ABSTRACT
Observations done between November 1992 and June 1993 showed that 2% of the individuals of Diadema setosum inhabiting Singapore waters were infested by a pedunculate barnacle of the genus Microlepas (from 1 to 3 Microlepas sp. per echinoid infested). Barnacles always occur at the top of blunt-tipped primary spines to which they attach through a pedunculate attachment organ penetrating into the spine skeleton. Infested spines were always shorter than non-infested ones. Removal of barnacles was never followed by the regeneration of previously infested spines. Section of an infested spine a centimeter below its tip is not followed by spine regeneration. These results show that barnacles inhibit the growth of the spine they infest and that this inhibition actually persists after removal of the symbiote.
