ABSTRACT. The tropical nudibranch Phestillu 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 PO&S 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 I_-
aspartic and r-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 initlal 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.
ABSTRACT. The tropical nudibranch Phestillu 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 PO&S 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 I_-
aspartic and r-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 initlal 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.
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