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

Scyllaeidae is a small group of nudibranchs comprising three genera (, , and ) 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 from and . The genus , commonly known as the sargassum nudibranch, currently consists of only two valid species, and . These species inhabit seaweed spp., feeding on hydroids. During a biodiversity survey conducted in April 2023, seven sargassum nudibranch individuals were collected from the seaweed spp. at a depth of 2 meters in Tai She Wan through SCUBA diving. Initially, the specimens were misidentified based on their resemblance to and previous local records. However, thorough morphological and molecular examinations confirmed them to be , 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 to be the sister group of . 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 in this paper, integrating valuable morphological information for future species identification purposes.