Mollusca
Nudibranchs & relatives
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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 other 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  additionally, some 66 species are known from Oregon, 46 of them from the Cape Arago area.

Goddard   1994   Veliger 27: 143
MacDonald   1993   Malacologia 24: 114

ANIMATION of the snail's odyssey © Thomas Carefoot 2026
map used by the snail in A SNAIL'S ODYSSEY

To navigate through the ODYSSEY:

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  • OR: play the animation to the left
  • OR: follow the snail's ODYSSEY by CLICKING on any X-marked invertebrate on the map above

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.

Clade Heterobranchia

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 species2 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 taylori3 along most of the coast and, in southern California, Aplysia4californica and A. vaccaria

Order Sacoglossa/Ascoglossa (lit. “shield tongue” and “bag/bladder tongue”, respectively), about two 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

"Lower Heterobranchia"

Family Pyramidellidae (including Odostomia columbiana, in a large genus of ectoparasitic, shelled marine snails)

   

NOTE¹ overall, about 250 species of shallow-water, benthic opisthobranchs are known from the west coast of North America, of which about half may be reasonably common (Goddard 2004). It should be noted that more recent classificatory systems consider the Opisthobranchia to be just one of three major subdivisions of the Clade Heterobranchia (the other subdivisions are Pulmonata and "Lower Heterobranchia"), but portions of the older classification as shown above are retained in the ODYSSEY for their familiarity.

NOTE² the ubiquitous species Aeolidia papillosa originally named by Linnaeus in1761 and long thought to be a single cosmopolitan species inhabiting mostly northern temperate climes, is now considered to be a species complex.  The old name is retained for Atlantic and eastern Pacific representatives (Alaska, British Columbia, and Washington), while those in California and Oregon are now A. loui sp. nov. and those in the U.K. and northern Europe are A. filomenae sp. nov.    Chilean specimens revert back to their original designation of A. campbellii named by Cunningham in 1871 (Kienberger et al. 2016).  The World Register of Marine Species (WoRMS) has recently recognised this change. The accompanying photographs show representative morphotypes of A. papillosa.

NOTE³ Phyllaplysia taylori  lives on the blades of eelgrass Zostera marina.  It was at one time re-described as P. zostericola (McCauley 1960) but, for whatever reason, the old name P. taylori dating from1900 is still its official designation.

NOTE4 two major species of sea hares inhabit southern California and Baja California, Mexico.  Aplysia vaccaria is reknowned for its large size, reaching up to 14kg or more in live mass.  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) has made 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 parvula is a cosmopolitan tropical/subtropical species that has recently been reported in southern California waters, but its extent of distribution is not known (Behrens 2004).  Another putative species Aplysia reticulopoda was earlier described for southern California (Beeman 1960), and it is now  recognised as a valid species.  DNA sequences of life stages of A. californica including egg, larva, juvenile, and adult have been published (Fiedler et al. 2010)

McCauley   1960   Proc Cal Acad Sci 29: 549
Behrens   2004   Proc Cal Acad Sci 55 (2): 11
Beeman   1963   Veliger 5: 145
Fiedler et al.   2010   Comp Biochem Physiol D 5: 165
Goddard   2004   Can J Zool 82: 1954
Kienberger et al.   2016   Zoological J Linn Soc 177: 481

Research Study 1

Fig. 1.  Southern California morph Hermissenda opalescens.  If this is a representative imge, then there are several obvious differences in body proportion, and ceratal color and distribution, that easily distinguish it from H. crassicornis
Courtesy The authors (all photos)




Fig. 2.  Distributed from northern California to Alaska, the morph Hermissenda crassicornis has distinct white striping along the anterior surface of each ceras.  Other differences in body shape and size make this morph readily distinguishable from H. opalescens (Fig. 1)

Along the same lines, two researchers from California State Polytechnic University, Pomona determine through genetic and morphological analyses that the aeolid nudibranch historically known as Hermissenda crassicornis is actually represented by three species, H. crassicornis, and two new ones H. opalescens and H. emuri.  The historically recognised H. crassicornis has the broadest distribution on the west coast, from northern California to Alaska, while the new species H. opalescens1 is noted as being distributed from the Sea of Cortez to northern California.  The third species2 Hermissenda emuri is found not on the west coast of North America, but in the Japan/Russia area of the western Pacific, and will not be considered further here (but see Fig. 3).  The researchers use a combination of DNA sequencing3 in four gene fragments, and morphological comparisons of jaws, radulae, and overall colour patterns.  Results of the phylogenetic analyses reveal three main clades corresponding to the three geographical areas just mentioned.  Radula morphology is similar in all three clades, but some small differences exist in denticle size and number that are consistent within the species.  Colour patterns are different among the clades (Figs. 1 - 3).  The findings in the study have important implications for past and future research on the genus, especially ones along the coast of California, for it can't be known for certain which species was being used in a certain past investigation.  The authors provide a thoughtful discussion of the naming history of the two Californian species spanning more than one and one-half Centuries

NOTE1  recent discovery of this new species as far north as Vancouver Island considerably extends its distributional record 

NOTE2  the authors clarify a point regarding speciation that may be of interest to readers not familiar with the jargon.  Species that are morphologically indistinguishable are known as cryptic, while those that are later only distinguishable by molecular technology or other means are known as pseudocryptic

NOTE3   gene fragments sequenced include mitochondrial 16S, COI, nuclear H3, and nuclear 18S



Fig. 3.  Japan morphs Hermissenda emuri.   Note the uniform brown/orange coloration of the cerata and lack of white striping.  Also, in this clade the cerata are in distict clumps

NOTE  the image above is missing an 'm' in its genus spelling...Odyssey error!

Lindsay & Valdez   2016   PLoS ONE 11(4): e0154265

Research Study 2

In a follow-up paper one of the previous authors with 9 other Canadian and Californian  scientists have confirmed through similar genetic and morphological methodology that the aeolid nudibranch Hermissenda crassicornis is represented by three species, H. crassicornis and the new species H. opalescens and H. emuri.  The historically recognised H. crassicornis has the broadest distribution, from N. California to Alaska, while the new species H. opalescens is distributed from the Sea of Cortez to mid-Vancouver Island, Canada.  The two species can be differentiated by ceratal colour pattern (Fig. 1), foot dimensions (Fig. 2), and overall body size (Fig. 3).  These two species are confirmed as differing genetically.  

NOTE  despite the authors confirming in the body of their paper that Hermissenda opalescens' distribution extends as far north as Vancouver Island, they appear to have carelessly left the older idea of its northern boundary being northern California in Table 1 of their paper.  In defense of the authors, they do consider that the more northern presence of the species may relate to happenstance transport of larvae, with the implied suggestion that their presence there may not be permanent

Fig. 1.  Species collected from the west coast of Vancouver Island, B.C. display differences in ceratal striping: present in H. crassicornis on the Right, and absent in H. opalescens on the Left
Fig. 2.  Comparative foot shapes in Hermissenda crassicornis and H. opalescens
Fig. 3. Comparison of overall body sizes in Hermissenda crassicornis H. opalescens
Merlo et al.   2018   FACETS 3: 764–776 DOI: 10.1139