LARVAL METAMORPHOSIS OF THE OPISTHOBRANCH MOLLUSK ADALARIA-PROXIMA (GASTROPODA, NUDIBRANCHIA) - THE EFFECTS OF CHOLINE AND ELEVATED POTASSIUM-ION CONCENTRATION

Veliger larvae of the nudibranch mollusc Adalaria proxima are triggered to metamorphose to the benthic form by the adult prey bryozoan, Electra pilosa. Ion substitution and supplementation experiments with artificial sea water (ASW) have, however, shown that metamorphosis can be induced by elevation of the potassium ion concentration alone. Approximately 19 mM K+ ASW (10mM 'excess') was found to elicit maximal metamorphic responses: 29 and 39 mM K+ ASW had no inductive effect. Choline chloride was also found to induce metamorphosis in a dose-dependent manner, with 10-4 M ineffective, 10-3 M approximately threshold, and 5x10-3 M to 10-2 M optimal. Concentrations of choline > 10-2 M were sub-lethally toxic. That the absence of larval metamorphosis on exposure to 29 and 39 mM K+ ASW was due to inhibition is inferred from interaction experiments with choline: at these concentrations of K+, metamorphosis in response to choline could be abolished. Timed-exposure experiments indicated that artificial induction elicited by elevated K+ and choline involve either separate mechanisms, or different parts of the same pathway. Thus, whilst larvae required to be continuously exposed to 19 mM K+ ASW in order to complete metamorphosis, those exposed to 10-2 M choline would subsequently complete development in normal ASW following only 1-2 h exposure to the inducer. Preliminary experiments failed to specify further the nature of the natural inducer, beyond the confirmation that live intact colonies of the bryozoan Electra pilosa will trigger larvae to metamorphose.