Despite the ubiquity of sea anemones (Cnidaria: Actiniaria) in tropical ecosystems, our understanding of their biodiversity and taxonomy is limited. Here we re-establish the identity of an intertidal zooxanthellate species, Phymanthuspinnulatus Martens in Klunzinger, 1877. Originally described from a single preserved specimen in the Berlin Museum by CB Klunzinger, his brief footnote lacked crucial details to positively identify the species. Our redescription is based on more than 50 living individuals of P.pinnulatus collected from its type locality, Singapore. These were examined and compared with type materials of the species and its congeners. Specimens of P.pinnulatus differ from syntypes of species described as Phymanthuslevis Kwietniewski, 1898 from Indonesia, as well as Phymanthussansibaricus Carlgren, 1900 and Phymanthusstrandesi Carlgren, 1900, both described from East Africa. Phymanthuspinnulatus was encountered on the lower intertidal, among coral rubble and between rocky crevices. It is vibrantly coloured and has 96 marginal tentacles with branching outgrowths along each, resulting in a 'frilly' appearance. The anemone has a flat expanded oral disc, with discal tentacles that are inconspicuous and reduced, unlike syntypes of its congeners. Details of its live appearance, musculature, and cnidom are also provided for the first time. Overall, types of cnidae and capsule sizes differ from other known species of Phymanthus documented elsewhere. It is inferred that P.pinnulatus has a wide distribution that extends eastwards from Singapore, as far as Ambon and the Torres Straits. Some individuals reported as Phymanthusmuscosus Haddon and Shackleton, 1893 and Phymanthusbuitendijki Pax, 1924 are probably P.pinnulatus. This morphological analysis provides new insights into the characters used to delimit P.pinnulatus, clarifies its geographical distribution, and contributes to an ongoing revision of the genus Phymanthus.
Keywords: Actinoidea; Indo-Pacific; Southeast Asia; intertidal; zooxanthellae.