Hermissenda opalescens (Cooper, 1862) Common name(s): Southern opalescent nudibranch
Synonyms: Aeolis opalescens, Flabellina opalescens, part of Hermissenda crassicornis
Phylum Mollusca  Class Gastropoda  Subclass Opisthobranchia  Order Nudibranchia  Suborder Aeolidacea  Superfamily Euolidoidea  Tribe Cleioprocta  Family Facelinidae
	Hermissenda opalescens in a San Simeon, CA tide pool
	(Photo by:  Dave Cowles, 1999)

Description:  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 opalescens 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, but there is no longitudinal white band along the anterior side of each ceras.  Body to about 80 mm long, easily recognized when in juvenile states due to presence of orange areas on back borderd by bright light-blue lines. 

How to Distinguish from Similar Species:Hermissenda crassicornis, the northern opalescent nudibranch, is very similar except that it has a distinct longitudinal white band along the anterior margin of each ceras. Only recently were these two species distinguished  by molecular means (Lindsay and Valdes, 2016), so a large number of papers and guides, including Kozloff's key, prior to that date confused the two.

Geographical Range:  Sea of Cortez, Mexico to Bodega Bay, CA

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.

Biology/Natural History:  This 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, and dead animals of any sort.  Will also eat other Hermissenda individuals.  Mating animals are most often found in southern California in winter.  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 one another fights will break out, which involves lunging and biting.  Encounters most likely to induce a fight are those of mutual head contact.  The individual whose head is closest to the other's tail or side will usually get the first bite in, and 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)

References:

General References:
  Behrens, 1991
  McDonald and Nybakken, 1980
  Morris et al., (1980)

Scientific Articles:
Goddard, Jeffrey H., Terrence M. Gosliner, and John S. Pearce, 2011.  Impacts associated with the recent range shift of the aeolid nudibranch Phidiana hiltoni (Mollusca: Opisthobranchia) in California.  Marine Biology DOI:10.1007/s00227-011-1633-7

Lindsay, Tabitha, and Angel Valdes, 2016. The model organism Hermissenda crassicornis (Gastropoda: Heterobranchia) is a species complex. PLoS One 11:4:e0154265. doi: 10.1371/journal.pone.0154265

Web sites:

General Notes and Observations: Locations, abundances, unusual behaviors:

Hermissenda opalescens from San Simeon, CA, May 1996.  Photo by Dave Cowles

Authors and Editors of Page:
Dave Cowles (2018):  Created original page
CSS coding for page developed by Jonathan Cowles

Salish Sea Invertebrates web site provided courtesy of Walla Walla University