EXCESS POTASSIUM INDUCES LARVAL METAMORPHOSIS IN FOUR MARINE INVERTEBRATE SPECIES

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Abstract
An increase in the concentration of K+ in defined seawater medium induces settlement and metamorphosis in larvae of the marine molluscs Phestilla sibogae, Haliotis rufescens, and Astraea undosa, and in larvae of the marine annelid Phragmatopoma californica. The effect is dose-dependent, optimal at approximately double the normal concentration of K+ in seawater, and specific for the K+ ion. The ability of K+ to directly influence cell membrane potential is proposed as an explanation for its broad effectiveness as a metamorphic inducer for larvae that recruit to different habitats. Depolarization of externally accessible, excitable cells thus is suggested to be a mechanism common to the induction of settlement and metamorphosis of a number of species. For Phestilla and Haliotis, the inducive effect of excess K+ is additive with that of the substratum-derived inducers or analogs. The sensitivity of induction by K+ to external tetraethylammonium (TEA, a K+ -channel blocker) reported previously for Haliotis (Baloun and Morse, 1984) is not present in Phestilla or Phragmatopoma. Results presented here indicate that the addition of excess K+ may provide a widely useful technique for inducing metamorphosis, and for analyzing the mechanisms which govern this process, in other marine invertebrate larvae.