Faculty of Fisheries, Kagoshima University, Shimoarata, Kagoshima 890-0056, Japan
Putative chemoreceptors in the tentacles and the snout of the abalone Haliotis discus hannai were examined with scanning electron microscopy and light microscopy, and their role in inducing feeding was tested behaviorally. The cephalic, epipodial and mantle tentacles bear papillae with ciliary tufts at their tips. Chemoreceptors with 14-21 ciliated cells were found at high density on the snout. When the tentacles were touched with a bamboo rod or a piece of plastic sheet, the abalone contracted the tentacle stimulated, indicating mechanoreception. The contact of a piece of brown algae Laminaria japonica to any of three tentacles elicited a typical feeding response: shell lifting, pedal sole expansion and lifting, and food seizing with pedal sole. The response disappeared when the algae was separated from the tentacles, indicating that the ciliated cells were contact chemoreceptors as well as mechanoreceptors. A possibility of distant chemoreception was examined in a Y-maze in which seawater flowed over the algae and was directed towards the abalone. The algal stimulant triggered the food searching movement of the abalone which randomly chose both the stimulant containing water from the algal containing side and the control water without stimulant. This implies the possession of distant chemoreception possibly by the osphradium, but the abalone lacks a function to detect the origin of the smell.