FORM AND FEEDING MECHANISM OF A LIVING PLANCTOSPHAERA-PELAGICA (PHYLUM HEMICHORDATA)

We describe aspects of the anatomy and suspension-feeding mechanism of a single Planctosphaera pelagica captured from the plankton in June 1 992 off Bermuda in the western Atlantic. We also describe several unus ual features of the larva, including its occurrence in surface waters, unusually large size, and limited swimming ability. Our account of th e form and feeding behavior of P. pelagica is the first based on obser vations of a specimen captured and observed alive. Our limited observa tions suggest that the planctosphaera may use a suspension-feeding mec hanism much like that of the other feeding deuterostome larvae (the pl uteus and bipinnaria larvae of echinoderms and the tornaria larva of e nteropneust hemichordates) known to capture food particles using a sin gle ciliated band. Although we could not observe cilia directly, the m ovement of dye streams and food particles and the structure of the cil iated band suggest that some particles may be captured at the ciliated band by the reversal of ciliary beat. The planctosphaera possesses ma ny prominent mucous glands near the food grooves. This suggests an imp ortant role of mucus in the biology of the larva, but we were not able to observe directly any role of mucus in particle capture.