Morphology and Phylogenetic Position of the Sargassum Nudibranch Scyllaea fulva Quoy & Gaimard, 1824 (Nudibranchia: Scyllaeidae): First Record in Hong Kong.

Scyllaeidae is a small group of nudibranchs comprising three genera (Scyllaea, Notobryon, and Crosslandia) with striking morphological similarities, making their identification challenging based on external features alone. Previous studies have highlighted the significance of central radular teeth in distinguishing Notobryon from Scyllaea and Crosslandia. The genus Scyllaea, commonly known as the sargassum nudibranch, currently consists of only two valid species, Scyllaea pelagica and Scyllaea fulva. These species inhabit seaweed Sargassum spp., feeding on hydroids. During a biodiversity survey conducted in April 2023, seven sargassum nudibranch individuals were collected from the seaweed Sargassum spp. at a depth of 2 meters in Tai She Wan through SCUBA diving. Initially, the specimens were misidentified based on their resemblance to Notobryon wardi and previous local records. However, thorough morphological and molecular examinations confirmed them to be Scyllaea fulva, representing the first record of this species in Hong Kong. Notably, our specimens lacked the blue spots observed in specimens from Thailand and the West Pacific Ocean, as reported in previous studies. Internally, a pineapple-like structure formed a honeycomb pattern on the surface of the masticatory edge of the jaw flap, with the presence of central radular teeth. A Maximum Likelihood tree analysis revealed Crosslandia to be the sister group of Scyllaea. Comparative analysis of intra-specific distances between individuals from the Philippines, French Polynesia, and Hong Kong indicated a close relationship between the Hong Kong specimens and those from the Philippines. Furthermore, we provide a detailed description of the external and internal morphology of Scyllaea fulva in this paper, integrating valuable morphological information for future species identification purposes.