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The hydrodynamic properties of the carapace of the swimming crab, (Callinectes sapidus, Portunidae) and two dissimilar benthic species, Cancer productus and Lopholithodes mandtii, are investigated employing flow visualization and low-speed wind tunnel techniques. Drag and lift forces for different orientations are determined for angles of attack ranging from 0–20. For Callinectes and Cancer productus drag is least at all angles of attack for sideways placement of the carapace, which corresponds to the position during locomotion in both species. Values of the drag Coefficient (based on plan form area) for Reynolds numbers greater than 104 for C. sapidus and C. productus are of the order of 0.2 0.35. Maximum values of the lift coefficient are of the order of 1.2 and 0.6 -0.7 for C. supidus and C.productus, respectively, giving maximum lift to drag ratios of about 4.0 for C. sapidus and 2.0 for C. productus. In contrast, L. mandtii is characterized by relatively high drag coefficients, low lift coefficients (about 1.0 and 0.2, respectively) and a lift to drag ratio that is an order of magnitude less than that of C. productus.

The adaptive significance of the different lift to drag ratios of the animals is discussed in the context of simple hydromechanical models of carapace design in relation to flow velocity. It is argued that the carapace of Callinectes is adapted for minimum resistance and to generate sufficient lift to counter excess weight over buoyancy (specific gravity, S.G. = 1.14) at low forward speeds. Assuming that all of the lift required is produced passively by the carapace, minimum swimming speeds of the order of 0.15 ms-1 are calculated.

The model applied to the benthic species focuses on two critical velocities, the slipping speed (speed at which lateral displacement first occurs) and the lift-off speed (speed at which the animal is lifted off the bottom). The ratio of lift-off speed to slipping speed for C. productus is approximately 1.9 and similar to that for plaice. Values for L. mandtii are about 3.3. It is suggested that the carapace of C. productus is adapted to maximize slipping speed and that displacement in L. mandtii is resisted actively by holding on to the substratum. The influence of flow on crabs in general is briefly discussed in relation to morphological design and habitat.