Echinoderms piggybacking on sea cucumbers: benign effects on sediment turnover and movement of hosts

Sea cucumbers (Holothuroidea) are known to host ectocommensal animals but echinoderm epibionts have never been reported nor their effects on hosts appraised quantitatively. At one location in New Caledonia, we found a high number of ophiuroids (Ophiothela cf. danae) and synaptid sea cucumbers (Synaptula media and Synaptula sp.) living on the bumpy external body wall of sea cucumbers, Stichopus herrmanni. Rates of midday sediment defecation (mean: 23 g h⁻¹) and short-term displacements (mean: 60 cm h⁻¹) by the hosts were not significantly affected by the number of echinoderm commensals they carried. The frequent relationships at the location suggest that such facultative commensalism between echinoderms could be more common than previously understood. Appreciable numbers of scaleworms, crabs and shrimps on the sea cucumbers show that sea cucumbers can act as hosts to echinoderms and other epibiotic taxa, bolstering the notion that they play important ecological roles in reef ecosystems.