MANIPULATING EXTERNAL CA2+ INHIBITS PARTICLE CAPTURE BY PLANKTOTROPHIC ECHINODERM LARVAE

Planktotrophic larvae of echinoderms feed by filtering suspended particles from seawater using a ciliary reversal mechanism. The control of ciliary beat depends on calcium ion flux across the membranes of ciliated cells in many organisms. In an earlier study, T. H. J. Gilmour (1985. Can. J. Zool. 63: 1354-1359) observed that the calcium channel blocker verapamil failed to prevent particle capture by echinoid larvae, and he interpreted this result as indicating that ciliary reversals are not important in particle capture by these larvae. I have also found that echinoid and asteroid larvae feed normally (and have high rates of clearance) in concentrated solutions of verapamil in seawater, but particle captures by these larvae are qualitatively identical to those by the same larvae before exposure to verapamil and they appear to occur by ciliary reversal. Clearance rates are greatly depressed, however, for larvae in Ca2+ -free artificial seawater and for larvae in artificial seawater containing 9 mM Co2+. Most particle captures under these conditions do not appear to occur by the reversal of ciliary beat. These observations suggest that verapamil fails to block calcium channels in the membranes of ciliated cells of echinoderm larvae, but that other methods of preventing transmembrane Ca2+ flux do interfere with ciliary reversals and particle capture.