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Settlement & metamorphosis

  Settlement & metamorphosis are dealt with in this section, while topics of MATE SELECTION & COPULATION, EGG-LAYING, EMBRYONIC DEVELOPMENT, HATCHING & LARVAL DEVELOPMENT, SETTLEMENT CUES, and ONTOGENETIC DEVELOPMENT OF BEHAVIOUR are considered elsewhere.
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Research study 1

schematic showing culture data on length and size with age for sea hares Aplysia californicaIn what is thought to be the first time a marine gastropod with long-duration planktotrophic development has been successfully and reliably cultured, researchers at the Marine Biological Laboratory, Massachusetts rear Aplysia californica from egg through metamorphosis. After 34d of larval development on a diet of unicellular alga Isochrysis galbana (22oC), and stimulated by the presence of the red alga Laurencia pacifica, the veligers settle and metamorphose.  Within 3-4d after the onset of metamorphosis the juveniles begin feeding on Laurencia and other red seaweeds and, by 120d after hatching and at a mass of about 400g, are sexually mature. Of 6 species of west-coast algae tested as metamorphosis inducers, only Laurencia pacifica is successfulDevelopment of rearing techniques for A. californica has been a truly ground-breaking achievement and has enabled research specimens for neurophysiological research to be cultured essentially ad libitumKriegstein et al. 1974 Proc Nat Acad Sci, USA 71: 3654.

NOTE  the authors note that 4 individuals of A. californica were previously cultured to reproductive maturity at the Pacific Bio-Marine Supply Co., California but that the techniques did not prove reliable

NOTE  a culture facility, the National Resource for Aplysia, now exists on the campus of the University of Miami’s Rosenstiel School of Marine & Atmospheric Science, and provides annually about 30,000 sea hares of all ages and sizes to neuroscience researchers around the world over

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Research study 2
  series of drawings of settlement and metamorphosis in an opisthobranch mollusc
During metamorphosis in nudibranchs there is, as in most other marine invertebrates, a considerable re-arranging of tissues and structures.  An individual changes within a day or so from a free-swimming, phytoplankton-eating, shelled organism to a crawling, predatory, shell-less form.  All soft tissues are resorbed and used; only the larval shell is cast off.  Bonar 1978 p. 177 In, Settlement and metamorphosis of marine invertebrate larvae (Chia & Rice, eds.) Elsevier, N.Y.
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Research study 3

photograph of nudibranch Dirona albolineata
drawing of post-metamorphic nudibranch Dirona albolineataA post-metamorphic 5mm-long Dirona albolineata is already feeding on adult-type foods (snails, bryozoans, hydroids).  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

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Research study 4

photograph of juvenile nudibranch Hermissenda crassicornis
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

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Research study 5
  photograph of nudibranch Tritonia diomedea courtesy Russ Wyeth & Owen Woodward
The subtidal nudibranch Tritonia diomedea can be readily cultured in the laboratory.  Development at 12°C leads to completion of metamorphosis within 34d.  Metamorphosis will take place in the absence of a specific substratum.  Kempf & Willows 1977 J Exp Mar Biol Ecol 30: 261. Photograph courtesy Russ Wyeth & Owen Woodward.
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Research study 6

photograph of nudibranch Melibe leonina courtesy Charles Seabourne, Los AngelesStudies on larval development of specimens of Melibe leonina collected in the San Juan Archipelago and southern Vancouver Island regions show a similar pattern to graph showing shell length with age of the nudibranch Melibe leoninathat 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 early 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 photograph of a juvenile nudibranch Melibe leonina courtesy Bickell & Kempf 1983 Biol Bull 165: 119left 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

photo series showing metamorphosis of the nudibranch Melibe leonina

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Research study 7

graph showing seasonal recruitment of sea hares Aplysia californica at Santa Catalina Island, CaliforniaSettlement 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 photograph of sea hare Aplysia californica courtesy Kevin Lee, Fullerton, Californiathey 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 crawling in the reverse direction on the frond 2X

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Research study 8

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 photograph of several cephalaspideans Bulla gouldiana courtesy Kevin Lee, Fullerton, Californiacurrents 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

Other than breeding aggregations, more than 1-2 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

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Research study 9

graph showing growth of veliger larvae of the nudibranch Janolus fuscusgraph showing growth of juvenile nudibranchs Janolus fuscusA thorough and detailed account of development from egg to juvenile is provided for the arminacean nudibranch Janolus fuscus by researchers at the Oregon Institute of Marine Biology.  Janolus feeds on arborescent bryozoans such as Bugula pacifica, and in the Oregon area breeds from spring through summer.  Egg masses typically contain about 20,000 embryos.  At 12°C in the laboratory embryos hatch as veliger larvae in about 2wk.  Veligers average about 140µm diameter at hatching, but size is dependent upon duration in the capsule (see graph on Left).  After about 50d in culture the researchers are able to induce 2 individuals to settle onto a portion of Bugula pacifica colony. Settlement of a competent veliger onto a colony of Bugula whose lophophores (feeding zooids) will represent the first food of the juvenile is not without risk.  The authors describe a newly settled and crawling veliger being stabbed by the defensive avicularium of one colony, and another veliger being attacked, although not captured, by a lophophore. The 2 settled individuals metamorphose within 2d after settling.  Growth of juveniles is maintained in culture over a subsequent 5mo period to a length of almost 6cm (see graph on Right).  Growth and life span in culture is similar to that of new recruits of Janolus collected in the field in February and maintained under identical culture conditions in the laboratory, suggesting that a “subannual” life cycle is probably typical for the species.  Wolf & Young 2012 Biol Bull 222 (2): 137.photograph of a nudibranch Janolus fuscus with a parasitic copepod attached

NOTE  veligers in culture are fed on the flagellates Rhodomonas lens and Isochrysis galbana

NOTE  a defensive zooid of certain types of bryozoan. Bugula has a “bird’s-head” avicularium, so named because of its shape in the form of a beak and its elevation above the colony on a stalk.  A strong muscle enables the avicularium to stab downwards in defense of the colony

View of nudibranch Janolus fuscus from above. The head
end with both rhinophores visible is at the Left. The white
structures are the egg sacs of a parasitic copepod whose body
is buried in, and feeding on, the flesh of the nudibranch 2X



The following photographs show different stages of development:

photograph of trochophore stage of nudibranch Janolus fuscus photograph of a veliger larva of the nudibranch Janolus fuscus photograph of juvenile nudibranch Janolus fuscus feeding on a bryozoan zooid photograph of juvenile of nudibranch Janolus fuscus crawling
Trochophore stage showing ciliated prototroch for propulsion and feeding Pre-competent veliger showing regions of gut, partially retracted mantle fold, and velum Post-metamorphic juvenile eating the lophophore tentacles of a bryozoan 8d juvenile crawling. Note the developing cerata and rhinophores
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