Buoyancy in Porcellana platycheles

Intermoult Porcellana platycheles (Pennant) are slow moving, crawling crabs, incapable of swimming upwards from the substrate. Newly moulted porcelain crabs, however, are capable of effective upward swimming. An increased efficiency of tail-fan propulsion in newly moulted crabs was thought unlikely, and it was suspected that newly moulted P. platycheles were more buoyant than intermoult crabs. Results are presented to show that this is the case, and calculations were performed which indicate that fluid uptake at moult is insufficient to produce the observed increase in buoyancy. It is concluded that loss of heavy minerals and scleroprotein at moult cause the increased buoyancy. The adaptive significance of the enhanced swimming ability in newly moulted crabs is thought to lie in an improved ability to escape predation by intertidal fish and crabs.