ABSTRACT
Decapod crustaceans inhabit aquatic environments that are frequently subjected to changes in salinity and oxygen content. The physiological responses of decapod crustaceans to either salinity or hypoxia are well documented; however, there are many fewer reports on the physiological responses during exposure to these parameters in combination. We investigated the effects of simultaneous and sequential combinations of low salinity and hypoxia on the cardiovascular physiology of the Dungeness crab, Cancer magister. Heart rate, as well as haemolymph flow rates through the anterolateral, hepatic, sternal and posterior arteries were measured using a pulsed-Doppler flowmeter. Summation of flows allowed calculation of cardiac output and division of this by heart rate yielded stroke volume. When hypoxia and low salinity were encountered simultaneously, the observed changes in cardiac properties tended to be a mix of both factors. Hypoxia caused a bradycardia, whereas exposure to low salinity was associated with a tachycardia. However, the hypoxic conditions had the dominant effect on heart rate. Although hypoxia caused an increase in stroke volume of the heart, the low salinity had a more pronounced effect, causing an overall decrease in stroke volume. The patterns of haemolymph flow through the arterial system also varied when hypoxia and low salinity were offered together. The resulting responses were a mix of those resulting from exposure to either parameter alone. When low salinity and hypoxia were offered sequentially, the parameter experienced first tended to have the dominant effect on cardiac function and haemolymph flows. Low salinity exposure was associated with an increase in heart rate, a decrease in stroke volume and cardiac output, and a concomitant decrease in haemolymph flow rates. Subsequent exposure to hypoxic conditions caused a slight decrease in rate, but other cardiovascular variables were largely unaffected. In contrast, when low salinity followed acclimation to hypoxic conditions, apart from an increased heart rate, there were no other cardiovascular changes associated with the low salinity episode. The implications of these changes in cardiovascular dynamics are discussed in relation to physiological mechanisms and the ecology of decapod crustaceans, in hypoxic or low salinity environments.
Subject(s)
Brachyura/drug effects , Brachyura/physiology , Hemodynamics/drug effects , Hemodynamics/physiology , Hypoxia/physiopathology , Sodium Chloride/pharmacology , Animals , Cardiac Volume/drug effects , Environment , Heart Rate/drug effects , Hemolymph/drug effects , Hemolymph/physiology , Osmolar Concentration , Stroke Volume/drug effects , Time Factors , Water-Electrolyte Balance/drug effectsABSTRACT
Hemolymph samples were taken from small (< 100 g) individuals of Cancer productus following ca. 3 h air exposure (emersion) on the beach, `in situ', at Friday Harbor, Washington. Compared with crabs of similar size in sea water in the laboratory, these crabs emersed `in situ' had lower Pao2, and Pvo2, but no significant change in pH and a small, not significant, internal hypercapnia. Total CO2 (Cco2) content of the hemolymph was elevated by 70% (15.2 versus 9.0 mM), possibly as compensation for input of acid into the hemolymph. These responses are qualitatively similar to those resulting from similar treatment in the laboratory, but differ in the reduced magnitude of the internal hypercapnia and acidosis of the hemolymph. It is suggested that the particular conditions of emersion `in situ' permit some gas exchange with interstitial sea water. Interstitial sea water was found to be hypoxic (Po2 = 20-40 torr), which would limit oxygen supply yet permit CO2 excretion to continue, in agreement with the data.
