Biology/Natural History: It is one of the most abundant nudibranchs in Califonia. It eats hydroids, but the diet also includes small sea anemones, bryozoans, colonial ascidians (Aplidium solidum, botryllids), annelids, small crustacea, tiny clams, dead animals of any sort. Will eat other Hermissenda. In the Puget Sound, Hermissenda is the main predator of the sea pen Ptilosarcus gurneyi. Mating animals are most often found in southern California in winter, but are found year around in the Puget Sound (Washington). The egg string resembles linked pink sausages. They are commonly attached to algae and to blades of eelgrass. Each egg case usually contains one egg, but can contain up to four. Many studies have been carried out on Hermissenda, but the main area of focus is the eye. It has five cells, each about 75 um in diameter, which are large enough to receive a recording electrode. Within the cells it is suspected of containing symbiotic fungi. Hermissenda is an aggressive creature. When two individuals encounter fights will break out, which involves lunging and biting. Encounters most likely to induce a fight are those of mutual head on contact. The individual whose head is closest to the others tail or side will usually get the first bite in, this also means that they usually come out the winner. The copepod Hemicyclops thysanotus is often found adhering to the dorsal surface of Hermissenda.

The nudibranch Phidiana hiltoni may attack this nudibranch (Goddard et al., 2011)

As with other members of suborder Aeolidacea, this species has an anus on the right side, on a conspicuous papilla on the anterior half of the dorsum. The dorsum has numerous outgrowths (cerata) besides the rhinophores, usually arranged in transverse rows. The clavus of the rhinophores cannot be retracted into a sheath (there is no sheath). Hermissenda crassicornis has cerata without a sail-like ridge on the posterior side. None of the cerata are anterior to the rhinophores. The anterolateral margins of the foot are elongated into prominent "pedal tentacles". A mid-dorsal orange band begins just anterior to the rhinophores on the dorsum and passes back at least to the region of the first cerata. This band is usually bordered by a broad, opaque white or luminescent light blue band which begins on the oral tentacles and continues back to the tip of the tail. The distal parts of the cerata are orange, with white at the tip. Body to about 80 mm long, easily recognized when juvenile states due to presence of orange areas on back borderd by bright light-blue lines. Note in the photos below that some have smooth-appearing rhinophores while in others the rhinophores appear annulate.

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)

Biogeographic Regions: nearctic (Native )

Geographical Range: Sitka (Alaska) to Puertecitos (Baja California), more abundant in center of range.

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)

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)

Depth range based on 2 specimens in 1 taxon.

Environmental ranges
  Depth range (m): 1 - 1.3

Graphical representation

Depth range (m): 1 - 1.3
 
Note: this information has not been validated. Check this *note*. Your feedback is most welcome.

Depth Range: Low intertidal zone, subtidal to 35m

Habitat: Common in spring and summer, varied habitats, usually found in rocky pools, marina floats, pilings, and mud flats.

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)

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)

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)