Taxonomy and Molecular Phylogeny of the Sea Anemone Macrodactyla (Haddon, 1898) (Cnidaria, Actiniaria), with a Description of a New Species from Singapore.

Sea anemones (Cnidaria, Actiniaria) are a successful group of marine invertebrates found in a diverse range of environments globally. In spite of their ubiquity, identities for many sea anemones remain unverified, especially those from the Indo-West Pacific region. Here, we clarify the taxonomy of the poorly known , a shallow-water species first described from the Torres Straits in northern Australia. We re-describe based on new morphological and molecular data gathered from the type specimen, other museum vouchers, and from fresh material collected from Singapore. We tested the monophyly of using three mitochondrial (12S, 16S and ) and one nuclear (28S) marker based on three congeners, recovering this genus to be polyphyletic. As a consequence, we transferred to the genus , and describe a new species, sp. nov. While both and sp. nov. share the same arrangement and number of complete mesenteries, a similar distribution of cnidae, and are not symbiotically associated with any other biota, sp. nov. has perforated, lobe-like verrucae on its column, and lacks nematocyst batteries on its tentacles, unlike . These two species also occur in similar habitats in Singapore. Finally, because strongly resembles , the latter species flagged as a danger to public health due to its ability to inflict painful stings, we tested the relationship between these species and found them not to be closely related. However, tentacles of , like , are densely covered with nematocyst batteries and harbour large nematocysts; we infer that may also be capable of delivering stings that endanger public health. This study builds upon a growing number of studies that aim to ascertain identities and systematics of sea anemones historically reported from the Indo-West Pacific. Our findings will facilitate accurate species identification, which is crucial for advancing research, formulating conservation measures, and protecting public health.