STUDIES ON THE EXTERNAL MORPHOLOGY, LOCAL DISTRIBUTION, AND BIONOMICS OF THE SPIDER CRAB HYASTENUS SEBAE BUITENDIJK (DECAPODA, BRACHYURA) AT SINGAPORE

Abstract

Adult Hyastenus sebae lives in the littoral zone where the higher water temperature favours a continual breeding activity. The juveniles live in the outer zone which is possibly favourable for moulting, as the salinity is less affected by the weather and land drainage. Hyastenus sebae are present on Enhalus aceroides covered with a thick deposit of detritus. The blades of this plant provide suitable accommodation for the animals moving or still. Sedentary most of the time, Hyastenus sebae is sluggish when it moves. Except for the chelipeds, its limbs are adapted for locomotion on the blade, and at the same time for securing the animal while it moves. Its steady and determined gait is necessary to cope with the resistance and turbulence of the medium in which it lives. This gait uses a powerful gear, which is also suitable for 7 slow progression over the ground. The incapacity for swift movement does not raise any food problem as Hyastenus sebae does not have to hunt. Hyastenus sebae feeds mainly on vegetable detritus, but algae is also a common food item for Hyastenus sebae in different localities. However, the quantity as well as the genus of the algae consumed vary according to their availability and abundance in each locality. Hyastenue sebae, however, is an omnivore that has a restricted diet because its sluggishness has prevented it from scavenging in a wider field. Hyastenus sebae is not an easy prey for other animals, as its sluggishness allows vegetable detritus to deposit on its body, or barnacles or algae to grow on it thus blending it with the background. These are effective camouflage against detection by predators. Despite its camouflage Hyastenus sebae runs the risk of detection when it feeds with the movement of its mouthparts and chelipeds. Feeding time is considerably reduced by relieving the mouthparts of masticatory function, which is performed by the gastric mill. This organ enables the animal to ingest as big a piece of food as the mouth permits, and to cope with the tough food that is available, Hyastenus sebae breeds continually. The mature crabs are capable of mating at any time after the moult of puberty. A large number of eggs is laid at one time, and this offsets the high mortality among the larvae. The eggs are carried on the pleopods beneath the abdomen until they hatch. Larvae hatch out at the end of fourteen days. Hatching lasts about three hours and occurs at night for safety from predators. The growth of the body and limb characters is allometric in both sexes. Positive allometry is more marked in the breadth of the abdomen of the female, and in the chela-propodus length of the male which results in the most obvious male characteristic viz. the distinctly larger cheliped. The absence of moulting signs among the mature population indicate that the moult of puberty is also the terminal ecdysis for Hyastenus sebae.