Statocyst Control of Uropod Righting Reactions in Different Planes of Body Tilt in the Norway Lobster, Nephrops Norvegicus

Abstract

The righting reactions of the uropod exopodites of the Norway lobster, Nephrops norvegiens, induced by stimulation of the statocysts, were studied during both imposed body tilts in different vertical planes and freely expressed manoeuvres. The opening and closing movements of the uropod exopodite were brought about by the reciprocal activity of the dorsal abductor muscles and the reductor muscles, respectively. The uropods were held symmetrically open when the animal was upright, but adopted an asymmetrical pattern, with the downward uropod open and the upward uropod closed, during imposed body roll. In an imposed pitch of the body, the uropods closed symmetrically on headdown movement and opened symmetrically on head-up movement. The response pattern which occured in roll persisted through intermediate vertical planes to within 10° of true pitch. Removal of the statolith from a single statocyst caused the zone of symmetrical uropod responses to shift towards the operated side, but did not alter its angular dimensions. Bilateral statolith removal abolished the uropod reaction to tilt. Animals released in mid-water exactly in the pitch plane recovered their upright posture by a pitching movement, using symmetrical motor reactions of the abdomen and its appendages. Animals released at all other possible orientations used an initial rolling movement, involving an asymmetrical disposition of the appendages. The chelipeds did not contribute significantly to righting in roll, but both the lateral beating of the swimmerets and the asymmetrical disposition of the uropods produced righting torques as the animal descended through the water. These results are discussed in terms of the hydromechanical effect of asymmetrical uropod postures, and the functional significance of the distinct switching between symmetrical and asymmetrical patterns. Implications for the mechanisms of statocyst control of uropod righting reactions, in terms of both the magnitude and the direction of body tilt, are also considered.