[3] The maximum length recorded for the California sea hare is 75 cm (30 in) when crawling, thus fully extended, although most adult specimens are half this size or smaller.

A California sea hare is typically reddish-brown to greenish-brown, but the color varies based on the algae it ingests.

Its diet consists primarily of red algae such as Laurencia pacifica, Plocamium pacificum, and Ceramium eatonianum, which give the animal its typically reddish or pinkish coloration.

[13] When it is considerably disturbed, the sea hare is capable of releasing two different kinds of ink from different locations within its mantle cavity, much in the way an octopus does.

[14] Inking provides protection from spiny lobsters, a major predator of sea hares, by means of three mechanisms:[14] chemical deterrence, sensory disruption, and phagomimicry.

Ink creates a dark, diffuse cloud in the water that disrupts the sensory perception of the predator by acting as a screen or decoy.

The opaline, which affects the senses dealing with feeding, causes the predator to instinctively attack the cloud of chemicals as if it were indeed food.

[14][15] A. californica has become a valuable laboratory animal, used in studies of the neurobiology of learning and memory, and is especially associated with the work of Nobel laureate Eric Kandel.

[16] Its ubiquity in synaptic plasticity studies can be attributed to its simple nervous system, consisting of just 20,000 large, easily identified neurons with cell bodies up to 1 mm in size.

[17] Despite its seemingly simple nervous system, though, A. californica is capable of a variety of nonassociative and associative learning tasks, including sensitization, habituation, and classical and operant conditioning.