Daily burrowing cycle and feeding activity of juvenile sea cucumbers Holothuria scabra in response to environmental factors

This study investigated the daily activities of juvenile sea cucumbers Holo thuria scabra Jaeger. All individuals exhibited daily burrowing and feeding rhythms in response to environmental factors. The cycle of the smallest ju venile, > 10-40 mm, was linked to light; they began to burrow around sunris e and emerged close to sunset. Their burrowing activity was inhibited by co ntinuous darkness. Juveniles >40-140 mm responded to changes in temperature ; they burrowed earlier around 03:30 h, seeking shelter in the sediment as temperature declined, and emerged around mid-day. The maintenance of a cons tant warm temperature prevented them from burrowing. For all juveniles, tim e spent on the surface corresponded with feeding and periods of locomotion; while burrowed they remained stationary, did not feed and had a low intest inal transit. Organic matter content in the intestine was also found to var y with the daily cycle. Decreases in salinity from 35 to 30, 25 and 20 part s per thousand induced the burrowing of all juveniles within minutes, but t hey began to re-emerge after a few hours. Acclimation occurred most rapidly at salinity 30 parts per thousand and was slowest at salinity 20 parts per thousand. Conversely, nearly 40% of the juveniles were unable to cope with a decrease to salinity 15 parts per thousand. Juveniles of all sizes demon strated a strong selectivity for sediment characteristics. In two substrate selection experiments, their preference for sand with a grain size around 0.4 mm and for organically rich material was firmly expressed within an hou r. The general behavior of juvenile H. scabra reflects their ability to inh abit shallow sandy areas with high terrigenous inputs and variable environm ental conditions. (C) 1999 Elsevier Science B.V. All rights reserved.