The association between a dendronotid nudibranch and the soft coral Parerythropodium fulvum fulvum (Forskål, 1775) is described from the Red Sea. The nudibranch is a new species of the genus Marioniopsis Odhner, 1934 (Nudibranchia: Dendronotacea). This species feeds on the alcyonacean octocoral P. f. fulvum, and the slugs do not take zooxanthellae from their prey. The nudibranchs are randomly distributed on the soft coral host, usually one slug per 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 characterized 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 in four to five rows. The radula presents a median tricuspid tooth, a simple first lateral, and the rest of laterals hamate, without denticles. Marioniopsis fulvicola sp. n. is usually brown-yellow in colour, with brown transverse stripes forming darker patches between the gill clusters. Rhinophores and gills have a bluish coloration.
- Red sea