INDUCTION OF METAMORPHOSIS IN LARVAE OF THE OYSTER CRASSOSTREA-GIGAS USING NEUROACTIVE COMPOUNDS

Chemical (neuroactive compounds at different concentrations and exposure times) and physical (water agitation, light) factors with potential effects on the metamorphosis of larvae of the oyster Crassostrea gigas (Thunberg) larvae have been studied. The neurotransmitters L-dihydroxyphenylalanine (DO), epinephrine (EP), norepinephrine (NE), and acetylcholine (AC) have been identified as very active inducers of metamorphosis, whilst serotonin (SE), dopamine (DA) and potassium (K) were less effective inducers. The gamma-aminobutyric acid (GA) and ammonium (AM) were found ineffective at the concentrations teste:d. Exposure to 10(-4) M EP for 15 min was sufficient to promote > 80% metamorphosis within 48 h, whereas NE required 2 h to exert comparable induction. Maximum induction by DO ( > 50%) was achieved after 2 h exposure to 10(-4) M. However, unlike EP and NE, DO was lethal at that concentration in the long term. Maximum induction by AC ( similar to 30% metamorphosis) was achieved at a concentration of 10(-4) M. In. contrast to other neurotransmitters, AC induced settle ment behaviour, cementation and eventual metamorphosis, yielding postlarvae which were all attached to the substratum. EP and NE triggered the morphogenetic changes, by-passing settlement and leading to postlarvae not cemented to the substratum. DO induced mostly attached spat at low concentrations (10(-5) M) and unattached spat at high concentrations (10(-4) M), and a similar pattern was apparent for the weaker inducers SE and DA. Regarding physical factors, a highly reflectant surface significantly increased the percentage of attached spat obtained, compared to a dark bottom. No consistent effect of water current or light was detected on the production of unattached spat. The three different forms of induction are discussed in relation to different regulatory pathways of settlement and metamorphosis.