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29 Interesting Facts About the California Sea Hare

Leena Palande
Despite having a very simple nervous system, 'California sea hare' exhibits amazing behavioral patterns. Here are number of fascinating facts about this little-known marine animal.

Did You Know?

As sea hares form a line to mate, each becomes a male for the one in front and female for the one behind. Thus, each sea hare can play the role of both genders. This mating chain can yield millions of eggs.
There exist numerous species of sea hare, but California sea hare is the most well-known species amongst all. The scientific name of this sea hare is Aplysia californica. A. californica is a marine opisthobranch gastropod. It is also known as California sea slug, as it is a form of sea slug, but it is usually larger than sea slugs.
Sea hares are often found in regions with thick vegetation. They camouflage themselves and expel different types of inks for defense. In aquarium, they are fed kelp and nori. As they have very large neurons, they are used for neurobiological investigations. Thus, they are important laboratory animals.

Scientific Classification

Kingdom: Animalia
Phylum: Mollusca
Class: Gastropoda
Superfamily: Aplysioidea
Family: Aplysiidae
Genus: Aplysia
Species : A. californica


✦ They are found in sheltered, rocky habitats, mainly in the photic zone, as they eat algae. They are usually found in coastal environment.
✦ Adults are often found in shallow water within the middle and lower intertidal zones, while the younger ones inhabit deeper waters (18-20 m. deep).
✦ They are found on the Pacific coast from Humboldt Bay in Northern California to the Gulf of California, Baja California, Mexico. They are seen off the beaches of Florida too. In fact, they are found throughout the globe.


✦ They usually have a lifespan of one year. However, sea hares older than one year have been found. Cooler temperature that delays breeding is responsible for an extended lifespan. Dead sea hares are often washed ashore in large numbers. Hundreds and thousands of them die at a time.

Characteristic Features

✦ Because of the pair of tentacles called 'rhinophores' on top of its head, a California sea slug looks like a sitting rabbit or hare. Therefore, they are known as sea hares. The rhinophores help detect chemicals in the surrounding areas. This way, they perceive their food, companions, and predators too.
✦ They have another pair of short tentacles on either side of mouth. The anterior pair of tentacles help in motility. The posterior pair of tentacles is used for smelling.
✦ The sea hare has a small, thin, internal shell that protects the gill and the heart.
✦ The body is mottled, soft, and unsegmented. The mantle, that covers the gills, has two external wing-like folds called parapodia. These structures extend almost the entire length of its body. These extensions meet near the center.
✦ The eyesight is poor. The small black eyes help to detect light from the dark only.
✦ They have a sucker-like mouth. There is a large muscular foot on the underside of the body.
✦ This slug-like animal is brown or reddish. Sometimes, it looks yellowish or pinkish. It acquires the color of the algae that it eats.
✦ A seventy-five cm long (when fully extended) sea hare is the longest ever seen sea hare. Normally, an adult is 15-38 cm long.
✦ Those which are about 31 cm long may weigh around 100 g. Smaller ones may weigh about 20-50 g. Depending upon the species, some adults can weigh up to 7 kg.
✦ When threatened or disturbed, the sea hare releases a thick cloud of colorful ink, somewhat like an octopus. It secretes two different kinds of inks from two different spots. Reddish-purple slime released from the purple ink gland, is thrown at the predator. This confuses the predator and the sea hare gets time to escape.
A pigment present in the red algae that they eat imparts purple color to the ink. The milky white ink is released from the opaline gland. It contains the aversive chemical opaline, because of which they taste awful. Therefore, predators do not get attracted towards them. Both inks act as an irritant against potential predators.
✦ In the vicinity of a seaweed, it is difficult to recognize a sea hare unless it is moving. This is due to the color camouflage that occurs after eating the seaweed.


✦ Like all Aplysia species, the California sea hares are herbivorous. They eat brown, red algae and the soft frilly stuff found in rocky shelters. It mainly incorporates red algae like Laurencia pacifica, Plocamium pacificum, and Ceramium eatonianum, in its diet. Because of these algae, it looks reddish or pinkish brown.


✦ Each sea hare acts like both male and female, as it has both sexual organs.
✦ Three or more sea hares come together and form a line or circle for mating. Each one breeds with two others at the same time. A California sea hare has been seen forming mating chains with up to 20 animals. Coupling may last for hours or even for days.
✦ They usually gather together into eelgrass beds and mate early in the morning during summers.
✦ Normally, mating triggers egg laying, but sea hares kept in isolation for up to 3-4 months also have laid eggs. However, they do not self-fertilize eggs.
✦ During a single period, it may lay up to eighty million eggs. However, most of the eggs are eaten by other sea animals.
✦ The eggs are laid in the form of strings or masses. The mass can be as big as a grapefruit. The eggs are yellow-green and are often attached to eelgrass. They turn brown after 8 to 9 days and hatch in 10-12 days.
✦ The first stage in the developmental process of the larvae involves day 1-37 (after hatching) and is called planktonic stage. The second stage is known as metamorphic stage which involves day 37-45, and the period between the day 45-80 is known as juvenile stage.
Larvae reach reproductive maturity within 85 days after hatching (133 days after deposition of the fertilized eggs). The nervous system develops within 140 days.

Significant Uses

✦ California sea hare does not have a brain. It has a simple nervous system which consists of just 20,000, large, easily identifiable nerve cells (neurons). Though it has a simple nervous system, A. californica exhibits intelligence through a variety of actions. It has become a valuable laboratory animal, as it has helped neuroscientists for over 50 years.
Researchers have used it for studying the phenomenon of learning and memory. It has played an important role in 'Nobel' prize-winning research in neuroscience. The research was carried out by Nobel Laureate Eric R. Kandel.
✦ Other studies which involve use of Aplysia have helped to discover drugs that can be used to reverse memory loss in patients suffering from degenerative mental illnesses. Scientists may introduce these drugs after proper clinical trials.
✦ Scientists have discovered that the ink released by a sea hare contains an anti-bacterial protein 'escapin', which is structurally similar to toxins found in some poisonous snakes.
As this protein prevents formation of bacteria, scientists hope that it can be used to prevent biofilm formation on marine products, ship hulls, and fishermen's nets or traps. This can prove to be a perfect environment-friendly alternative to currently used toxic metals like copper.


✦ The toxins in the body of a sea hare keep the predators away. Regular consumption of algae helps generate those toxins.
✦ The giant green anemone 'Anthopleura xanthogrammica' is one of the main predators of sea hares. When it catches a sea hare, it consumes only 70-85% of it, and regurgitates the remains. While eating, as soon as the sea hare's digestive gland and its associated toxins get exposed, the anemone regurgitates all the remaining undigested parts.
✦ The coral fish-humphead wrasse, the giant 11-armed starfish-Coscinasterias calamaria, lobsters, a bailer shell 'Melo amphora', and the opisthobranch Navanax inermis which usually eat juveniles are some other important predators of sea hares.
As described already, sea hares are important to both environment and scientific study. Protecting wetlands against development can help protect the homes of many animals. Thus everything created by mother nature does play an important role in ecology and improving the quality of life in several ways.