The angular acceleration receptor system of diverse cephalopods

Abstract

The system for monitoring angular acceleration is described in 59 genera of cephalopods. The dimensions are measured and volumes calculated. The volume of the statocyst is always small relative to the size of the adult animal but is smallest in animals that move fast, such as ommastrephids, loliginids and sepiids; it is larger in neutrally buoyant squids and very large in cirroctopods. In many genera the flow of endolymph is restricted by knobs, the anticristae, usually arranged on a standard plan. These reach their greatest extent in non- buoyant squids and sepiids, where some of them join to form incomplete semicircular canals in the horizontal plane, providing the lower sensitivity appropriate to rapid turning to right or left. In buoyant squids and octopods the cavity is less obstructed. The system for adjusting sensitivity is not the same in all three planes: there are not three canals as in vertebrates. In the pitching and rolling planes the channels for endolymph flow are wide, presumably providing the higher sensitivity appropriate to slower movements. Sensitivity is perhaps also adjusted by variations in the cupulae, as it is in Octopus . The crista of pelagic octopods such as Argonauta and Japetella is divided into nine sections, as in the typical benthic forms. In cirroctopods the crista is not divided in this way and indeed shows no interruption of the rows of hair cells, even at the turns. In some cirroctopods and in Vampyroteuthis there are several anticristae, an apomorphic feature that they share with the decapods; but they have perilymph- and endolymph-like octopods. In the more active cranchiid squids the anticristae are arranged on a helical course, perhaps serving to provide appropriate sensitivity during turns from the head-down position. The statocyst of Spirula differs from both sepiids and teuthids.