Competence and metamorphosis in the long-term planktotrophic larvae of thenudibranch mollusc Hermissenda crassicornis (Eschscholtz, 1831)

Hermissenda crassicornis is a eurytrophic carnivorous opisthobranch mollusc that possesses long-term planktotrophic larvae. No morphological criteria for determining larval competence in this slug are reliable; thus competenc e has been established by inducing metamorphosis with the hydroid Tubularia crocea. The competence period lasts approximately three weeks at 12-17 deg rees C, the best time for inducing metamorphosis being from days 31-50 with in a given egg mass, and more precisely from day 41 to 50, as concluded fro m several egg masses. Within-clutch and between-clutch variability exists i n both Tubularia-induced and spontaneous metamorphosis. Variability within egg masses in Hermissenda is probably due to the gradual acquisition of met amorphic competence. Spontaneous metamorphosis increases significantly as l arvae age within a given egg mass. Metamorphosis is non-specific, since the larvae are able to metamorphose with several natural inducers, such as oth er hydroids (Pennaria sp.) and anemones (Metridium senile and Haliplanella luciae). Combinations of Tubularia with glutamate at 10(-4) M or with Mg2at 20 mM significantly increased the metamorphic yields, while NH4+ was ine ffective. K+ at 30 mM and Cs+ at 1 mM also increased the metamorphic yields , although Cs+ at 30 mM was toxic. Several combinations of the natural indu cers and some cations also produced high metamorphic yields, up to 25%. The effect of epinephrine at 10(-5) M in increasing the metamorphic yields, wh en used in combination with Tubularia, was equal to that obtained when usin g aged epinephrine, indicating that the oxidized products of epinephrine ar e probably responsible for this activity. Increasing the temperature from 1 2 degrees C up to 17 degrees C also had a significant effect in increasing the metamorphic yields with several inducers. The extracts of Tubularia wit h several organic solvents did not have an inducing effect. The natural ind ucer from Tubularia seems to be a water-soluble compound. Aqueous extracts of Tubularia at high concentrations (about eight-times the normal amounts) produced yields up to 50+/-8.3% metamorphosis, i.e., five-times the maximum yields ever reported for Hermissenda. (C) 1998 Elsevier Science B.V. All r ights reserved.