Emergence behavior of juvenile Tachypleus tridentatus under simulated tidal conditions in the laboratory and at two different sediment temperatures

Abstract

Previous local studies estimated that the densities of emerged, feeding, juvenile Tachypleus tridentatus (prosomal width = 17.1-36.9 mm), obtained from surface counts at Ha Pak Nai, Hong Kong, varied from 4 individuals·1,000 m-2 in summer (July-September) to 0 individuals·1,000 m -2 in winter (December) 2002. To determine if such figures reflected true densities, juveniles were kept in tanks with sediment from the nursery ground at temperatures of between 15-20°C (winter) and 25-30°C (summer) under simulated tidal cycles. After a week's acclimation, their emergences were recorded, as was the depth of sediment to which they burrowed. No individuals emerged under imitated conditions of low tide at winter temperatures whereas 23% emerged at summer ones, indicating that sediment temperatures override circatidal activities when they fall below 20°C. The estimated abundance of juveniles on a nursery beach in the summer of 2002 should therefore be 4.16/0.23 = 18 individuals·1,000 m-2. During the imitated low tide, nearly all juveniles, which did not emerge at the substratum surface, buried themselves to a depth of <3 cm, irrespective of sediment temperature. Our results also showed that only 5% of the tested juveniles, regardless of temperature, were ever identified above the substratum during high tides. Overall, the present study confirms that field estimations of juvenile T. tridentatus abundance should include temporal patterns because emergence varies with temperature and tidal state. © Springer Science+Business Media, LLC 2009. All rights reserved.