Cardiovascular Responses Resulting from Variation in External Salinity in the Dungeness Crab, Cancer magister

Abstract

Haemolymph flow was assessed simultaneously in the five arterial systems leaving the heart of the crab Cancer magister, allowing calculation of stroke volume and cardiac output via heart rate. Scaphognathite beat frequency was determined via pressure measurements from within the branchial cavity. During acute exposure (6 h) to 50% dilution of normal seawater, heart rate increased, but a concurrent decrease in mean stroke volume resulted in an overall decrease in cardiac output. Initial heart rates were regained after several hours, on return to 100% seawater. Haemolymph flow rates through the anterior, anterolateral, and sternal arteries decreased during low-salinity exposure. Flow rates through the hepatic arteries remained stable, whilst flow in the posterior aorta increased. Haemolymph flow rates in all but the anterolateral arteries returned to pretreatment levels when the salinity was raised. Changes in the scaphognathite beat frequency were similar in direction to those of heart rate. In a second series of experiments, animals were acclimated to 50% seawater for 4 d and then returned to 100% seawater. After 4 d in 50% seawater, heart rates were still strongly elevated. Stroke volume was depressed but to a lower extent than observed following 6 h of exposure. On return to 100% seawater, heart rate decreased but stroke volume increased, with the result that no significant change in cardiac output occurred. It is apparent from these results that change in heart rate cannot be used to predict changes in stroke volume in decapod crustaceans. Arterial flow levels were not significantly depressed following 4 d of exposure to 50% seawater and, with the exception of a transient increase of flow in the posterior aorta, did not change significantly on return to 100% seawater. As with acute exposure, changes in scaphognathite rate again paralleled changes in heart rate. The relevance of these circulatory changes is discussed in terms of maintenance of the crabs' internal environment and behavioural or physiological adaptation.