Daniel L. Curtis

Behavioral Influences on the Physiological Responses of Cancer gracilis, the Graceful Crab, During Hyposaline Exposure

The relationship between the behavioral and physiological responses to hyposaline exposure was investigated in Cancer gracilis, the graceful crab. The status of C. gracilis as an osmoconformer was confirmed. Survival decreased with salinity: the LT50 in 50% seawater (a practical salinity of 16, or 16‰) was 31.5 ± 22.7 h and in 25% seawater (a salinity of 8) was 8.0 ± 0.7 h. When exposed to a salinity gradient, most crabs moved towards the highest salinity. However, in the salinity range of 55% to 65% seawater, they became quiescent. This “closure response” was also evident at low salinities: the mouthparts were tightly closed and animals remained motionless for 2 to 2.5 h. During closure, crabs were able to maintain the salinity of water within the branchial chambers at a level that was about 30% higher than that of the surrounding medium. The closure response was closely linked to a short-term decrease in oxygen uptake. During closure, oxygen within the branchial chamber was rapidly depleted, with oxygen uptake returning to pretreatment levels upon the resumption of activity. In addition to the short-term decrease in oxygen uptake, there was a longer-term bradycardia, which may serve to further reduce diffusive ion loss across the gills. By exhibiting a closure response during acute hyposaline exposure and an avoidance reaction during prolonged or severe hyposaline exposure, C. gracilis is able to use behavior to exploit areas prone to frequent episodes of low salinity.

Extracellular digestion during hyposaline exposure in the Dungeness crab, Cancer magister, and the blue crab, Callinectes sapidus.

Extracellular digestive processes were examined in the Dungeness crab, Cancer magister and the blue crab, Callinectes sapidus, during hyposaline exposure. Both species are found in estuaries as adults, but vary in their ability to balance the cardiovascular and respiratory demands of concurrent osmoregulation and digestion. The weak osmoregulator, C. magister, is unable to balance the demands of osmoregulation and digestion. Concordant with observed decreases in oxygen consumption and mechanical digestion, proteolytic digestion within the foregut and hepatopancreas was delayed, resulting in a relative reduction of circulating amino acids post-feeding in low salinity. In contrast, the efficient osmoregulator, C. sapidus, balances the demands of osmoregulation and digestion, and mechanical digestion continues unabated in low salinity. Protease activity in the gut fluid and hepatopancreas showed either no change or a reduction over time. The transport of amino acids into the cells post-feeding is opposed by an efflux of amino acids at the cellular level, and resulted in a build up of amino acids in the hemolymph. Despite differences in the extracellular responses to low salinity exposure following feeding, both species were able to maintain high digestive efficiencies.

A POSSIBLE FEEDING CONTROL MECHANISM IN DUNGENESS CRABS DURING HYPOSALINE EXPOSURE

Abstract The Dungeness crab, Cancer magister, is classed as a weak osmoregulator. Nevertheless, this species will enter into low salinity regimes of estuaries during times of high food abundance. The present study investigated the possible regulatory role of neurohormones on feeding behaviour during acute low salinity exposure. When the crabs eyestalks were ablated, removing the terminal medulla and the associated X-organ/sinus gland complex, they consumed more food and fed for a longer period of time compared with intact animals. Eyestalk ablated animals would even attempt to feed in freshwater, whereas intact animals would only consume food in salinities above 40% SW. The results suggest that feeding behaviour during low salinity exposure in C. magister is regulated by an inhibitory neurohormonal mechanism. This mechanism may help animals balance the demands of competing physiological processes.