An in situ study of respiratory variables in three British sublittoral crabs with different routine rates of activity

Abstract

Abstract To assess the respiratory physiology of marine crabs in situ, velvet swimming crabs, Necora puber, edible crabs, Cancer pagarus, and spider crabs, Maja squinado were sampled in the sublittoral zone by subaqua divers. Measurements included routine rates of oxygen uptake (ṀO2) and analysis of oxygen levels and acid–base variables of haemolymph samples withdrawn from crabs immediately after capture and analysed on the shore. Samples were then transported back to the laboratory for analysis of ion and metabolite levels along with in vitro determinations of maximum oxygen carrying capacity, protein and haemocyanin concentrations. Mean values for the major respiratory variables seemed to depend on the routine rates of activity of each species, with N. puber, an active swimming crab, characterised by the highest ṀO2, circulating oxygen levels, and oxygen carrying capacities. Haemolymph PCO2 values and lactate levels were the lowest measured, indicating high ventilation rates and a lower anaerobic component to metabolism. In comparison, M. squinado, a relatively inactive, slow moving crab, had the lowest ṀO2, haemolymph oxygen levels, and oxygen carrying capacity. Haemolymph PCO2 values were the highest, indicating a low ventilation rate while lactate levels were relatively high indicating a high level of anaerobic metabolism. The relatively slow moving, crepuscular crab, C. pagurus had intermediate values for all the variables studied. Despite differences in absolute levels of oxygen carrying capacity the majority of oxygen transported in the haemolymph was bound to haemocyanin in all three species, with high levels of % saturation in both post (80–85%) and prebranchial (50–63%) haemolymph. The amount of oxygen delivered to the tissues was proportionally higher in N. puber reflecting the high oxygen demand and aerobic scope of this species. There was a close positive correlation between maximum oxygen carrying capacities and protein, Cu2+ and haemocyanin levels in all three species. Circulating [Mg2+] levels reflected interspecific differences in routine activity with lower [Mg2+] values in N. puber compared to C. pagurus and M. squinado. In general, the respiratory variables obtained from animals sampled in situ differed from the measurements taken from crabs held in the laboratory. This investigation emphasises the importance of in situ studies to characterise routine levels of oxygen uptake, oxygen transport and acid–base status in crabs.