IONIC CONTROL OF SETTLEMENT AND METAMORPHOSIS IN LARVALHALIOTIS RUFESCENS(GASTROPODA)

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Abstract
An increase in K+ concentration in defined sea water medium is shown to induce settlement and metamorphosis in larvae of the marine gastropod mollusk, H. rufescens. A decrease in external K+ concentration can inhibit larval response to GABA, stereochemically specific inducer of metamorphosis in H. rufescens. Stimulation of the metamorphic response by GABA or by increased K+ may depend on transmembrane movement of ions, since induction is sensitive to neuropharmacological blockers of ion conductance. Sulfonylisothiocyanostilbene (SITS, an anion exchange blocker) inhibits the larval response to GABA, but does not affect induction by increased external K. Larval response to K is inhibited by tetraethylammonium (TEA, a K channel blocker), and induction of metamorphosis by GABA is independent of the presence of TEA. Most concentration manipulations of other predominant cation components of sea water are not inductive or inhibitory. The actions of GABA and increased K+ as inducers are sensitive to changes in external Ca2+. K may act by directly depolarizing excitable cells involved in larval percepton of inductive stimuli. Activation of metamorphosis by GABA may depend similarly on a depolarizing ion movement at GABA-sensitive cells. Depolarization by manipulation of the ionic environment may offer a general technique for inducing metamorphosis in various marine invertebrate larvae.

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