A new genus and species of bathyal cidaroid (Echinoidea: Ctenocidaridae) from the Mar del Plata submarine canyon, Southwest Atlantic, with insights into its phylogenetic affinities

Abstract

The deep ocean represents the largest yet least studied habitat on Earth, and the deep sea off Argentina is no exception. Here, we describe Bathycidaris argentina gen. nov., sp. nov. (Cidaroida: Ctenocidaridae) from the Mar del Plata submarine canyon, Southwest Atlantic, at depths between 1099 and 1950 m. The new taxon is characterized by: a low test, slightly domed aborally; apical disc between 46% and 61% of horizontal test diameter; oblique and non-confluent pore pairs; aboral primary spines rod-like and spinulated; oral primary spines slightly flattened distally, with finely serrated margins; and globiferous pedicellariae, lacking a terminal tooth. Phylogenetic analyses based on mitochondrial cytochrome c oxidase subunit I sequences, using Bayesian inference and maximum likelihood, recover the new taxon as a strongly supported monophyletic lineage within Ctenocidaridae, consistent with intergeneric and species-level genetic divergence. The new species occurs within the core of the Circumpolar Deep Water, suggesting potential connectivity with sub-Antarctic and Antarctic faunas, in addition to possible local isolation associated with canyon topography. This study also highlights unresolved phylogenetic relationships within Ctenocidaridae and underscores the need for a comprehensive systematic reassessment of the family. Overall, our findings contribute to the understanding of deep-sea echinoid diversity and biogeographical patterns in the Southwest Atlantic.