OXYGEN-BINDING BY AMPHIPOD (CRUSTACEA) HEMOCYANINS

Haemocyanin (Hc) occurs in the haemolymph of all gammaroidean amphipods examined to date. Amphipod haemolymph is characterised by its comparatively low protein content, much of which is Hc, resulting in a low O2 carrying capacity. However, Hc has been shown to function as a respiratory pigment in vivo in at least some of the larger species during sustained activity. Amphipod Hcs seem to exist as hexamer aggregates. The number and types of subunit(s) present has still not satisfactorily been determined. The functional properties of amphipod Hcs are qualitatively similar to those of decapod and isopod crustaceans. They possess a moderate to large Bohr effect. Hc O2 affinity tends to be moderate in gammarid amphipods but higher than found in talitrid amphipods. Temperature affects O2 binding by some Hcs but not others: such effects could not be correlated with the ecology of all of the species examined. The colonisation of land by talitrid amphipods is accompanied by a reduction in both Hc O2 affinity and the Bohr effect together with a decrease in Hc effector sensitivity. The Bohr effect for Hc for semi-/euterrestrial amphipods was remarkably similar (DELTA log P50/DELTA pH = -0.77 +/- 0.06) and consistently smaller than that observed for Hc from aquatic species (DELTA log P50 DELTA pH = -1.44 +/- 0.20). It was not possible to relate the functional differences in O2 binding evident between Hcs from aquatic (gammarid) and Hcs from semi-/euterrestrial (talitrid) amphipods to Hc structure. Similarly relating changes in Hc O2 binding to changes in respiratory exchange organs is difficult until we know the extent to which extrabranchial gas exchange occurs.