Influence of flow on locomotion, feeding behaviour and spatial distribution of a suspension-feeding sea cucumber

Although movement in response to environmental conditions represents a fundamental link between animal behaviour and population ecology, it is rarely investigated in suspension feeders because they are generally perceived as sessile. Here, the interplay between water flow and fine locomotor and feeding behaviours was experimentally investigated for the first time in a free-moving suspension-feeding sea cucumber (Cucumaria frondosa; Echinodermata, Holothuroidea) using time-lapse videography in a mesocosm setting. Individuals moved away from static conditions in the weakest flow treatment and fled the strongest flows (>40 cm s^-1) in the more dynamic treatments. The tentacles of individuals located in areas with flows of ≥40 cm s^-1 was aligned with the direction of the current, whereas in flows <40 cm s^-1, they were typically perpendicular to the direction of flow. Tentacle deployment and insertion rates (i.e. feeding rate) increased with flow, from 0.95 min^-1 at 10 cm s^-1 to 1.13 min^-1 at 40 cm s^-1 Three modes of locomotion were detected. Forward crawling was most frequent at flows ≤40 cm s^-1, passive rolling dominated at flows >40 cm s^-1 and active rolling occurred randomly at flows between 0 and 120 cm s^-1 Overall, the flow regime favoured by C.frondosa was determined to be between 21 and 40 cm s^-1, under which an optimal balance between efficient food capture and energy expenditure for attachment to the bottom was presumably found. These findings provide insight into the distribution and population dynamics of suspension-feeding holothuroids, and may also assist the fisheries management and aquaculture development of commercial species.