Class Ophiuroidea), like all echinoderms, lack centralized nervous systems, having instead five radially arranged nerve cords joined by a central nerve ring. Although operant and classical conditioning have been demonstrated in a limited number of studies in sea stars (Class Asteroidea), members of the other echinoderm classes remain relatively untested. We examined whether individuals of the ophiuroid species Ophiocoma echinata were able to learn an association between a period of darkness and the presentation of a food reward. Ophiuroids in an experimental group were trained by presenting food during a 30-minute period of darkness, while control group animals were fed under regular daytime room lights many hours after a period of darkness of the same duration. After the training period, the experimental group demonstrated they had learned to associate the two cues by regularly emerging during the dark period even when no food was presented. The untrained control animals, as well as pre-training experimental animals, did not emerge during the dark periods when no food was presented. Once trained, experimental animals emerged significantly more times than control animals during dark periods without food (trained emergences = 109; untrained emergences = 22; χ2 = 64.65, p = 0.0007). This study shows that classical conditioning is possible in a class of animals that lacks a centralized nervous system.