Semicircular canals in squids

Abstract

THE cephalopods and fishes are rivals as fast-moving predators and they show many parallel adaptations¹. We present here evidence that in squids and cuttlefishes the statocysts possess semicircular canals, though of rather imperfect form. Animals that move quickly need to monitor angular rotation in order to allow appropriate adjustments of the eyes as they turn. To provide this facility certain critical physical requirements must be met and semicircular canals provide this. Their small size ensures that during rotation the pattern of flow is dominated by viscous damping. Consequently the actual volume of fluid flow is small, permitting “accurate transduction of the volume displacement of fluid by means of a water-tight ‘swingdoor’ cupula having limited angular excursion in the ampulla”². Furthermore, in vertebrates, the low Reynolds number of the system (less than one) ensures that “the velocity of relative flow becomes strictly proportional to the inertial force driving it and hence to the angular acceleration of the head”. The statocysts of the fast-moving cephalopods have gone some way to meet these requirements by their shapes, and by the arrangement of the strange anticristae, projections that partly divide the cavity.