The west coast of North America is arguably the richest temperate area in the world for diversity of nudibranchs and other opisthobranchs. On a typical SCUBA-dive in British Columbia, a diver might expect to see a dozen or more species of nudibranchs, and perhaps one or two related opisthobranchs.  At least 70 species of nudibranchs are recorded from British Columbia, and a 1983 report lists 101 species along the California coast (50 genera and 32 families).

NOTE  lit. “naked gills”, referring to the outside, unprotected location of the gas-exchange organs in the group.  In shelled gastropods, the ctenidia or gills are within the mantle cavity and protected by the shell.  During the evolution of nudibranchs the ancestral gills were lost and new gas-exchanging surfaces arose around the anus (in dorids) or along the back (in aeolids) – for this reason these organs are not considered "true" gills

NOTE  see MacDonald 1983 Malacologia 24: 114. Some 66 species are known from Oregon, 46 of them from the Cape Arago area.  Goddard 1984 Veliger 27: 143.

Phylum Mollusca (lit. “soft” or “shellfish” L.)

Class Gastropoda (lit. “stomach foot” G.), referring to the body structure of viscera lying overtop of the muscular foot

SubClass Opisthobranchia¹ (lit. “behind gills” G.), referring to the  “gills” being located at the back of the animal.  Included in this Subclass is a number of Orders of largely shell-less, colorful, mostly predatory snails, mostly benthic but including some pelagic forms.  Only the benthic Orders are listed below.  Nudibranchia is the largest of these groups in terms of numbers of species.

Order Nudibranchia (lit. “naked gills” L.), rich diversity of species, including at least 50 that can be commonly observed intertidally or seen during SCUBA-dives. 

SubOrder Doridacea (lit. “sacrificial knife” G.) (=Holohepatica), perhaps referring to the shape of the foot, including many colorful species characterised by a dorsal anus near the posterior surrounded by secondarily-derived gas-exhange organs

SubOrder Aeolidiacea (lit. “quick-moving” G.), including many colorful species characterised by an anus near the anterior-right dorsal side and dorsal cerata

Smaller SubOrders are the Arminacea (including species of Armina, Janolus, and Dirona) and the Dendronotacea (incuding species of Tochuina, Tritonia, Dendronotus, and other species)                                                                                                                                                     

Order Anaspidea (lit. “without shield” G.), representatives of these herbivorous sea hares include Phyllaplysia taylori² along most of the coast and, in southern California, Aplysia³ californica and A. vaccaria

Order Sacoglossa/Ascoglossa (lit. “shield tongue” and “bag/bladder tongue”, respectively G.), about 2 dozen species of plant-suckers, including Elysia hedgpethi, Alderia modesta, A. willowi, Stiliger fuscovittatus, and species of Hermaea

Order Cephalaspidea (lit. “head shield” G.), shell may or may not be visible; common species include Haminoea visicula, H. virescens, H. japonia, Gasteropteron pacificum, Aglaja ocelligera, Navanax inermis, Bulla gouldiana, and Philine bakeri

Order Notaspidea (lit. “back shield” G.), including Berthella californica and Pleurobranchaea californica

NOTE¹ overall, about 250 species of shallow-water, benthic opisthobranchs are known from the entire west coast of North America, of which about half may be reasonably common. Goddard 2004 Can J Zool 82: 1954. It should be noted that more recent classificatory systems consider the Opisthobranchia to be an Order, rather than a SubClass.  Until this change is commonly accepted, the older SubClass designation will be retained in the ODYSSEY

NOTE² Phyllaplysia taylori lives on the blades of eelgrass Zostera marina.  It has been re-described as P. zostericola but, for whatever reason, the new name seems not to have caught on and zostericola is presently considered a junior synonym for the species.  McCauley 1960 Proc Cal Acad Sci 29: 549.

NOTE³ these species of sea hares in inhabit southern California and Baja California, Mexico.  Aplysia vaccaria is reknown for its large size, reaching up to 14kg or more.  Aplysia californica is known for its use in neurobiological studies of learning and memory. Discrete ganglia with relatively few large-sized neurons combined with a repertoire of several simple behaviours, some of them of the “on-off” type (e.g., inking) make A. californica an ideal model system for neurobiological studies.  Thousands of publications, several books, numerous research symposia, at least one Nobel prize, hundreds of PhD degrees, and a sea-hare rearing facility in Miami, Florida to provide specimens for study, are the products of several decades of intense research on this species.  The scientific rewards of the research have been enormous, including many fundamental discoveries on the mechanism of habituation and other learning processes. Despite all of this, surprisingly little work has been published on their field biology, especially for A. vaccaria. A third species Aplysia reticulopoda was earlier described from southern California (Beeman 1963 Veliger 5: 145), but is currently not recognised as a valid species. DNA sequences of life stages of A. californica including egg, larva, juvenile, and adult have recently been done (see Fiedler et al. 2010 Comp Biochem Physiol D 5: 165)