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Aquarium of the Pacific - Online Learning Center - Species Print Sheet

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Aquatic SpeciesCalifornia Brown Sea Hare

Aplysia californica InvertebratesMollusks, marine

california brown sea hare | Jay Tayag
california brown sea hare - popup
Jay Tayag

Species In-Depth | Print full entry

At the Aquarium

California brown sea hares are on occasionally on exhibit at the Aquarium. In keeping with their vegetarian diet in the wild, our sea hares are fed kelp and nori. They also graze on algae that grows naturally in their Aquarium habitat.

Geographic Distribution

Most of the California coast and some areas of the Gulf of California


Sea hares are found in sheltered coastal waters, usually where vegetation is thick. The depth of water where they can be found depends on whether they are adults or juveniles. Adults prefer shallow middle and lower tidal zones. Juveniles inhabit deeper waters to depths up to 18.3 m (60 ft).

Physical Characteristics

California brown sea hares have two pairs of tentacles on the top of the head, one pair near the mouth and the other behind the eyes. The anterior are an aid in locomotion, and the posterior are used for smelling. The mantle, which covers the gills, has external wing-like extensions that extend almost the entire length of the animal’s body, up over each side of the back meeting at or close to the center. These extensions are called parapodia.

The color of these sea hares is dependent on the color of algae that they eat. Coloration ranges from reddish-brown to greenish-brown with some mottling.


Although California brown sea hares can be as long as 40 cm (16 in) and weigh up to 2.3 kg (5 lb), most are only half this length and weight.


These animals are herbivores that change their food preferences as they grow. As larvae, they settle out on red algae in deeper water to eat, gradually moving into shallow water as they become adults. In water closer to shore they forage on eelgrass and tougher brown and green algae. They use a pair of jaws and a rasp-like radula to graze.

Brown sea hares have a very complicated digestive system that includes an alimentary canal, a crop where horny plates chew the food, and a multi-chambered stomach where digestion takes place.


Sea hares are hermaphrodites with a full set of both male and female reproductive organs; however, they do not self-fertilize. Any individual sea hare can act as either a male, a female, or simultaneously as both. At breeding time they come into eelgrass beds to mate, often in large numbers that pile up in lines or circles to make a chain of mating animals. The sea hare at the front of the line is female only. The ones that follow “her” are male to the animal in front of it and female to the one behind. The sea hare at the end of the line bringing up the rear is male only.

Up to 80 million jelly-encased, yellow eggs are laid in long strings that can form a mass as large in diameter as a grapefruit. The mass is usually attached to eelgrass where the eggs turn brown in 8-9 days and hatch in 10-12 days.

Free–swimming larvae are planktonic for about 30 days and then settle on algae, (usually red algae), where they gorge and metamorphose, doubling their weight every 10 days for the next three months.


Although predominately quiet, brown sea hares exhibit a behavior called head waving in which the head is moved side to side in a series of movements separated by pauses. During head waving the anterior two-thirds of the animal’s body is free to move, but most movement takes place immediately behind the head portion of the animal’s body.


This sea hare species releases a cloud of irritating, slimy, reddish-purple ink from a gland in the mantle cavity. The color comes from a concentrated secretion of compounds derived from their red algae diet. Some researchers believe the ink is a screen or decoy to ward off predators while others believe it is merely used to release waste by-products of the animal’s diet.

California brown sea hares can adapt to a loss of 30% of their body weight due to drying out.


Sea hares have a short life. They usually die after laying eggs but their life can be extended in water that is 14-25o C (57-77o F). Cooler water delays spawning and extends longevity somewhat.


California brown sea hares are important laboratory animals, valuable in neurobiology. They have very large neurons, the largest in the animal kingdom, and very few of them, making it possible to identify individual nerve cells that are responsible for specific behaviors. They have been and are being used extensively in studying memory, behavior, and learning. To eliminate the variables created when wild-caught animals are used in research, a national federally funded laboratory has been developed that has shipped over 25,000 sea hares bred and raised in a controlled environment meeting National Institutes of Health standards.

These animals are not on the IUCN Red List.

Special Notes

The California black sea hare, Aplysia vaccaria, is the largest gastropod in the world. It can grow to be as long as 75 cm (29 in) in length with a weight of up to 13.6 kg (30 lb). Unlike the California brown sea hare, it does not eject ink.

Most sea hare species expel a cloud of purple ink, a behavior similar to that of their more advanced inking relatives, octopus and squid. Some scientists believe this is an evolutionary link among these mollusks, none of which have an external shell.