Air Breathing by the Purple Shore Crab,Hemigrapsus nudus(Dana). III. Haemocyanin Function in Respiratory Gas Transport

Abstract

The functional properties of the haemolymph and haemocyanin (Hc) of Hemigrapsus nudus were investigated with respect to the amphibious behaviour of this crab. The O₂ affinity of the Hc was decreased at higher temperature (∆H = −30 to −40 kJ mol⁻¹). Haemocyanin-O₂ affinity was sensitive to pH (Bohr factor = −0.843 at 15° C) but essentially insensitive to L-lactate (Δ log P50}/Δ log [lactate] = −0.053). The pH sensitivity of Hemigrapsus Hc was dependent on temperature (Bohr factor = −0.44 at 5°C, −0.90 at 25° C). Increasing CO₂ had no specific effect, nor was Hc-O₂ affinity increased by increasing levels of dopamine. Urate markedly increased the O₂ affinity of Hemigrapsus Hc (Δ log P50/Δ log [urate] = −0.203), and together with H+, it would seem to be the major organic effector of Hc-O₂ affinity. Increasing haemolymph Ca both potentiated Hc-O₂ affinity and decreased the magnitude of the Bohr shift. The potentiating effect was more pronounced at lower than normal Ca concentration, while Mg had little effect. The CO₂ capacitance of the haemolymph was more dependent on temperature than could be accounted for by the reduction in CO₂ solubility at higher temperature (CO₂ capacitance = 0.44-0.91 mmol L⁻¹ mmHg⁻¹). Consistent with a relatively low haemolymph protein concentration, the nonbicarbonate buffer was low (Δ[]/ΔpH = − 4.80), and thus changes in H+ concentration would promote large changes in pH and Hc-O₂ affinity. A Haldane effect was detected at 25° C Relating these data to in vivo measurements showed that acid-base balance in Hemigrapsus emerging from 10° C water into 25° C air was likely managed separately from CO₂ excretion. In crabs breathing 10° C water, the Hc transported 50% of the used O₂, while in animals breathing 25° C air the amount of O₂ so transported rose to 85%. Emersion-induced acidosis initially promoted a large decrease in O₂ affinity, reduced oxygenation, and promoted a decrease in the arterial-venous O₂ content difference. However, a subsequent metabolic alkalosis and an elevation in haemolymph urate reestablished the transport role of the Hc. Thus the pH sensitivity, especially at high temperatures, and the effect of urate seem crucial to the amphibious behaviour of Hemigrapsus, whose modulator systems are quite unlike those found in land crabs. Further work is required to determine whether these features represent a transitional stage between water and air breathing or whether they are specific adaptations to the supratidal environment or indeed are peculiar to grapsid crabs.