Mechanoreceptors in the fins of the cuttlefish, Sepia officinalis

Little is known about mechanoreceptors in cephalopods. The anatomical data are fragmentary; Graziadei (1964) described multipolar nerve cells in the arms of Octopus and the lips of Sepia and similar structures have been described in the mantle of Octopus (Sereni & Young, 1932) and Eledone (Alexandrowicz, 1960). While there is good physiological evidence for the existence of mechanoreceptors in the mantle of Octopus (Gray, I960; Wilson, 1960; Boyle, 1976), mechanoreception in the mantle and fins of decapods has not been investigated. Here we present physiological evidence that there are receptors along the entire length of the cuttlefish fin that respond to mechanical stimuli. We also identify unusual structures in the fin whose distribution is consistent with their being these receptors. The nerves to the fins of Sepia ojficinalis pass through a foramen in the mantle on each side and radiate out beneath the skin in a series of approximately fifteen bundles (Tompsett, 1939). The mantle foramen was exposed by cutting away the overlying skin under MgCb anaesthesia (Messenger, Nixon & Ryan, 1985). Animals were held in a Perspex holder that clamped onto the mantle over the cuttlebone allowing free movement of the fins as well as normal respiratory movements. A suction electrode was attached to an intact fin nerve and its spontaneous activity was monitored using an Isleworth A103, or a Tektronix 122 preamplifier. Regular, patterned bursts of potentials were recorded in each of the fifteen identifiable nerve bundles and were visually correlated to the beating of the fin itself (Fig. 1 A). The nerve was then cut and each severed end attached to a suction electrode. The proximal end showed activity similar to that of the intact nerve while the distal end was silent. Stimulation of the animal induced more vigorous beating and increased the efferent activity recorded from the proximal cut end. During this fin beating we also recorded activity in the distal portion of the nerve at the bottom of the downstroke of that region of the fin innervated by the nerve (Fig. IB). Decapitated preparations (N = 8) were used to study this afferent activity in more detail. The mantle was secured with pins onto a Sylgard dish and the fin was spread flat and secured with pins on a raised platform of Sylgard. A suction electrode was