Feeding and digestion in low salinity in an osmoconforming crab, Cancer gracilis. I. Cardiovascular and respiratory responses.

The osmoregulatory physiology of decapod crustaceans has received extensive attention. Within this field there is a growing body of literature on cardiovascular and respiratory responses to low salinity. Most species exhibit a tachycardia coupled with an increase in ventilation rate and oxygen uptake. However, these previous experiments were conducted on animals that were starved prior to experimentation in order to avoid increases in metabolism associated with digestive processes. Because organisms are not necessarily starved prior to experiencing environmental perturbations, results from previous experiments may not represent natural physiological responses. The present study investigated how an osmoconforming decapod, the graceful crab Cancer gracilis, balanced the demands of physiological systems (prioritization or additivity of events) during feeding and digestion in a low salinity environment. Cancer gracilis exhibited a typical increase in oxygen uptake and less pronounced increases in cardiovascular variables (heart rate, stroke volume, cardiac output) during feeding in 100% seawater. In 3-day starved crabs, exposure to 65% seawater resulted in a pronounced bradycardia, with a concomitant decrease in cardiac output and haemolymph flow rates and a temporary decrease in oxygen uptake. When crabs were exposed to low salinity, 3 h and 24 h after food ingestion, heart rate increased slightly and cardiac output and ventilation rates remained stable. Although oxygen uptake decreased transiently, feeding levels were quickly regained. During a recovery phase in 100%SW there was an overshoot in parameters, suggesting repayment of an oxygen debt. Thus, it appears that feeding and digestion are prioritized in this species, allowing it to survive acute exposure to hyposaline water. Furthermore, the results show that the nutritional state of an animal is important in modulating its physiological responses to environmental perturbations. This underscores the importance of studying physiological responses at the whole organism level under conditions closely approximating those of the natural environment.