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

Gastric evacuation and gut contraction rates were followed in the graceful crab Cancer gracilis during exposure to low salinity. Crabs were fed a radio-opaque meal and then exposed to 100% seawater (SW), 80%SW or 60%SW; passage of digesta was followed using a fluoroscope. Exposure to low salinity increased the time for food passage through the gut system. Times for emptying of the foregut, midgut and hindgut varied in a dose-dependent manner. In the lowest salinity, crabs regurgitated food from the foregut after approximately 6 h. This may act as a protective response, clearing the gut and avoiding subsequent increases in metabolism associated with digestion. Contraction rate of the cardiac stomach and gastric mill was sporadic and there was no significant change with salinity. In contrast, contractions of the pyloric region were more constant and rapid. Pyloric contractions decreased at each salinity within 2-4 h after feeding. Contraction rates of the pyloric chamber were significantly lower in 60%SW compared with 100%SW and 80%SW. During a salinity cycle there was also slowing of gut contractions and food passage through the gut system. Pre-treatment levels were only regained slowly when the animals were returned to 100%SW. Cancer gracilis was able to slow digestion during low salinity exposure, which may spare resources for other systems. However, the crabs could not halt digestion completely and may be committed to protein synthesis once intracellular digestion has begun.