A taxonomic revision of the sponge-associated genus Thoracactis Gravier, 1918 (Anthozoa: Zoantharia) based on an integrated approach

The integrated approach of molecular phylogenetic and morphological analyses has revolutionized the systematics and our understanding of the evolutionary relationships of marine taxa. One such group is the hexacorallian order Zoantharia Rafinesque, 1815. The monotypic genus Thoracactis Gravier, 1918 has been little investigated since its placement within the order Zoantharia more than 100 years ago. Here, we examined museum specimens collected from the Cape Verde Islands (eastern Atlantic) and newly collected specimens from Brazil (southwestern Atlantic), using a combined molecular and morphological approach. Our results conclusively show Thoracactis to be referable to the family Parazoanthidae. Morphological data show that Thoracactis topsenti Gravier, 1918, the type species of this monotypic genus, has a cyclically transitional arrangement of its sphincter muscle, and this arrangement has previously been reported from the Parazoanthidae. Thoracactis can be distinguished from other hexasterophoran glass-sponge-associated genera (Churabana Kise, Montenegro &Reimer, 2022, Parachurabana Kise, 2023, and Vitrumanthus Kise, Montenegro &Reimer, 2022) by a combination of morphological, ecological and molecular phylogenetic data. In addition, molecular phylogenetic analyses clearly indicate that Thoracactis topsenti is placed within Parazoanthidae. These results are yet another demonstration of the utility of comprehensive combined approaches. From now, research attention should focus on the revision of remaining taxonomic questions within the family Epizoanthidae, with the goal of a comprehensively revised suborder Macrocnemina within reach.