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Animal profile

California Sea Hare

Aplysia californica

GastropodaShallow rocky subtidal and intertidal zones with abundant red algae

Field profile

The California sea hare is a large marine gastropod that can exceed thirty centimetres and several kilograms. It grazes on red algae, which give it its reddish-brown colouring, and releases purple ink when disturbed. It has become one of the most important animals in neuroscience.

Scope note

This is an editorial profile, not a diagnostic field-identification guide or a substitute for current conservation assessments. Follow the cited sources on each fact for the evidence trail.

Published field notes

2026-08-30

California Sea Hare

A sea slug with visible neurons

Aplysia californica has around twenty thousand central neurons, some nearly a millimetre across and individually identifiable from animal to animal. That combination made it the organism in which the cellular basis of simple learning was first worked out.

Read with evidence →

California Sea Hare | Daily Animal Facts