Periodic movement, recruitment and size-related distribution of the sea cucumber Holothuria scabra in Solomon Islands

Abstract

Field studies of the sea cucumber Holothuria scabra conducted in Kogu Veke, Solomon Islands, showed monthly recruitment of newly-settled juveniles on seagrass and indicated that size distribution was a function of substratum type and depth. Adults >250 mm body length were found mainly on sand, with <5% organic matter (OM), at depths of >1–3 m. Individuals >10–250 mm were found mostly in 30–120 cm of water, on mud and muddy sand with OM content between 5 and 10%. Specimens >40–150 mm were also found in the intertidal zone, sometimes burrowed on exposed sandflats at low tide. Holothuria scabra avoided substrata of fine silt or shell and coral pebbles, and sediment with an organic content ≥30%. Juveniles ≤100 mm burrowed at sunrise and surfaced at sunset, whereas individuals >100 mm burrowed and surfaced a few hours earlier. Holothuria scabra tended to burrow when salinity decreased, whereas increased water temperatures reduced normal burrowing behaviour. Spatial distribution, observed during tank experiments, suggested that adult H. scabra aggregated prior to spawning and in response to the lunar cycle. The formation of pairs, trios or larger groups increased during the new moon and was most common just before the full moon. Newly-settled juveniles up to ca. 9 mm were found on seagrass leaves. Typically, maximum densities and smallest recruits were observed a couple of weeks after the full moon, lower densities and slightly larger recruits were found a few days later. Juveniles with a mean length around 65 mm released on sand moved less and grew faster than juveniles released in seagrass beds or on substrata of shells and crushed coral.