The taxonomic challenge posed by the Antarctic echinoids Abatus bidens and Abatus cavernosus (Schizasteridae, Echinoidea)

Cryptic species have been repeatedly described for two decades among the Antarctic fauna, challenging the classic model of Antarctic species with circumpolar distributions and leading to revisit the richness of the Antarctic fauna. No cryptic species had been so far recorded among Antarctic echinoids, which are, however, relatively well diversified in the Southern Ocean. The R/V Polarstern cruise PS81 (ANT XXIX/3) came across populations of Abatus bidens, a schizasterid so far known by few specimens that were found living in sympatry with the species Abatus cavernosus. The species A. cavernosus is reported to have a circum-Antarctic distribution, while A. bidens is only recorded with certainty in South Georgia and at the northern tip of the Antarctic Peninsula. Based on genetic and morphological analyses, our results clearly show that A. bidens and A. cavernosus are two distinct species. The analyzed specimens of A. bidens group together in two haplogroups separated from one another by 2.7 % of nucleotide differences. They are located in the Weddell Sea and in the Bransfield Strait. Specimens of A. cavernosus form one single haplogroup separated from haplogroups of A. bidens by 5 and 3.5 % of nucleotide differences, respectively. The species was collected in the Drake Passage and in the Bransfield Strait. Morphological analyses differentiate A. bidens from A. cavernosus. In contrast, the two genetic groups of A. bidens cannot be differentiated from one another based on morphology alone, suggesting that they may represent a case of cryptic species, common in many Antarctic taxa, but not yet reported in Antarctic echinoids. This needs to be confirmed by complementary analyses of independent genetic markers.