Aplysia californica

Geographic Range

The California sea hare, Aplysia californica, is found along the California coast from Humboldt Bay in northern California to the Gulf of California, Baja California, Mexico. (Angeloni, et al., 2003)


Aplysia californica is in greatest abundance within sheltered rocky shore coastal environments. Distribution by depth depends on the specimen’s age. Juveniles occupy deeper waters (up to 18 meters) whereas adults primarily prefer shallow water within the middle and lower intertidal zones. ("California Brown Sea Hare", 2014)

  • Range depth
    18 (high) m
    59.06 (high) ft

Physical Description

Aplysia californica is a large mollusk, capable of growing as long as 40 cm and weighing as much as 2.3 kg; however, most average 20 cm long and 1 kg in weight. Body coloration is cryptic, and generally ranges from reddish-brown to greenish-brown. Coloration of A. californica depends largely on the particular species of algae fed upon by an individual. The overall body texture is delicately fleshy. The head region bears four tentacles. Two are on the top of the head behind the eyes as well as two in front of the face slightly above the mouth. The species' nickname (sea hare) is due to these tentacles which resemble rabbit ears. On the dorsum of the bilaterally symmetrical body are two wing-like folds called parapodia which cover the gills but allow water to pass through. As a type of large sea slug, A. californica lacks a hard calcified outer shell. However, it does possess a flat internal shell which encases the internal organs including the heart. On the underside of the body is a large muscular foot used for locomotion. ("California Brown Sea Hare", 2014; "The Rosenstiel School Of Marine and Atmospheric Science", 2009)

  • Sexual Dimorphism
  • sexes alike
  • Range mass
    2.3 (high) kg
    5.07 (high) lb
  • Average mass
    1 kg
    2.20 lb
  • Range length
    40 (high) cm
    15.75 (high) in
  • Average length
    20 cm
    7.87 in


Aplysia californica rapidly develops over the course of a year. After larvae hatch from egg masses, each free-swimming microorganism is known as a trochophore larva. These larvae swim with hair-like cilia. The larvae drift for 34 days or more until they settle on red algae and undergo metamorphosis into the adult form. After metamorphosis, the feeding juveniles rapidly grow in weight for three months. After 120 days, juveniles may become sexually mature. (Angeloni, et al., 2003; "California Brown Sea Hare", 2014; Pawlik, 1989)


Aplysia californica is hermaphroditic, meaning each animal has both female and male sex organs. Sea hares mate promiscuously in multiple events with conspecifics, when individuals locate each other through chemical cues, congregate, and form a “daisy chain”. During a daisy chain, several sea hares line up and each animal may be either receiving or delivering sperm (or both) based on their current sex and position within the chain. Mating season occurs during spring and summer. ("The Rosenstiel School Of Marine and Atmospheric Science", 2009)

The eggs are fertilized within a sperm storage cavity also known as the seminal receptacle before they are laid in bright pink, long gelatinous strings. These noodle-like egg masses may hold millions of embryos. Eggs usually hatch after approximately twelve days, releasing planktonic larvae. Mating occurs during spring and summer. Egg laying is initiated by copulation. ("The Rosenstiel School Of Marine and Atmospheric Science", 2009)

  • Breeding interval
    Aplysia californica mates once a year.
  • Breeding season
    Mating takes place during spring and summer.
  • Range number of offspring
    80 million (high)
  • Range gestation period
    10 to 12 days
  • Average age at sexual or reproductive maturity (female)
    1 years
  • Average age at sexual or reproductive maturity (male)
    1 years

After an individual A. californica lays its egg mass, which likely contains provisioning, there is no further investment in the offspring. ("The Rosenstiel School Of Marine and Atmospheric Science", 2009)

  • Parental Investment
  • pre-hatching/birth
    • provisioning


Sea hares’ lifespans are usually fairly short as they typically live only a year. Most sea hares perish after reproduction. However, because water temperatures delay reproduction, cooler waters can somewhat lengthen lifespan. ("California Brown Sea Hare", 2014)

  • Average lifespan
    Status: wild
    1 years
  • Average lifespan
    Status: captivity
    1 years


One of the more notable behavioral aspects noticed in sea hares is their ability to release a deep purple ink from within the parapodia when threatened. The deep purple color is derived from pigments within the red algae in their diet. The ink has been shown to be distasteful and acts as a defense mechanism to ward off predators. In order to avoid being attacked, sea slugs are mainly active at night. Sea hares give a cautionary avoidance response when in contact with certain sea stars and predaceous opisthobranchs. This response consists of withdrawing the head rapidly and engaging in an escape response. (Angeloni, et al., 2003; "Opisthobranch parasites - the copepod Ismaila", 2010; Nusnbaum and Derby, 2010)

  • Average territory size
    unknown cm^2

Home Range

Aplysia californica has no specific home range.

Communication and Perception

Sea hares congregate to mate, and thus they perceive conspecifics via tactile and chemical cues. These slugs have photoreceptors that detect variances in the intensity of white light. However, they are less able to identify red wavelengths. Overall, sea slugs respond more to chemical changes in seawater. A sensing structure located near the gills called the osphradium detects dissolved chemicals and is the major organ of olfaction. (Angeloni, et al., 2003)

Food Habits

Aplysia californica is a herbivore and primarily feeds on several kinds of red algae (Laurencia pacifica, Plocamium pacificum and Ceramium spp.) as well as sea lettuce (Ulva spp.) and eelgrass (Zostera marina) from within tidepools in the middle intertidal zone. Aplysia uses its toothed radula to graze attached algae. After the material has been temporarily stored in the esophagus, it is then passed through a muscular stomach lined with pyramidal teeth which further grind up the food matter. The food is then mixed with various digestive fluids and wastes are excreted out the anus located near the parapodia. The larvae obtain nourishment by consuming phytoplankton. (Angeloni, et al., 2003; Winkler and Dawson, 1963)


Due to their lack of an outer calcareous shell, sea hares utilize cryptic coloration, nocturnal behaviors, and ink release as defense mechanisms. Sea hares are also distasteful to many predators due to chemicals known as secondary metabolites which they create from chemicals obtained from their algal diet. This chemical adaptation also makes sea hare tissue somewhat toxic to certain animals and fairly effective against most predators. It has been discovered that the mucus covering the mantle also acts a deterrent against predatory crustaceans and some fish. Two of the main predators are the giant green sea anemone, Anthropleura xanthogrammica, and the ophistobranch slug, Navanax inermis. (Angeloni, et al., 2003; "Opisthobranch parasites - the copepod Ismaila", 2010)

  • Anti-predator Adaptations
  • cryptic
  • Known Predators

Ecosystem Roles

Aplysia californica is a primary consumer due to its herbivorous diet of algae. As such, it is a key organism in the intertidal ecosystem, affecting the density and abundance of its algal prey. There is little specific information on parasitic symbiosis in A. californica, other than one report of a microsporidian protozoan infecting the nuclei of its neurons. Other shell-less opisthobranchs are known to serve as hosts for certain copepods in the family Splanchnotrophidae. ("Opisthobranch parasites - the copepod Ismaila", 2010; Krauhs, et al., 1979)

Commensal/Parasitic Species
  • protozoan, Microsporidium aplysiae

Economic Importance for Humans: Positive

Aplysia californica is a highly valuable laboratory animal especially in the field of neurobiology and more specifically in studying memory and learning behaviors. Sea hares possess the largest neurons in the animal kingdom, as well as very few of them, so it is feasible to identify the specific nerve cells responsible for specific mechanisms. ("California Brown Sea Hare", 2014)

  • Positive Impacts
  • research and education

Economic Importance for Humans: Negative

There are no known adverse effects of Aplysia californica on humans.

Conservation Status

Although there is no current management or conservation plan for this species, individuals are protected if they occur within the boundaries of marine protected areas.

Other Comments

The purple ink released by these animals smells pleasantly of cedar (P. Detwiler, pers. comm.).


Samantha Dice (author), San Diego Mesa College, Paul Detwiler (editor), San Diego Mesa College, Angela Miner (editor), Animal Diversity Web Staff.


Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

World Map


Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.


uses smells or other chemicals to communicate


the nearshore aquatic habitats near a coast, or shoreline.


having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.


animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature


union of egg and spermatozoan


An animal that eats mainly plants or parts of plants.

induced ovulation

ovulation is stimulated by the act of copulation (does not occur spontaneously)

internal fertilization

fertilization takes place within the female's body

intertidal or littoral

the area of shoreline influenced mainly by the tides, between the highest and lowest reaches of the tide. An aquatic habitat.


seaweed. Algae that are large and photosynthetic.


A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.


having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.


active during the night


reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.


chemicals released into air or water that are detected by and responded to by other animals of the same species


photosynthetic or plant constituent of plankton; mainly unicellular algae. (Compare to zooplankton.)


an animal which has a substance capable of killing, injuring, or impairing other animals through its chemical action (for example, the skin of poison dart frogs).


the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

seasonal breeding

breeding is confined to a particular season


remains in the same area


offspring are all produced in a single group (litter, clutch, etc.), after which the parent usually dies. Semelparous organisms often only live through a single season/year (or other periodic change in conditions) but may live for many seasons. In both cases reproduction occurs as a single investment of energy in offspring, with no future chance for investment in reproduction.


reproduction that includes combining the genetic contribution of two individuals, a male and a female


lives alone


uses touch to communicate


that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).


uses sight to communicate


Aquarium of the Pacific. 2014. "California Brown Sea Hare" (On-line). Aquarium of the Pacific. Accessed February 17, 2014 at http://www.aquariumofpacific.org/onlinelearningcenter/species/california_brown_sea_hare.

Australian Museum. 2010. "Opisthobranch parasites - the copepod Ismaila" (On-line). The Sea Slug Forum. Accessed May 09, 2014 at http://www.seaslugforum.net/find/ismaila.

Universty of Miami. 2009. "The Rosenstiel School Of Marine and Atmospheric Science" (On-line). The National Resource for Aplysia. Accessed April 01, 2014 at https://cc1.rsmas.miami.edu/slugs/.

Angeloni, L., J. Bradbury, R. Burton. 2003. Multiple mating, paternity, and body size in a simultaneous hermaphrodite, Aplysia californica. Behavior Ecology, 14/4: 554-560.

Audesirk, T. 1979. A field study of growth and reproduction in Aplysia californica. The Biological Bulletin, 157/3: 407-421. Accessed February 18, 2014 at http://www.biolbull.org/content/157/3/407.full.pdf.

Krauhs, J., J. Long, P. Baur. 1979. Spores of a New Microsporidan Species Parasitizing Molluscan Neurons. The Journal of Protozoology, 26/1: 43-46.

Macginitie, G. 1934. "The egg laying activities of the sea hare Tethys californicus" (On-line). Accessed February 16, 2014 at http://www.biolbull.org/content/67/2/300.full.pdf.

Morris, R., D. Abbott, H. Eugene, R. Beeman, G. Williams. 1980. Intertidal Invertebrates of California. Stanford, CA: Stanford University Press.

Nusnbaum, M., C. Derby. 2010. Effects of Sea Hare Ink Secretion and Its Escapin-Generated Components on a Variety of Predatory Fishes. The Biological Bulletin, 218: 282-292. Accessed February 17, 2014 at http://www.biolbull.org/content/218/3/282.full.pdf+html.

Pawlik, J. 1989. Larvae of the sea hare Aplysia californica settle and metamorphose on an assortment of macroalgal species. Marine Ecology Progress Series, 51: 195-199. Accessed February 17, 2014 at http://people.uncw.edu/pawlikj/1989MEPSPaw.pdf.

Winkler, L., E. Dawson. 1963. Observations and Experiments on the Food Habits of California Sea Hares of the Genus Aplysia. Pacific Science, 17: 102-105. Accessed February 17, 2014 at http://scholarspace.manoa.hawaii.edu/bitstream/handle/10125/4930/vol17n1-102-105.pdf?sequence=1.