Chemoreception in the nudibranch gastropod Phestilla sibogae

Abstract

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 pathways in P. sibogae focussed on two pairs of cephalic tentacles, the rhinophores and the oral tentacles. Two nerves from each oral tentacle pass directly to the brain, whereas multiple nerves from the rhinophores converge on paired rhinophoral ganglia that connect to the cerebral ganglia. Chemical sensitivity, determined by changes in rates of discharge, was monitored by suction electrodes attached to cut ends of rhinophoral or oral-tentacle nerves while the excised structure was perfused with extracts of Porites spp., non-food corals, l-amino acids or glutamate-receptor modulators. Although the oral tentacles were relatively 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 weak 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 disappeared 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 chemosensory organs for P. sibogae. Glutamate receptors pharmacologically similar to the kainic acid type of vertebrates are present.