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Species Database

Hermissenda crassicornis - Horned Aeolid

Horned Aeolid image

Geographic range:

Kodiak Island, Alaska to central California.

Key features:

White longitudinal lines on cerata, absent on similar species (H. opalescens). Translucent or light blue body with white lines along margin and out onto cephalic tentacles. A bright orange, diamond-shaped line runs between the annulated rhinophores. Cerata are usually orange with light tips.

Similar species:

Phidiana hiltoni -- Hilton's Phidiana
Hermissenda opalescens -- Opalescent Aeolid


bay (rocky shore), bay (sandy shore), estuary, exposed rocky shore, exposed sandy beaches, kelp forest, protected rocky shore, protected sandy beaches

Primary common name:

Horned Aeolid

Synonymous name(s):

Phidiana crassicornis

General grouping:

Nudibranchs or sea slugs

ITIS code:


Geographic Range

Range Description:

Hermissenda crassicornis can be found from Kodiak Island, Alaska to central California.

This species was recently subdivided into three species: Hermissenda emurai found from Japan to Russia; Hermissenda crassicornis found from Alaska to central California; and Hermissenda opalescens found from northern California to the Sea of Cortez, Mexico.

Intertidal Height

Lowest intertidal height:

meters OR -2 feet

Highest intertidal height:

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 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:

The body of Hermissenda crassicornis is a distinctive bluish white with neon white lines near the base of the foot 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 clusters on each side of the dorsum. Hermissenda crassicornis has two annulate rhinophores and two cephalic tentacles that have a blue line.


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 crassicorniss 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 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. Amazingly, these nudibranchs are able to ingest the stinging cells of their prey without discharging the nematocysts, and then use those same stinging cells for their own defense by flailing the cerata and discharging the nematocysts against potential predators.

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.

Monterey Bay National Marine Sanctuary:

There are two species of Hermissenda in central California, where the range of the more northern species Hermissenda crassicornis overlaps with the range of the more southern species Hermissenda opalescens. Whiite lines extending up the cerata distinguish Hermissenda crassicornis from the southern Hermissenda opalescens.
Click on an image below to view a larger version in the SIMoN Photo Library. You will also be able to view important information on each photo such as photographer, date, caption and more.
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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.
Lindsay, T. and A. Valdes. 2016. The model organism Hermissenda crassicornis (Gastropoda: Heterobranchia) is a species complex. PLoS ONE 11(4):1-17.
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.