Competence and metamorphosis in the long-term planktotrophic larvae of the nudibranch 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 competence has been established by inducing metamorphosis with the hydroid Tubularia crocea. The competence period lasts approximately three weeks at 12-17 degrees C, the best time for inducing metamorphosis being from days 31-50 within a given egg mass, and more precisely from day 41 to 50, as concluded from several egg masses. Within-clutch and between-clutch variability exists in both Tubularia-induced and spontaneous metamorphosis. Variability within egg masses in Hermissenda is probably due to the gradual acquisition of metamorphic competence. Spontaneous metamorphosis increases significantly as larvae age within a given egg mass. Metamorphosis is non-specific, since the larvae are able to metamorphose with several natural inducers, such as other hydroids (Pennaria sp.) and anemones (Metridium senile and Haliplanella luciae). Combinations of Tubularia with glutamate at 10(-4) M or with Mg2+ at 20 mM significantly increased the metamorphic yields, while NH4+ was ineffective. 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 inducers and some cations also produced high metamorphic yields, up to 25%. The effect of epinephrine at 10(-5) M in increasing the metamorphic yields, when used in combination with Tubularia, was equal to that obtained when using aged epinephrine, indicating that the oxidized products of epinephrine are probably responsible for this activity. Increasing the temperature from 12 degrees C up to 17 degrees C also had a significant effect in increasing the metamorphic yields with several inducers. The extracts of Tubularia with several organic solvents did not have an inducing effect. The natural inducer 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 rights reserved.