Placida dendritica Alder and Hancock, 1843

Common name(s): Branched placida

Synonyms:Hermaea dendritica, Hermaea ornata Placida dendritica
Phylum Mollusca 
Class Gastropoda 
Family Limapontiidae 
Placida dendritica, 5 mm long, found flushing through the Rosario seawater system.  This is likely an immature individual as it has much less than 50 cerata.
(Photo by: Dave Cowles, August 2014)

Description:  As a Sacoglossan, this nudibranch-like species has no external shell nor cephalic shieldRhinophores, if present,  are solid or rolled, not perfoliate, with longitudinal ridges, or with vertical pinnate plumes, and the clavus is not distinct from the stalk.  The rhinophores, if present, cannot be retracted into a sheath.  The dorsum of Sacoglossans may or may not have elongated outgrowths such as cerata.  The anus is on the dorsal midline, just posterior to the rhinophoresPlacida dendritica has up to about 50 well-developed cerata and also rhinophores, but the foot does not have parapodial outgrowths.  The body is approximately cylindrical, and the posterior foot tapers gradually to form a pointed tail well behind the last cerata.  The cerata are found on both the anterior and posterior half of the dorsum, and often occur in clusters of 3-5.  The rhinophores are rolled tightly for nearly their entire length.  The body is pale yellowish, with a branching green or red pattern on the dorsum and up into the cerata, formed by branches of the gut.  Total length to 1.4 cm but usually 8 mm or smaller.

How to Distinguish from Similar Species:Hermaea oliviae and H. vancouverensis have thicker cerata. Stiliger fuscovittatus has solid rhinophores.

Geographical Range:  Vancouver Island, Canada to Baja California, Mexico.  Also has been found in Southeast Alaska.  Also in Japan and much of the North Atlantic, and the Mediterranean.

Depth Range:  Mid and low intertidal

Habitat:  Generally on Bryopsis corticulans,Codium fragile,or C. setchelli algae.

Biology/Natural History:  Like other sacoglossans, the radula of this species is composed of individual, bladelike teeth used to slit open algal cells.  The animal sucks out the cell contents.  Individuals found on C. fragile seem to strongly prefer that species and may starve on C. setchelli, while those from C. setchelli had little preference for one algal host over the other.  Trowbridge (1991) found that the sacoglossan forms feeding groups when feeding on C. fragile but not when feeding on Bryopsis corticulans.  Members of groups grew faster than isolated individuals if they were all the same size or if they were smaller than the other sacoglossans in the group.  Evertsen and Johnsen (2009) found that they did not retain functional chloroplasts from their food (C. fragile).  On the Oregon coast there is substantial recruitment of this species into protected bays, and peak abundance is in June.  The thalli of C. fragile could be heavily covered with new sacoglossans at a rate of 200 to 400 sacoglossans/thallus per month.  The animals grew from initial settlement to sexual maturity in less than a month.  Adults also moved rapidly among different thalli of C. fragile (Trowbridge, 1992)



 

References:

Dichotomous Keys:
  Carlton, 2007
  Kozloff, 1987, 1996

General References:
  O'Clair and O'Clair, 1998

Scientific Articles:
Evertson, Jussi, and Geir Johnsen, 2009.  In vivo and in vitro differences in chloroplast functionality in the two North Atlantic sacoglossans (Gastropoda, Opisthobranchia) Placida dendritica and Elysia viridis. Marine Biology 156: pp 847-859

McLean, N., 2005.  Phagocytosis of chloroplasts in Placida dendritica (Gastropoda: Sacoglossa).  Journal of Experimental Zoology 197:3 pp 321-329.  DOI 10.1002.jez1401970304

Trowbridge, Cynthia D., 1991.  Group membership facilitates feeding of the herbivorous sea slug Placida dendritica.  Ecology 72:6 pp 2193-2201

Trowbridge, C.D., 1992.  Phenology and demography of a marine specialist herbivore: Placida dendritica (Gastropoda: Opisthobranchia) on the central coast of Oregon.  Marine Biology 114: pp 443-452

Vardaro, Rita, Vizenzo Di Marzo, and Guido Cimino, 1992.  Placidenes: cyercene-like polypropionate y-pyrones from the mediterranean ascoglossan mollusck Placida dendritica.  Tetrahedron letters 33:20 pp 2875-2878

Web sites:
 


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

We found this sacoglossan, probably not fully grown, in seawater from our seawater system in August, 2014.  The animal must have been sucked into the seawater intakes, which are about 1.5 m off the bottom and 5-6 m from the surface, so it may have been pelagic or was feeding on algae coating the intake.  I have not seen Codium fragile near the intakes so the animal was most likely pelagic.  Trowbridge (1992) notes that the species has rapid recruitment and that adults are quite mobile as well, so I suspect it was traveling through the water column.  The animal is very active and mobile--hard to photograph because it moves so fast.

Placida dendritica
Another view of the same individual as it charges around the dish.  The green gut, which extends up into the cerata, can be clearly seen.


Authors and Editors of Page:
Dave Cowles (2014):  Created original page
CSS coding for page developed by Jonathan Cowles (2007)

Salish Sea Invertebrates web site provided courtesy of Walla Walla University