Settlement preferences and early migration of the tropical sea cucumber Holothuria scabra

Settlement and post-settlement processes of the sea cucumber Holothuria sca bra Jaeger were studied in the laboratory. Independent and paired choice ex periments revealed that several substrates could induce metamorphosis into pentactulae, but that specific substrates favoured settlement. Leaves of se agrass Thalassia hemprichii, with or without their natural bio-film, yielde d the highest settlement rates (4.8-10.5%). T. hemprichii was preferred as a settlement substrate over sand, crushed coral, several other plant specie s and artificial seagrass leaves with or without a bio-film. Only settlemen t on the seagrass, Enhalus acoroides, was similar to that recorded for T. h emprichii. In the absence of a substrate, the larvae delayed settlement for nearly 96 h and survival was less than 0.5%. Sand and crushed coral, eithe r alone or together, induced settlement from < 1.5% of the available larvae . The pentactulae found on sand, coral and in bare containers were 10-35% s maller than those on T. hemprichii leaves. Soluble extracts from T. hempric hii and E. acoroides successfully induced metamorphosis and settlement on c lean plastic surfaces. Newly settled juveniles remained on the seagrass lea ves for 4-5 weeks before migrating to sand at around 6 mm in length. Prior to this, the juveniles spent 4-5 days moving on and off the leaves. Once on the sand, the juveniles became deposit-feeders, but did not show the typic al burrowing behaviour of older specimens until they reached around Ii mm i n length. The larvae of H. scabra appear to actively select seagrass leaves , possibly through chemical detection. We hypothesise that larvae settling on seagrass have an increased chance of growth and survival because they ar e provided with a suitable substrate on which to grow, and a bridge to sand substrates as they become deposit-feeders. (C) 2000 Elsevier Science B.V. All rights reserved.