An association between a dendronotid nudibranch (Mollusca, Opisthobranchia) and a soft coral (Octocorallia, Alcyonaria) from the Red Sea

The association between a dendronotid nudibranch and the soft coral Pareryt hropodium fulvum fulvum (Forskal, 1775) is described from the Red Sea. The nudibranch is a new species of the genus Marioniopsis Odhner, 1934 (Nudibra nchia: Dendronotacea). This species feeds on the alcyonacean octocoral P. f . fulvum, and the slugs do not take zooxanthellae from their prey. The nudi branchs are randomly distributed on the soft coral host, usually one slug p er colony, and 24.5% of the soft coral colonies found are occupied by slugs . The nudibranch matches the colour of its prey and the gill clusters have a shape similar to that of the soft coral polyps, and because of these they are very cryptic on its surface. Marioniopsis fulvicola sp. n. is characte rized by a thin, elongated body with a smooth dorsal surface, a narrow foot , seven to nine clusters of gills, three to four velar processes per side, 22-32 stomach hard plates, and jaws with more than 100 denticles arranged i n four to five rows. The radula presents a median tricuspid tooth, a simple first lateral, and the rest of laterals hamate, without denticles. Marioni opsis fulvicola sp. n. is usually brown-yellow in colour, with brown transv erse stripes forming darker patches between the gill clusters. Rhinophores and gills have a bluish coloration.