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.

• Biogeographic Regions
• pacific ocean pacific ocean
  • native native

Habitat

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.

• Habitat Regions
• temperate temperate
• saltwater or marine saltwater or marine

• Aquatic Biomes
• benthic benthic
• coastal coastal

• Other Habitat Features
• intertidal or littoral intertidal or littoral

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.

• Other Physical Features
• ectothermic ectothermic
• bilateral symmetry bilateral symmetry
• poisonous poisonous

• Sexual Dimorphism
• sexes alike

Development

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.

• Development - Life Cycle
• metamorphosis metamorphosis

Reproduction

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.

• Mating System
• polygynandrous (promiscuous) polygynandrous (promiscuous)

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.

• Key Reproductive Features
• semelparous semelparous
• seasonal breeding seasonal breeding
• simultaneous hermaphrodite
• sexual sexual
• induced ovulation induced ovulation
• fertilization fertilization
  • internal internal
• oviparous oviparous

After an individual A. californica lays its egg mass, which likely contains provisioning, there is no further investment in the offspring.

• Parental Investment
• pre-hatching/birth
  • provisioning

Lifespan/Longevity

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.

Behavior

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.

• Key Behaviors
• nocturnal nocturnal
• motile motile
• sedentary sedentary
• solitary solitary

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.

• Communication Channels
• visual visual
• tactile tactile
• chemical chemical

• Other Communication Modes
• pheromones pheromones

• Perception Channels
• visual visual
• tactile tactile
• chemical chemical

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.

• Primary Diet
• herbivore herbivore
  • algivore

• Plant Foods
• algae
• macroalgae macroalgae
• phytoplankton phytoplankton

Predation

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 .

• Anti-predator Adaptations
• cryptic cryptic

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 .

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.

• 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.).

Additional Links

Encyclopedia of Life

Contributors

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

References

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 .

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/ .