CHEMORECEPTION IN THE NUDIBRANCH GASTROPOD PHESTILLA-SIBOGAE

The tropical nudibranch Phestilla sibogae feeds exclusively on corals of the genus Porites, which it locates and recognizes by chemical cues . Morphological and physiological analyses of chemosensory neural path ways in P. sibogae focussed on two pairs of cephalic tentacles, the rh inophores and the oral tentacles. Two nerves from each oral tentacle p ass directly to the brain, whereas multiple nerves from the rhinophore s converge on paired rhinophoral ganglia that connect to the cerebral ganglia. Chemical sensitivity, determined by changes in rates of disch arge, was monitored by suction electrodes attached to cut ends of rhin ophoral or oral-tentacle nerves while the excised structure was perfus ed with extracts of Porites spp., non-food corals, L-amino acids or gl utamate-receptor modulators. Although the oral tentacles were relative ly insensitive to the substances tested, responses in rhinophores were positive to extracts of Porites compressa and a non-food coral, were biphasic to L-aspartic and L-glutamic acids and were perceptible but w eak to several other amino acids. Rhinophores responded positively to the glutamate-receptor modulators D-glutamate, 6,7-dinitroquinoxaline- 2,3-dione (DNQX) and kainic acid but not to N-methyl-D-aspartate (NMDA ); the initial inceased firing-rate response to L-glutamate disappeare d when the glutamate was dissolved in magnesium-or cobalt-substituted calcium-free seawater. For the substances tested here, including food corals and a set of amino acids, the rhinophores are the main chemosen sory organs for P. sibogae. Glutamate receptors pharmacologically simi lar to the kainic acid type of vertebrates are present. (C) 1997 Elsev ier Science Inc.