Four new species of the little-known genus Baccalaureus Broch, 1929 (Thecostraca: Ascothoracida) endoparasitic in zoantharians from Taiwan: integrative taxonomy, functional morphology, diversity, and distribution

Abstract Baccalaureus is a genus of specialized ascothoracidan endoparasites that inhabit the gastral cavity of zoantharian corals. Twelve species have been recognized in the genus. The present study, employing an integrative taxonomic approach, examines both the morphological and molecular divergence of Baccalaureus specimens collected from Palythoa corals in Taiwan. This analysis reveals four new species: Baccalaureus oliviformis sp. nov., Baccalaureus brevipes sp. nov., Baccalaureus arietinus sp. nov., and Baccalaureus spiralis sp. nov. These species are diagnosed based on the morphology of the carapace, paired thoracopods, penis rudiment, and furcal rami. Based on sequence divergence in COI and the results obtained from automatic partitioning analysis (ASAP) and Poisson tree processes (PTP) model methods, the samples of Baccalaureus collected in Taiwan were assigned to four or six groups. Diagnostic morphological characters for Baccalaureus include carapace shape, the structure of anterior thoracic horns, furcal rami, thoracopods 1, 5, and 6, and the development of seminal papillae and the penis rudiment. The study also compares host specificity interactions among members of the family Lauridae, including the genera Baccalaureus , Laura , Polymarsypus , and Zoanthoecus , offering new insights into the biology of these parasitic crustaceans. Previous records of all these zoantharian-infesting genera with localities, depths, and host taxonomy are summarized, showing the diversity of host taxa, wide geographical, and vertical ranges of the parasites.