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,PhymanthuspinnulatusMartens 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 ofP.pinnulatuscollected from its type locality, Singapore. These were examined and compared with type materials of the species and its congeners. Specimens ofP.pinnulatusdiffer from syntypes of species described asPhymanthuslevisKwietniewski, 1898 from Indonesia, as well asPhymanthussansibaricusCarlgren, 1900 andPhymanthusstrandesiCarlgren, 1900, both described from East Africa.Phymanthuspinnulatuswas 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 ofPhymanthusdocumented elsewhere. It is inferred thatP.pinnulatushas a wide distribution that extends eastwards from Singapore, as far as Ambon and the Torres Straits. Some individuals reported asPhymanthusmuscosusHaddon and Shackleton, 1893 andPhymanthusbuitendijkiPax, 1924 are probablyP.pinnulatus. This morphological analysis provides new insights into the characters used to delimitP.pinnulatus, clarifies its geographical distribution, and contributes to an ongoing revision of the genusPhymanthus.
