Morphology and phylogeny of two new species of deep-sea mushroom soft corals (Octocorallia, Corallidae, Anthomastinae) from the Southern Mid-Atlantic Ridge

Members of the subfamily Anthomastinae Verrill, 1922, commonly known as mushroom soft corals, are characterized by their capitate or mushroom-shaped red colonies and large autozooids. Deep-sea mushroom corals of this subfamily remain poorly documented in the South Atlantic. This study describes two new Anthomastinae species, Neoanthomastus longistylus sp. nov. and Anthomastus mirabilis sp. nov., from the Southern Mid-Atlantic Ridge at depths of 1,553 –2,145 m.

Neoanthomastus longistylus sp. nov. is characterized by a slender stalk, siphonozooids distributed on the capitulum and the upper third of the stalk, and tentacular sclerites that are predominantly straight rods and rods with one end curved. Anthomastus mirabilis sp. nov. is distinguished by the presence of dumbbells in its tentacles, clubs and dumbbells in the anthocodial wall, and the absence of spindles. The mitochondrial genome of N. longistylus sp. nov. contains 14 PCGs, 2 rRNAs, and 1 tRNA, while that of A. mirabilis sp. nov. comprises 17 PCGs, 2 rRNAs, and 1 tRNA, with three duplicated genes – ND 3, ND 6, and ND 4 L – that are identical in sequence.

Phylogenetic analyses based on the partial mtMutS gene and 13 PCGs from the mitochondrial genomes (mtMutS not included) reveal the phylogenetic relationships within the subfamily Anthomastinae. These are the first records of both genera from the Southern Mid-Atlantic Ridge, enriching the known species diversity and providing critical baseline data for future biodiversity assessments.