Notes on animal associations. 4. The starfish, Pentaceros hedemanni (Lutken) and the Hesionid polychaete, Podarke angustifrons (Grube)

Abstract

The occurrence of commensals in echinoderms is well known and examples are too many to be listed here. Association between starfishes and polychaetes especially polynoids is common (Hyman 1955). Hesionids and syllids also have been recorded from echinoderms. While watching some specimens in the Institute's aquarium my attention was drawn to a Pentaceros hedemanni harbouring in one of its ambulacral grooves a polychaete which was subsequently identified as a hesionid, Podarke angustifrons (Grube). The worm was found to emerge from the grooves winding its way through the podia and move about in the adambulacral region, rarely proceeding beyond the infra-ambulacral region. The starfish was attached to the vertical wall of the aquarium and though the movement of the podia did not appear to disturb the polychaete, it reacted to any outside disturbance and retreated quickly to the ambulacral groove. The accompanying photographs (Plate I) show the polychaete in natural condition. Subsequently when broken open the starfish was found to harbour in all three specimens of P. angustifrons. Only very few of the starfish had the commensal and usually only one was on each, and never more than three.