Changes in apparent water permeability during the moulting cycle in the western rock lobster

Abstract

The kinetics of water exchange, as measured by 3H 2O fluxes, were examined in the western rock lobster, Panulirus longipes (Milne Edwards) during stages C 4, D 3, D 4, D 4-E and B 1 of the moulting cycle. A series of samples of blood and external water was taken during each experiment and the inward and outward rate constants found using the SAAM 25 computer program, assuming a simple 2-compartment model with reversible exchange. Both inward and outward turnover rates remained constant up to the swelling prior to ecdysis when there was an increase of 203% inwards compared with 133% outwards. The latter increased in stage B 1, so that both rates were comparably high, probably due to the increased permeability of the soft integument. Further analysis showed that the rock lobster does not behave as a single compartment with respect to tritiated water. Two compartments were resolved by graphical analysis, one with fast and the other with slow exchange. It is suggested that blood and tissue ‘free’ water comprise one compartment and chemically ‘bound’ water the other, with fast exchange between the free and bound water and relatively slow exchange between free body water and the external medium.