Size-dependent predation on juvenile sandfish, <i>Holothuria scabra</i> by seagrass-associated crabs

Abstract

High mortality of juvenile <i>Holothuria scabra</i> in seagrass nursery areas has been attributed to predation. In this study, laboratory and field experiments were conducted to determine the effect of the size of two seagrass–associated crabs, <i>Thalamita crenata</i> and <i>Dardanus megistos</i>, on the survival of juvenile sandfish at different sizes (1–10 g). Small juvenile sandfish (1–2 g) offered in field trials were more preferred by small<i> Thalamita</i> and large <i>Dardanus</i>. Non-lethal damage caused by both crabs were also evident, indicating differences in the mode of attack and consumption. The incidence of non-lethal wounds in medium and large juveniles in the laboratory indicate higher incidences of unsuccessful attacks by the crabs. Differences in consumption rate may be related to cheliped morphology and handling techniques, and activity patterns affecting encounter rates. Mortality rates of juvenile sandfish due to <i>T. crenata</i> (CW: 4.30–7.05 cm) ranged from 10–43%, while those by <i>D. megistos</i> (SL: 0.90–2.3 cm) was even higher at 34–53% after 24 h trials. This is the first report that a hermit crab <i>D. megistos</i> is a significant predator of juvenile sandfish. Results provide new insights on crab–soft-bodied prey interactions in tropical seagrass areas. These are also important considerations in stock restoration and ocean-based culture. Given that juveniles (≤10 g) are very vulnerable to predation by these common crab species, growing sandfish to larger sizes is essential prior to release in the field.