Hermissenda crassicornis

Geographic Range

Hermissenda crassicornis are commonly found in areas of the West Coast of North America from Kodiak Island, Alaska to Baja California, Mexico. They are benthic organisms that can be found crawling on rocks, seaweed, and various other substrates on the ocean floor.

(Meinkoth 1981)


H. crassicornis can be found around tidepools, and on rocks, pier pilings, and mudflats. It can also be found from low-tide line water to water 110 feet deep.

(Pearse,et al 1989)

Physical Description

The Hermissenda crassicornis is one of the most beautiful of all the invertebrates. Its length is 3 and 1/4 inches long, on average, and its width, 3/8 inches. The distinctive colors of the H. crassicornis are bluish-white with an orange line down the middle of its back. The margins have pale electric blue lines. These colors are mainly carotenoids and carotenoproteins. There are two pair of tentacles (rhinophores) located on the top of the head. The first pair has blue lines, and the second pair is bluish with raised rings. In the middle, there are numerous finger-like projections, called cerata, in two clusters on each side of the back. These projections are brilliantly colored in bright orange with a white tip on each ceratum. The coloration of the H. crassicornis serves as a great identification factor for the species.

Unlike most other Mollusks, the H. crassicornis lack a shell, mantle cavity, and original gill.

(Meinkoth 1981)


Hermissenda crassicornis is a hermaphrodite, possessing both male and female organs. Self-fertilization is a very rare occurance among the H. crassicornis. Most mate, and then lay their eggs near their food source. The H. crassicornis can lay a wide range of eggs on a strand, containing from a few to a million. The maturation of the egg is highly influenced by temperature and can take as little as 5 days or as long as 50 days. The development of the egg is most favorable in warm temperatures. The egg develops into a larval stage called a veliger that floats around on the ocean floor until environmental conditions cause the veliger to settle and develop into adult form.

Recent experiments have determined that the H. crassicornis produce long-term planktotrophic velliger larva. They are unable to metamorphosize for at least three weeks. The best time for inducing metamorphosis is at approximately day 41 to 50 in an egg mass. This varies, however, within the different egg masses. Studies show that this is most likely due to the gradual aquisition of metamorphic competence.

(Avila 1998, Nordin 1996)


The Hermissenda crassicornis lacks a shell to protect itself and thus takes on different types of defense. One means of defense is warning coloration. The orange color of H. crassicornis indicates to predators that it is distasteful and is poisonous. Another means of defense is contained in the cerata found on the back of the H. crassicornis. The cerata contain stinging cells to keep predators away. These stinging cells developed from the nematocysts of sea anemones and hydroids that the H. crassicornis feed on. These nematocysts are undigestible in H. crassicornis, leading them to be stored in the cerata. During digestion, the nematocysts of prey remain undischarged and travel through interconnecting tubules from the diverticula into the cerata.

Not having a shell allows the H. crassicornis to have increased mobility. The increased mobility can also act as a defense, allowing them the ability have quicker escape swimming.

(Brusca and Brusca 1990, Pearse, et al 1989)

Food Habits

Hermissenda crassicornis feed on a wide variety of animals, such as hydroids, sponges, corals, and many other types of invertebrates. In some instances, they can become cannibalistic, eating other nudibranchs. The H. crassicornis will fight each other, struggling to bite chunks of tissue from each other.

Studies have been done that indicated that H. crassicornis locates its food by chemotaxis. This study also looked at the most profitable food items for H. crassicornis The feeding experiments demonstrated that a diet of Turbularia resulted in a higher growth rate in the H. crassicornis than any of the other organisms used in the experiments, such as Ciona viscera or mussels. These last two organisms are most likely used for maintenance diets.

(Tyndale, et al 1994)

Economic Importance for Humans: Positive

Hermissenda crassicornis is a great subject for study on the cellular level. H. crassicornis has been used as a biomedical research model for many physiological properties, such as learning and memory studies. It has also been used as a model for lead toxicity studies. In these studies, lead significantly reduced the ability for H. crassicornis to undergo associative conditioning and effected the ability for H. crassicornis to aquire associative learning. The behavioral studies can be related to identifiable cells in the ganglia of the head.

(Tyndale, et al 1994, Kuzirian, et al 1996)


Gena McKinley (author), Southwestern University, Stephanie Fabritius (editor), Southwestern University.



living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

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.


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

native range

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


Avila, C. 1998. Competence and metamorphosis in the long-term planktotrophic larvae of the nudibranch mollusc Hermissenda crassicornis. Journal of Experimental Marine Biology and Ecology, 231: 81-117.

Brusca, .., .. Brusca. 1990. Invertebrates. Massachusetts: Sinauer Associates, Inc..

Kuzirian, .., .. Child, .. Epstein, .. Smith, .. Tamse. October 1996. Lead affect learning by Hermissenda Crassicornis. Biological Journal, 191: 260-61.

Meinkoth, .. 1981. The Audubon Society Field Guide to North American Seashore Creatures. New York: Alfred A. Knopf, Inc..

Nordin, K. April 14, 1996. "Ocean Info. Nudibranchs" (On-line). Accessed February 12, 2000 at http://oceanlink.island.net/oinfo/nudibranchia.html.

Pearse, .., .. Pearse, .. Buchsbaum, .. Buchsbaum. 1989. Living Invertebrates. Massachusetts: Blackwell Scientific Publications.

Tyndale, .., .. Alvia, .. Kuzirian. October 1994. Food Detection and preferences of the nudibranch mollus Hermissenda crassicornis: Experiments in a Y-maze. Biological Bulletin, 187: 274-75.