Crude homogenates and plasma-membrane-enriched fractions were prepared from the epithelium of the gills, epipodites and branchiostegites of intermoult European lobsters Homarus gammarus, and Na+/K+-ATPase, Ca2+- ATPase and Na+/Ca2+ exchange activities were quantified

described as a stenohaline, non-regulating crustacean (Gilles, 1975; Péqueux, 1995). Indeed, this lobster may be considered to be an osmoconformer when living in sea water; however, when confronted with dilute sea water, the lobster hyperregulates its haemolymph osmolarity and by doing so it is able to survive salinities as low as 17 ‰ (Charmantier et al., 1984a,b). Because these animals may experience significant salinity changes in their natural habitat, such hyperregulatory activity is apparently of physiological significance. Clearly, Homarus gammarus is not a strictly stenohaline species as previously suggested. The branchial cavity of Homarus gammarus harbours three structures that could be involved in ion transport underlying hyperosmoregulatory activity (Haond et al., 1998): the (trichobranchiate) gills, the epipodites and the branchiostegites. All three structures, but in particular the epipodites and the inner side of the branchiostegites, bear epithelia with mitochondria-rich cells and elaborate plasma membrane invaginations that harbour abundant Na+/K+ATPase activity (Towle, 1984). Thuet et al. (1988) demonstrated increased Na+/K+-ATPase activity in a mixed preparation of gills and epipodites of postlarvae of H. gammarus transferred to dilute sea water, and this increase correlated with an increase in osmoregulatory capacity; since the gills and epipodites were pooled in their study, no conclusion could be drawn about the exact origin (i.e. gills, epipodites or both) of the increased activity. When we challenged H. gammarus with dilute sea water, hypertrophy of epithelia was observed in the epipodites and branchiostegite, but not in the gills. It was concluded, therefore, that the gills are not involved in the hyperosmoregulatory response (Haond et al., 1998), whereas the epipodites and branchiostegites could be. Lobsters, such as H. gammarus and 213 The Journal of Experimental Biology 203, 213–220 (2000) Printed in Great Britain © The Company of Biologists Limited 2000 JEB2281