A post-metamorphic 5mm-long Dirona albolineata is already feeding on adult-type foods (snails, bryozoans, hydroids).  Drawing from Hurst 1967 Veliger 9: 255.

Without the oral veil spread out on its anterior
end, it is difficult to tell which end is which in
this adult Dirona albolineata 1X

Veligers of Hermissenda crassicornis settle from the plankton after 5wk in response to chemical emanations from hydroid prey such as Obelia spp.   Metamorphosis is completed within 2-3d and within a few days the juvenile, at 0.4mm in length, begins to feed on hydroids.  Hermissenda’s complete lifespan is about 4mo.  Harrigan & Alkon 1978 Biol Bull 154: 430. 

NOTE at 15°C in laboratory culture

Studies on larval development of specimens of Melibe leonina collected in the San Juan Archipelago and southern Vancouver Island regions show a similar pattern to that described for other species of opisthobranchs that have planktotrophic larvae. The graph on the Right shows 3 stages of larval development (see blue dots): hatching (Day 0), eyespot-mantle retraction (Day 16-20 post-hatching), and commencement of metamorphosis (30-48d post-hatching).

The rudiments of cerata and oral hood appear in the late-stage larva.  Metamorphosis involves loss of shell, operculum, and velum.  Some parts of the larval stomach are retained along with right and left digestive diverticula.  The photo-series at the bottom shows metamorphic events during the 48-60h of metamorphosis (12-14°C). Shortly after metamorphosis there is a rapid expansion of the primary cerata and oral hood. 

Within 2-3d the juvenile is using its oral hood to catch prey (see figure on Left). Juveniles can be maintained on a diet of ciliates and copepod nauplii.  The authors note that a specific, external chemical cue does not appear to be required for metamorphosis.  Bickell & Kempf 1983 Biol Bull 165: 119. Colour photo of Melibe leonina courtesy Charles Seabourne, Los Angeles.

NOTE  at this time the epithelium of the mantle fold detaches from the rim of the shell and is pulled posteriorly.  The tissues coalesce and eventually grow into two protuberances – these are the rudiments of the primary cerata

Settlement of larval sea hares Aplysia californica on rocky reefs off the coast of Santa Catalina Island, California is primarily to the red alga Plocamium cartilagineum.  There are 2 settlement pulses, one in Jan-Feb probably from local spawnings the previous summer, and one in Jun-Sept possibly from larvae spawned in the region of Baja California and carried northwards in summercurrents. Individuals from the winter recruitment period are 500g or more in size by the time they reproduce in summer, while ones from the summer recruitment are only about 1g when they reproduce in summer.  Pennings 1991 J Exp Mar Biol Ecol 146: 253. Photograph courtesy Kevin Lee, Fullerton, California diverkevin.

Juvenile sea hare Aplysia californica of just a few cm
in size. Note the much smaller opisthobranch of uncertain
ID crawlingin the reverse direction on the frond 2X

The transient nature of nudibranch recruitment to our shores is familiar to marine biologists – sometimes they’re there, sometimes not.  Clearly, some link in the larval production/survival/transport chain is involved, but what it is, and how and when it works has not been identified.  Recently, sharp declines in numbers of nudibranchs in central California have been correlated with La Niña climatic events that disrupt onshore and alongshore currents that would otherwise bring larvae into favourable areas for settlement and metamophosis.  To investigate this, a research group from a variety of Californian and other institutions model distribution and abundance data for 56 nudibranch species collected between 1969-1995 with such ocean-climate variables as sea-surface temperature, sea-surface height, sea-level atmospheric pressure, and upwelling strength, to predict most favourable conditions for larval settlement.  The results of this wonderfully ambitious and broad-scale study support a cross-shelf and alongshore larval-advection theory to explain past and current fluctuations in abundances of nudibranchs in central California.  When sea levels are high and inshore cross-shelf currents predominate, as during El Niño conditions, recruitment is high; during the opposite conditions of La Niña events, inshore movements of larvae are not favoured, and recruitment is low (by up to 2 orders of magnitude).  Given that the latter conditions have predominated for several years, the authors hypothesise that nudibranch populations will likely recover when  El Niño conditions return.  Schultz et al. 2011 Limnol Oceanogr 56: 749. Photograph courtesy Kevin Lee, Fullerton, CA.

NOTE  major hypotheses involve changes in sea-surface temperature, nutrient levels for sustenance of larvae, and current direction and strength (cross-shelf larval advection)

NOTE  the normal summer flow of the California Current is southward and offshore, which increases coastal upwelling.  El Niño events transiently reverse the current to northward and onshore, which produces downwelling


More than one or two opisthobranchs being found together at
one time is usually rare. Perhaps the presence of several
dozens of the cephalaspidean Bulla gouldiana in this southern
California site has resulted from a favourable El Nino event 0.2X