Induction of metamorphosis in Hermissenda crassicornis larvae (Mollusca: Nudibranchia) by GABA, choline and serotonin

The nudibranch mollusc Hermissenda crassicornis is currently used as a biomedical model in neurobiological studies. It possesses planktotrophic larvae which metamorphose in the laboratory in the presence of the hydroid Tubularia crocea in yields of about 2-5% (Tamse et al., 1990). This study presents evidence that artificial metamorphic inducers such as GABA (gamma-aminobutyric acid), choline, and serotonin (alone or combined with the natural inducer T. crocea), can be used to improve the metamorphic success in H. crassicornis larvae. GABA at 10(-5) M and 10(-4) M, choline at 10(-3) M and 10(-4) M, and serotonin at 10(-5) M and 10(-4) M, were most effective, while serotonin at 10(-3) M was toxic. Larvae 47 and 63 days old metamorphosed at different rates when exposed to the same concentrations of chemicals, but these had no positive effect on young larvae, i.e., 28 and 39 days old. On the other hand, when young larvae were exposed to the natural inducer, an habituation phenomenon was observed, i.e., the larvae did not metamorphose. Exposing the larvae to the natural inducer after larval day 47 improved the metamorphic yield by about 100%. These results indicate that the optimum larval age for inducing metamorphosis in H. crassicornis is older than previously considered.