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

Abstract

SUMMARY 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.

Feeding and digestion in low salinity in an osmoconforming crab, Cancer gracilis II. Gastric evacuation and motility

“…New steady states of salinity were reached within 10·min and did not vary by more than 0.1‰ during experiments. In a second experiment, food passage was followed during a salinity cycle, representative of those experienced in the field (McGaw, 2006b). The crabs (N=10) were fed in 100%SW and allowed to digest for 3·h in 100%SW.…”

“…This is due to food handling and mechanical breakdown in the gut (Carefoot, 1990), and the subsequent intracellular protein synthesis (Houlihan et al, 1990;Mente, 2003). In crustaceans the specific dynamic action is associated with a two-to threefold increase in oxygen uptake that persists for up to 3 days (Houlihan et al, 1990;Burggren et al, 1993;McGaw and Reiber, 2000;Robertson et al, 2002;McGaw, 2006b). To support the increase in metabolic rate and to facilitate the uptake and distribution of absorbed nutrients, changes in cardiac function and haemolymph distribution also occur during the digestive process Reiber, 2000, McGaw, 2005;McGaw, 2006a;McGaw, 2006b).…”

“…There is a growing body of literature on the cardiovascular and respiratory responses of decapod crustaceans to low salinity (see McGaw, 2006b). However, these previous studies were conducted on unfed animals to ensure they were in a similar metabolic state and to avoid stimulatory effects associated with feeding and digestion (Wang, 2001).…”

“…In contrast to work on unfed osmoregulating crabs, which exhibit an increase in oxygen uptake and heart rate during low salinity exposure (King, 1965;Engel et al, 1975;Hume and Berlind, 1976;Taylor, 1977;McGaw and McMahon, 1996;McGaw and Reiber, 1998), unfed Cancer gracilis react with a pronounced decrease in cardiac and respiratory parameters. However, when postprandial Cancer gracilis encounter low salinity, the bradycardia is absent and the reduction in oxygen uptake is of shorter duration than in unfed crabs (McGaw, 2006b). This suggests that either digestive events are prioritized and digestion continues unabated in low salinity or, because of the elevated metabolism due to the specific dynamic action, increases in respiratory and cardiovascular parameters are necessary to sustain basic life functions.…”

“…This suggests that either digestive events are prioritized and digestion continues unabated in low salinity or, because of the elevated metabolism due to the specific dynamic action, increases in respiratory and cardiovascular parameters are necessary to sustain basic life functions. Since digestive events can pose an additional burden on an already taxed system (Legeay and Massabuau, 2000;McGaw, 2006a;McGaw, 2006b) the ability to alter or delay digestion may be an advantage. Therefore, the present study sought to determine how gastric processing and evacuation are affected or modulated by exposure to low salinity in an osmoconforming species of crab.…”