ABSTRACT
Decapod crustaceans have elaborate statocysts to detect changes in the body position. They utilize these organs to initiate and control various equilibrium reactions so that they can always maintain an upright posture. In macrurans, these reactions consist of asymmetrical movements of walking legs, swimmerets and uropods, as well as compensatory movements of the eyestalks and antennae. They have been previously described in terms of a simple input (i.e. the tilt angle)-and-output (i.e. the magnitude of the resultant reaction) relationship (e.g. Schöne, 1954). But do these equilibrium reactions occur whenever the animal tilts? We report here that one of them, the uropod steering movement, does not occur by itself and always occurs together with the abdominal movement which is unrelated to the equilibrium reaction. It is shown that this association of two independent movements is not mediated by any peripheral sense organ. Quantitative aspects of the uropod steering movement were described elsewhere (Yoshino, Takahata & Hisada, 1980).
