Aspects of sodium ion and water regulation in two species of Corophium (Crustacea: Amphipoda), Corophium curvispinum (var. devium) and Corophium volutator.

Corophium curvispinum is an E. European brackish-water species. A freshwater variety (devium) has recently invaded English waterways. Euryhaline Corophium volutator characteristically inhabits muddy estuaries. Rates of unidirectional trans-integumental fluxes of 22Na and 3H2O, medium imbibation rate, body fluid composition and trans-epithelial electrical potential of both species were studied in relation to environmental salinity. Corophium curvispinum marker clearance rate and ionic transport numbers of Corophium volutator integument were also determined. Both species are hyperosmotic regulators in media of low solute content. The major permeable areas of Corophium integument are the gills. Diffusional water permeability (Pd) of 15% S.W. acclimated Corophium volutator, similar to Pd of freshwater Corophium curvispinum, is lower than Pd of 85% S.W. acclimated Corophium volutator. Use of Pd as a measure of integumental water permeability is discussed. Sodium permeability (PNa) Corophium volutator, independent of acclimation salinity, is higher than PNa of Corophium curvispinum. Both species elaborate hypoosmotic urine when hyperosmotically regulating, Corophium curvispinum exhibiting markedly the greater urine-diluting capacity. Saturable sodium transport systems are demonstrated in both species, and are characterised by Vmax and Km. of Corophium curvispinum is markedly lower than that of Corophium volutator, whilst Vmax are similar. Differences between PNa, Vmax and Km of Corophium curvispinum from different field localities are described. Use of models (Goldman, electrical analogue) to describe ion movements across Corophium integument is discussed. Transient restriction of haemolymph circulation through Corophium volutator gills, following animal transfer to hyperosmotic salinities, is described. Structure of the antennary (excretory) organ of Corophium curvispinum is described, and contrasted with that of Corophium volutator. The adaptation of Corophium (curvispinum) to fresh water is discussed. It is suggested that contrasting sodium-regulatory capacities of Corophium curvispinum from different localities reflect evolutionary adaptation to fresh water presently occurring in this species. Osmoregulatory adaptations of Corophium volutator to fluctuating estuarine salinity are considered.