Is Large Body Size always Advantageous for Terrestrial Adaptation?

Abstract

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 those individuals with medium body size are more distant from the water. This unusual distribution pattern might reflect the habitat preference related to physiological requirement of crabs with different body sizes. In natural conditions, 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 negatively related to the body size. Both the rates of water loss and uptake decrease 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 air, 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 amount of water requirement, may have a shorter time needed for rehydration. However, 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 frequently and thus have a lower potential for terrestrial activity also. For medium sized crabs, the advantages of high water uptake rate, less water requirement and better ability to utilize interstitial water may allow them to emancipate from the water body. Due to the constraints of performance in water balance, the optimum size for extending terrestrial activity of S. intermedium is below 14 g in weight (=28.5 mm in carapace width) at which the crabs may have a higher potential for terrestrial activity. In the terrestrial adaptation regarding the water balance of S. intermedium, how to increase water 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 advantageous for the terrestrial adaptation of S. intermedium and possibly other crustaceans.