Circulatory, acid-base and respiratory responses of the purple shore crab Leptograpsus variegatus to immersion

Abstract

The supratidal shore crab Leptograpsus variegatus makes use of aquatic as well as terrestrial habitats. The respiratory responses of this species to 12 h immersion in 100% seawater and re-emersion were investigated. The major perturbation was in haemolymph CO 2, the PCO 2 declining to 2.2 torr after 2 h in water compared to >4 torr in air-breathing crabs. Similarly, CCO 2 decreased from 12.8 typical of air breathing to 5.0 mmol · l −1, more typical of water breathing species. This internal hypocapnia promoted an alkalosis which was immediately compensated by a metabolic acidosis that was not a product of anaerobiosis since l-lactate in the haemolymph remained low (0.14 mmol · l −1). When re-emersed, haemolymph CO 2 did not increase to pre-immersion values but, instead, evinced a smaller elevation after 3 h access to air. Control Leptograpsus that could choose between water and air showed intermediate but more variable haemolymph CO 2 values, suggesting that the animals ‘shuttle’ between air and water, behaviourally influencing internal CO 2 levels. The Leptograpsus exhibited both elevated heart and ventilation rates as well as increased oxygen consumption in water (max. 14.7 μmol · g −1 · h −1) compared to air (6.5 μmol · g −1 · h −1). Part of this increase in rate may be due to increased activity in submerged crabs. The arterial and venous PO 2(86 and 54 torr) and CO 2 (0.52 and 0.31 mmol · l −1) were unchanged by immersion. Re-emersion promoted a transient increase in venous haemolymph oxygenation with respect to the values after 2 h breathing air but not the initial value. The haemocyanin bound O 2, i.e. saturation, was also largely unchanged during immersion cf. air-breathing, which seems to be achieved not by modulation of the haemocyanin functioning but rather by maintaining a constant haemolymph environment. While immersion seems to increase respiratory demand, possibly via increased activity, this does not challenge respiratory gas exchange or transport. In contrast to species reported to date, Leptograpsus maintains extremely high PO 2 in both air and water. Further study of this truly amphibious crab may reveal more as to important intermediate conditions in the transition from water to life on land via the intertidal and supratidal habitats.