Assessing the enormous diversity of Southern Ocean benthic species and their evolutionary histories is a central task in the era of global climate change. Based on mitochondrial markers, it was recently suggested that the circumpolar giant sea spider Colossendeis megalonyx comprises a complex of at least six cryptic species with mostly small and non-overlapping distribution ranges. Here, we expand the sampling to include over 500 mitochondrial COI sequences of specimens from around the Antarctic. Using multiple species delimitation approaches, the number of distinct mitochondrial OTUs increased from six to 15–20 with our larger dataset. In contrast to earlier studies, many of these clades show almost circumpolar distributions. Additionally, analysis of the nuclear internal transcribed spacer region for a subset of these specimens showed incongruence between nuclear and mitochondrial results. These mito-nuclear discordances suggest that several of the divergent mitochondrial lineages can hybridize and should not be interpreted as cryptic species. Our results suggest survival of C. megalonyx during Pleistocene glaciations in multiple refugia, some of them probably located on the Antarctic shelf, and emphasize the importance of multi-gene datasets to detect the presence of cryptic species, rather than their inference based on mitochondrial data alone.

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Pallenopsis patagonica (Hoek, 1881) is one of the most taxonomically problematic and variable pycnogonid species, and is distributed around the southern South American coast, and the Subantarctic and Antarctic areas. We conducted a phylogenetic analysis of mitochondrial cytochrome c oxidase subunit I (COI) sequences of 47 Pallenopsis specimens, including 39 morphologically identified as P. patagonica, five Pallenopsis pilosa (Hoek, 1881), one Pallenopsis macneilli Clark, 1963, one Pallenopsis buphtalmus Pushkin, 1993, and one Pallenopsis latefrontalis Pushkin, 1993. Furthermore, we studied morphological differences between the different COI lineages using light and scanning electron microscopy, including also material from Loman's and Hedgpeth's classical collections, as well as Hoek's type material of P. patagonica from 1881. The molecular results unambiguously reveal that P. patagonica is a complex of several divergent clades, which also includes P. macneilli, P. buphtalmus, and P. latefrontalis. Based on the material available, two major clades could be identified, namely a 'Falkland' clade, to which we assign the nominal P. patagonica, and a 'Chilean' clade, which is distinct from the 'Falkland' clade. We describe the 'Chilean' clade as new species, Pallenopsis yepayekae sp. nov. Weis, 2013. All molecular results are confirmed by specific morphological characteristics that are discussed in detail and compared with Pallenopsis species closely related to the P. patagonica complex. Our results reveal that P. patagonica is a species-rich complex that is in need for a thorough taxonomic revision, using both morphological and genetic approaches.

Sea spiders (Pycnogonida) are a widespread and phylogenetically important group of marine arthropods. However, their biology remains understudied, and detailed information about their feeding ecology is difficult to find. Observations on pycnogonid feeding are scattered in the literature, often in older sources written in various languages, and have never been comprehensively summarized. Here we provide an overview of all information on feeding in pycnogonids that we have been able to find and review what is known on feeding specializations and preferences in the various pycnogonid taxa. We deduce general findings where possible and outline future steps necessary to gain a better understanding of the feeding ecology of one of the world’s most bizarre animal taxa.Electronic supplementary materialThe online version of this article (10.1186/s12983-018-0250-4) contains supplementary material, which is available to authorized users.

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