Article: The role of body extension in cephalopod locomotion

The extension of the body outward from the aperture of the shell is a common behavioural trait of Nautilus, and presumably of most fossil ectocochliate cephalopods as well. In order to evaluate the possible contribution of body extension to ectocochliate locomotion, the hydrodynamic properties of body extension have been examined through tow-tank and flow-channel tests on shell models and body prostheses. Extension of the body reduces the drag generated by a swimming cephalopod. The extent of this reduction depends on how fully the body is extended, and on its posture relative to the flow. For most cephalopods this effect was probably small, but for some with compressed shells, and perhaps for Nautilus, an energy saving in the order of 25% may have been attainable. Extension of tentacles into the flow causes serious drag increases and can quickly overcome the beneficial effects of body extension. To minimize its drag, Nautilus retracts its tentacles when swimming fast, and in this aspect of its locomotory behaviour mirrors fish and squids, which do the same thing with their fins.