STUDIES ON SOME BIOLOGICAL ASPECTS OF SESARMA (DECAPODA, BRACHYURA) OF SINGAPORE

Abstract

1) A total of about 36 Sesarma species have been recorded from Singapore of which 2 are from freshwater far from the sea. 2) Keys were given for the identification of the species. 3) An ecological study has been made on these crabs in 5 selected stations on Seletar River, 3 at Mandai Forest Reserve and 1 at Bukit Timah Hill. 4) The nature of the various stations were briefly described. 5) Chemical and physical analysis of the stations revealed that salinity is a predominant chemical factor governing the distribution of crabs however, the ecological ranges of the species may be reduced by some other factors set within the limits of salinity; such as habitats preferences etc. 6) A comparison of the salinity ranges of the crabs suggests that they fall into three man groups; a) freshwater species with capacity for adaptation in soft water, b) from found only in low-salinity brackishwater and c) those capable of living in high-saline brackishwater. 7) An analysis has been carried out on five crabs on their osmoregulation when exposed to a wide salinity. It is concluded that three of the five species, the capability of osmoregulation acts as factor limiting their penetration into the mangrove swamp proper. Habitat preferences probably limit the penetration of two species into the low-saline region. 8) The capabilities of three species to withstand desiccation under 100% and 75-78% at temperature 27-29?C have been investigated. It is concluded that most of the crabs including the land-crabs could not survive under such condition at 75% relative humidity indefinitely and their survival in dry conditions are probably depended on microclimate.