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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. [ABSTRACT FROM AUTHOR]

Sterechinus is a very common echinoid genus in benthic communities of the Southern Ocean. It is widely distributed across the Antarctic and South Atlantic Oceans and has been the most frequently collected and intensively studied Antarctic echinoid. Despite the abundant literature devoted to Sterechinus, few studies have questioned the systematics of the genus. Sterechinus bernasconiae is the only species of Sterechinus reported from the Pacific Ocean and is only known from the few specimens of the original material. Based on new material collected during the oceanographic cruise INSPIRE on board the R/V Melville, the taxonomy and phylogenetic position of the species are revised. Molecular and morphological analyses show that S. bernasconiae is a subjective junior synonym of Gracilechinus multidentatus (Clark). Results also show the existence of two genetically distinct subclades within the so-called Sterechinus clade: a Sterechinus neumayeri subclade and a subclade composed of other Sterechinus species. The three nominal species Sterechinus antarcticus, Sterechinus diadema, and Sterechinus agassizi cluster together and cannot be distinguished. The species Sterechinus dentifer is weakly differentiated from these three nominal species. The elucidation of phylogenetic relationships between G. multidentatus and species of Sterechinus also allows for clarification of respective biogeographic distributions and emphasizes the putative role played by biotic exclusion in the spatial distribution of species. [ABSTRACT FROM AUTHOR]

The Bellingshausen Sea is one of the most remote and least surveyed seas of the Southern Ocean, so that little was known about benthic communities and those factors that determine community structuring until recently. The present work aims at characterizing the structure and spatial distribution of echinoid assemblages in the Bellingshausen Sea, as well as identifying the environmental factors that determine assemblage structuring. Echinoids were collected at 32 stations using an Agassiz trawl, at depths of 86-3,304 m, during BENTART oceanographic expeditions led in 2003 and 2006. Sediment and bottom water properties were analysed using an USNEL-type box corer and a Neil Brown Instrument System Mark III CTD, respectively. Echinoids were found at all stations, except Peter I Island. Seventeen species were identified, representing 22 % of the echinoid species present in the Southern Ocean and increasing twofold the number of species recorded in the Bellingshausen Sea so far. The echinoid fauna is dominated by the very abundant species Sterechinus antarcticus. Depth is the key factor that determines the nature of echinoid assemblages, which are mainly divided into the continental shelf, the slope and the deep-sea basin. In addition, sediment properties, namely redox values, organic matter and mud content, best match species dispersion on the shelf. Sediment properties affect echinoid distribution depending on species food range and feeding strategy. As it might be expected, sediment properties more strongly influence specialist feeders (Schizasteridae and Cidaridae) than generalists (Echinidae). [ABSTRACT FROM AUTHOR]