Oxygen-binding by haemocyanins from an ecological series of amphipod crustaceans

Abstract

The oxygen-binding properties of haemocyanins (Hc) from three species of gammaridean amphipods, Gammarus locusta (L.) (subtidal), Echinogammarus pirloti (Sexton and Spooner), (intertidal, marine) and E. marinus (Leach) (intertidal, estuarine), one species of hyalid amphipod Hyale nilsonni Rathke (high intertidal, marine) and the talitrid amphipod Orchestia gammarellus (Pallas) (semi-terrestrial) have been studied. All the species were collected from the Firth of Clyde, Scotland, during the spring of 1992. The oxygen-carrying capacity of haemolymph from each species was low, although variable, and was correlated with the low concentration of Hc present. The Hc oxygen-affinity of native gammarid haemolymph was relatively high [partial pressure of oxygen required for half-saturation, P50=4 to 5 torr (0.53 to 0.67 kPa)] at their respective in vivo pH values. At equivalent pH, however, Hc from G. locusta displayed a lower O2-affinity than either Echinogammarus species. Gammarid Hcs had a large Bohr effect (Δ log P50/Δ pH=-1.16 to-1.47). Resuspended Hc isolated from whole H. nilsonni showed similar O2-binding properties to those of the gammaridean amphipods [P50=6.3 torr (1.44 kPa) at pH=8.0; Δlog P50/ΔpH=-1.20]. Comparable data for haemolymph from O. gammarellus showed that the Hc had a lower affinity for O2 [P50=14.1 torr (1.87 kPa) at in vivo pH] and exhibited a more moderate Bohr effect (Δ log P50/Δ pH=-0.79). To eliminate the possibility that these differences were due to the different haemolymph constituents, each of the Hcs were pelleted and resuspended in physiological saline. The differences noted above persisted, demonstrating that they were due to inherent O2-binding properties of the Hc molecules themselves. An increase in L-lactate resulted in an increase in Hc oxygen-affinity for both Echinogammarus species but not for O. gammarellus. This study has confirmed that there is a clear difference between Hcs from aquatic and semi-terrestrial amphipod genera. The results lend further support to the hypothesis that the move on to land by amphipod crustaceans is accompanied by a decrease in Hc oxygen-affinity, a decrease in the Bohr effect and a decrease in effector (in this case L-lactate) sensitivity.