Mushroom Soft Corals (Octocorallia: Coralliidae) From Seamounts in the Tropical Northwestern Pacific: Morphology and Phylogenetic Analysis Reveal a New Genus and Six New Species

Mushroom soft corals in the subfamily Anthomastinae are among the most remarkable octocorals found in the deep sea, characterized by their capitate or mushroom‐shaped red colonies and large autozooids. To date, their species diversity remains largely unknown due to limited research, and their phylogenetic relationships have yet to be explored. Based on samples collected from four seamounts in the tropical Northwestern Pacific, we establish a new genus and six new species within Anthomastinae: Neoanthomastus stellatus gen. et sp. nov., Neoanthomastus elongatus gen. et sp. nov., Anthomastus sphaericus sp. nov., Anthomastus tongi sp. nov., Pseudoanthomastus ornatus sp. nov., and Pseudoanthomastus applanatus sp. nov. We transfer four species of Anthomastus with the siphonozooids extending into the stalk to the new genus Neoanthomastus , and four additional species of Anthomastus to Pseudoanthomastus . A dichotomous key to all five known genera and 43 species of Anthomastinae is provided. Meanwhile, we utilize the concatenated nucleotides of 13 mitochondrial protein‐coding genes (PCGs), the full‐length coding regions of the mitochondrial MutS gene ( mtMutS ) and the barcodes of partial mtMutS to elucidate the phylogenetic relationships among all the five genera ( Anthomastus , Bathyalcyon , Heteropolypus , Pseudoanthomastus , and Neoanthomastus gen. nov.) and available species of Anthomastinae. The phylogenetic trees constructed from the three types of sequences suggest a hierarchical relationship where Neoanthomastus gen. nov. and Pseudoanthomastus form a clade that clusters with Bathyalcyon , which in turn with Heteropolypus , and finally with Anthomastus , all with high nodal supports. We also identify a second species in octocorals that lack the unique mtMutS . The study reveals a high diversity of mushroom soft corals and underscores the need for further systematic and zoogeographic research.