California Sea Hare, Aplysia californica

California Sea Hare, Aplysia californica. Mollusk collected off the beach of Punta Chivato, Baja California Sur, May 2023. Size: 15.2 cm (6.0). Collection, photographs and identification courtesy of Colin Campbell, DVM, Punta Chivato.

Phylogeny: The California Sea Hare, Aplysia californica, is a member of the Aplysiidae Family of Sea Hares.  The Aplysiidae Family contains ten genera and the genus Aplysia contains forty-five species. The name Aplysiidae comes from the Greek and Latin words meaning unwashed, possibly referring to the inability of these animals to hide in a shell.  They derive their common name from their rounded body outline and long projecting rhinophores attached to the head, which give them the shape of a sitting rabbit. They are also known as as the California Brown Sea Hare and in Mexico as Liebre de Mar de California.

Morphology: The California Sea Hare has a large humped body with flaps that project dorsally on each side. The interior shell is partially calcified. This species is typically reddish-brown to greenish-brown, but their color shifts to resemble the color of the algae they consume. The head exhibits two sets of tentacles. One set is found on top of the head behind the eyes and the other set is on the front of their face above the mouth. This often makes them blend into their habitat when surrounded by seaweed. They have a toothed radula that allows them to graze on algae attached to surfaces. Sea slugs do have brains. This species in particular has a brain of approximately 20,000 brain cells. They vary from 15 cm (5.9 inches) to 76 cm (30 inches) in length.

Habitat and Distribution:  The California Sea Hare is found on soft substrates. They live in the intertidal zone. The California Sea Hare is found in all Mexican waters of the Pacific Ocean with the exception that they are absent from south of Mazatlán, Sinaloa, along the central and southwest coasts of the mainland.

Diet:  The California Sea Hare and all members of the Aplysiidae Family are herbivorous, feeding on various species of algae including the the Hairy Pottery Seaweed or Staghorn Felt, Ceramium eatoniana, the Red Algae, Laurencia pacifica, and the Sea Comb, Plocamium pacificum.

Predators: The California Sea Hare is distasteful, or toxic, as a result of the algae it consumes, and is avoided by most predators. However, the Green Sea Anemone, Anthropleura xanthogrammica, and the ophistobranch, Navaranax, Navanaz inermis, are common predators of this sea hare. The California Sea Hare has the ability to release a dark purple ink to confuse their predators.

Reproduction:  The California Sea Hare is hermaphroditic. Reproduction is sexual with internal fertilization. When mating, they usually form chains of several individuals, functioning as both male or females. Their eggs are deposited as long, tangled skeins with yellow, yellow-green, or pink eggs. One individual can deposit up to 750,000 eggs per season. After emerging from the egg masses, the trochophore larvae remain free-swimming organisms for one to two months before settling on red algae to begin metamorphosis into their adult form. After that, they reach their reproductive peak at about seven months. The California Sea Hare has a life span of approximately one year. This species is an example of an r-selected species, which has high fecundity, small offspring, and short lifespans.

Ecosystem Interactions: California Sea Hares are known to host endoparasitic microsporidian protozoans. Otherwise, their parasitic, commensal, and symbiotic relationships have not been documented.

Human Interactions:  California Sea Hare eggs are commercially harvested and often consumed as a delicacy in the Philippines. Additionally, the California Sea Hare is used to study neurobiology due to its large neurons. In fact, it has the largest neurons in the animal kingdom. From a conservation perspective they have not been formally evaluated however they are fairly common with a relatively wide distribution and should be consider to be of Least Concern.

Synonyms:  Aplysia (Neaplysia) nettiae, Aplysia ritteri, Tethys (Neaplysia) californica, Tethys (Neaplysia) ritteri, and Tethys californicus