Is large body size always advantageous for terrestrial adaptation? A studyof water balance in a semi-terrestrial crab, Sesarmops intermedium (Crustacea : Grapsidae)

Sesarmops intermedium de Haan (1835) is a semi-terrestrial crab inhabiting the areas around a forest creek at Heng-chun Peninsula in southern Taiwan. Small and large individuals excavate tunnels near the waterline, while thos e individuals with medium body size are more distant from the water. This u nusual distribution pattern might reflect the habitat preference related to physiological requirement of crabs with different body sizes. In natural c onditions, the animals keep their haemolymph osmolality at a relatively low level near those of fully-rehydrated individuals. In dehydrated conditions , the haemolymph osmolality of the animal was much higher and was negativel y related to the body size. Both the rates of water loss and uptake decreas e with increasing body size. While, both the potential time exposed in air and the rehydration time increase with increasing body size. The low water loss rate of large individuals may prolong the potential time exposed in ai r, but this advantage may be offset by their low efficiency in water uptake simultaneously. Small crabs, with a high water uptake rate and a small amo unt of water requirement, may have a shorter time needed for rehydration. H owever, those small crabs, smaller than about 2 g in living weight, due to their extremely fast loss of body water, have to access water more frequent ly and thus have a lower potential for terrestrial activity also. For mediu m sized crabs, the advantages of high water uptake rate, less water require ment and better ability to utilize interstitial water may allow them to ema ncipate from the water body. Due to the constraints of performance in water balance, the optimum size for extending terrestrial activity of S. interme dium is below 14 g in weight (=28.5 mm in carapace width) at which the crab s may have a higher potential for terrestrial activity. In the terrestrial adaptation regarding the water balance of S. intermedium, how to increase w ater uptake efficiency and the capability to use various interstitial water on land seem to be more important than how to reduce the water loss rate. Large body size and hence the low water loss rate may not be always advanta geous for the terrestrial adaptation of S. intermedium and possibly other c rustaceans.