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Hermissenda crassicornis - Opalescent nudibranch

Geographic range:
Kodiak Island, Alaska to Punta Eugenia, Baja California, Mexico

Key features:
Translucent or clear body with white lines along margin and out onto oral tentacles. A bright orange, diamond-shaped line runs between the annulated rhinophores. Cerata are usually orange with light tips.

Similar species:
Phidiana pugnax -- Pugnacious Phidiana

bay (rocky shore), bay (sandy shore), estuary, exposed rocky shore, exposed sandy beaches, kelp forest, protected rocky shore, protected sandy beaches
Hermissenda crassicornis - Opalescent nudibranch image


Primary common name:
Opalescent Nudibranch
  ITIS code:
Synonymous name(s):
General grouping:
Nudibranchs or sea slugs

Geographic Range
Range description:
Hermissenda crassicornis can be found from Kodiak Island, Alaska to Punta Eugenia, Baja California, Mexico and also in the northern part of the Sea of Cortez.
Northern latitude extent:
  Southern latitude extent:
East longitude extent:
  West longitude extent:

Intertidal Height
Lowest intertidal height:
0 meters OR -2 feet
  Highest intertidal height:
0.60060060 meters OR 2 feet
Intertidal height notes:
Hermissenda crassicornis is common in the intertidal.

Subtidal Depth Range
Minimum depth:
0 meters OR 0 feet
  Maximum depth:
40 meters OR 133.2 feet
Subtidal depth notes:
Hermissenda crassicornis is common in the subtidal.

bay (rocky shore), bay (sandy shore), estuary, exposed rocky shore, exposed sandy beaches, kelp forest, protected rocky shore, protected sandy beaches
Habitat notes:
Hermissenda crassicornis can be found on hard and soft bottoms from the low-tide line down to 40 m deep. It occupies rocks, pilings, mudflats, bays, estuaries and tidepools.

Relative abundance:
Hermissenda crassicornis is one of the most abundant nudibranchs on the Pacific Coast.

Species Description
General description:
Hermissenda crassicornis is in the class Gastropoda, phylum Mollusca, order Nudibranchia and suborder Aeolidacea. Unlike most mollusks, nudibranchs lack a shell, mantle cavity, and original gill. All members of the suborder Aeolidacea, including Hermissenda crassicornis, are characterized by rows of cerata on the back. Cerata are dorsal appendages containing a central core that is a branch of the liver.
Distinctive features:
Hermissenda crassicornis's distinctive body is bluish white with neon blue edges and a yellow or orange midline stripe. The body is broadest just behind the head, tapering to a fine point at the rear. There are numerous bright orange, white-tipped projections, called cerata, in two clusters on each side of back. Hermissenda crassicornis has four antennae, of which, the first pair has blue lines and is widely spaced and the second pair is bluish with raised rings.
Hermissenda crassicornis can grow to a length of 83 mm and a width of 10 mm.

Natural History
General natural history:
The life span of Hermissenda crassicornis is less than one year and therefore it has to grow and reproduces quickly.

Hermissenda crassicornis defends itself through storing nematocysts acquired from its prey in its cerata. Nematocysts are stinging cells that work like little mechanical devices that become activated by some trigger or exposure to certain chemicals. The nematocysts are indigestible to Hermissenda crassicornis, and therefore, during digestion they travel through interconnecting tubules from the diverticula into the cerata and can later be used against predators or in aggressive encounters. Hermissenda crassicornis's bright colors, that are mostly due to carotenoid pigments, may warn predators for their chemical weapons.
Hermissenda crassicornis can be eaten by fish, such as the Mosshead Warbonnet, Chirolophis nugatory, or other sea slugs, such as Navanax.
Hermissenda crassicornis feeds primarily on hydroids, but may also eat sea anemones, bryzoans, sea pens, sea squirts, corals, sponges, small crustaceans, and even each other.

Feeding behavior
Feeding behavior(s):
Feeding behavior notes:
Hermissenda crassicornis is a generalist carnivore and can detect its prey items through chemical scents in the water. It is also known to be cannibalistic. When two individuals meet, they frequently fight and bite chunks of tissue from each other. Hermissenda crassicornis is also known to have particularly potent cerata. This may be due to the fact that they prey on organisms with particularly toxic nematocysts, such as solitary corals and strawberry anemones, Urticina lofotensis.
January - December  
Hermissenda crassicornis is a hermaphrodite, meaning it possesses both male and female organs, however, they very rarely self fertilize. Most instead chose to mate and during mating they may spend up to 30 minutes together, touching each other and aligning sexual pores, the actual exchange of sperm lasts only seconds. After sperm exchange, eggs are packaged into narrow coils resembling tiny, pink sausage links and affixed to eelgrass or algae. Quantity of eggs in a ribbon can range from a few to a million. Maturation of the eggs is highly influenced by temperature and can take anywhere from 5 — 50 days. Warm temperatures are most favorable for egg development. The egg matures into a larval stage called a veliger. Veligers will float around on the ocean floor until environmental conditions cause them to settle and develop into adult form. Reproduction can occur year round and the generation time, the time from egg to first reproduction, is about 2.5 months.
Alden, P., F. Heath, R. Keen, A. Leventer, and W. Zomlefer. 2002. National Audubon Society Field Guide to California. A.A. Knopf, New York, NY.

Gotshall, D. 2005. Guide to marine invertebrates : Alaska to Baja California. Sea Challengers, Monterey, CA. 117 p.

Langstroth, L. and L. Langstroth. 2000. A Living Bay: The Underwater World of Monterey Bay. University of California Press, Berkeley and Los Angeles, CA. 287 p.

McKinley, G. 2000. Hermissenda crassicornis, Animal Diversity Web. World Wide Web electronic publication., Accessed [06/11/06].

Meinkoth, N.A. 1998. National Audubon Society Field Guide to North American Seashore Creatures. A.A. Knopf, New York, NY. 813 p.

Ricketts, E. F., J. Calvin, and J.W. Hedgpeth. 1985. Between Pacific tides. Stanford University Press, Stanford, CA. 652 p.

Stiles, M. 2003. The Bold and the Beautiful: Hermissenda crassicornis. High Tide Times, Birch Aquarium. San Diego, CA. 2 p.

Monterey Bay Aquarium. Online Field Guide, 2008.
Accessed [04/27/06]
Accessed 01/30/2009 for Pelagic Cormorant
Accessed 8/19/09 for Bryozoan
Accessed 7/31/09 for Spiny brittle star
Accessed 3/31/09 for Sunflower star
Accessed 8/9/09 for red octopus
Accessed 8/19/09 for Decorator crab
Accessed 7/31/09 for warty sea cucumber

Data supplied by SIMoN Sanctuary Integrated Monitoring Network
Anthopleura xanthogrammica - Giant green anemone

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