Thecacera sesama sp. nov. (Nudibranchia, Polyceridae) from Taiwan, evident from morphology and phylogenetic analyses of the 16S rDNA and cytochrome c oxidase I gene.

Thecacera sesama Chan & Lee, sp. nov. (Nudibranchia, Polyceridae) is described from north-eastern Taiwan based on an integrative taxonomic approach combining morphological and molecular data. The new species is distinguished from its congeners by a unique colour pattern consisting of a translucent white body covered with numerous small, round, black pigment spots and fewer, larger, yellow spots and five gills. While sharing a similar spotted colour pattern with Thecacera pennigera, the new species can be clearly distinguished by its significantly smaller body size (maximum length < 3 mm). Phylogenetic analyses of two mitochondrial genes, cytochrome c oxidase subunit I (COI) and 16S rRNA, confirmed it as a new species in Thecacera. The molecular data places Thecacera sesama sp. nov. as a sister species to Thecacera picta, with a significant interspecific COI divergence of 14.17%. This discovery highlights the rich, yet under documented, marine biodiversity of Taiwan and underscores the value of combining traditional morphological examination with molecular phylogenetics for accurate species delimitation in cryptic nudibranch lineages.