APPARENT WATER PERMEABILITY AS A PHYSIOLOGICAL PARAMETER IN CRUSTACEANS

This article reviews the use of apparent water permeability (AWP) calc ulated from measurements of isotope-labelled water flux as a physiolog ical estimate of whole-body water permeability in aquatic invertebrate s. The rationale and practices of AWP calculations are described in an Appendix. AWP calculations have provided a wealth of information. How ever, the validity of the method and therefore also of the information obtained have been questioned. Consequently, the use of AWP data in d iscussions of osmotic and fluid homeostatic questions in aquatic inver tebrates is limited. This article reviews three decades of published e xperiments in which measurements of isotope-labelled water fluxes were used to estimate water permeability in aquatic invertebrates. Data on 24 species of arthropod, most of them decapod crustaceans, are presen ted. The combined data indicate that the results obtained by different investigators on the same species show good agreement, even though di fferent tracers and experimental methods have been applied. When avail able, results from other kinds of studies were used to evaluate the re sults obtained using the AWP measurements. The various results demonst rate that AWP is influenced not only by natural environmental factors, such as salinity and temperature, and by anthropogenic factors, such as potentially toxic trace metals, but that it is also regulated by in trinsic factors, such as ecdysis and life cycle stage. The results obt ained can often be explained as effects of components of the habitat o f the animal. Accordingly, studies on variations in AWP contribute to our understanding of the different physiological strategies used by sp ecies living in a changing environment. We conclude that calculations of AWP offer reliable, relevant physiological data in a range of crust acean species, as long as methodological limitations and uncertainties are kept in mind. In addition, we propose some possible new ways of a pplying AWP calculations to marine invertebrates other than crustacean s. A major part of this review describes results already obtained for the shore crab Carcinus maenas as this species is probably the animal on which most work has been carried out. We suggest topics for future work on this species and review the possibility of using AWP in C. mae nas as a biomarker of metal exposure.