Despite significant advances in invertebrate phylogenomics over the past decade, the higher-level phylogeny of Pycnogonida (sea spiders) remains elusive. Due to the inaccessibility of some small-bodied lineages, few phylogenetic studies have sampled all sea spider families. Previous efforts based on a handful of genes have yielded unstable tree topologies. Here, we inferred the relationships of 89 sea spider species using targeted capture of the mitochondrial genome, 56 conserved exons, 101 ultraconserved elements, and three nuclear ribosomal genes. We inferred molecular divergence times by integrating morphological data for fossil species to calibrate 15 nodes in the arthropod tree of life. This integration of data classes resolved the basal topology of sea spiders with high support. The enigmatic family Austrodecidae was resolved as the sister group to the remaining Pycnogonida and the small-bodied family Rhynchothoracidae as the sister group of the robust-bodied family Pycnogonidae. Molecular divergence time estimation recovered a basal divergence of crown group sea spiders in the Ordovician. Comparison of diversification dynamics with other marine invertebrate taxa that originated in the Paleozoic suggests that sea spiders and some crustacean groups exhibit resilience to mass extinction episodes, relative to mollusk and echinoderm lineages.

No abstract

Despite significant advances in invertebrate phylogenomics over the past decade, the higher-level phylogeny of Pycnogonida (sea spiders) remains elusive. This group of arthropods appeared early in the fossil record, with the oldest unambiguous fossils dating to the Silurian. Due to the inaccessibility of some small-bodied lineages, few phylogenetic studies have sampled all sea spider families. In addition, previous efforts based on a handful of genes have yielded unstable tree topologies from one analytical approach to the next. Here, we inferred the relationships of 89 sea spider species using targeted capture of the mitochondrial genome, 56 conserved exons, 101 ultraconserved elements, and three nuclear ribosomal genes. We inferred molecular divergence times by integrating morphological data for fossil species to calibrate 15 nodes in the arthropod tree of life. This integration of data classes resolved the basal topology of sea spiders with high support. The enigmatic family Austrodecidae was resolved as the sister group to the remaining Pycnogonida and the small-bodied family Rhynchothoracidae as the sister group of the Page 3 of 31 robust-bodied family Pycnogonidae. This stable, dated phylogeny of Pycnogonida enables confident polarization of of cephalic appendage loss across pycnogonid families, with the consistent lack of the adult chelifore in a grade of basally diverging lineages. Molecular divergence time estimation recovered a basal divergence of crown group sea spiders in the Ordovician. Comparison of diversification dynamics with other marine invertebrate taxa that originated in the Paleozoic suggests that sea spiders and some crustacean groups exhibit resilience to mass extinction episodes, relative to mollusk and echinoderm lineages.

Selected sea spider specimens of Antarctic Colossendeis species collected during the Italica XIX cruise and the Polarstern cruise ANT XXIII/8 were examined to provide new information about the external and internal anatomy of the basal parts of the palps and ovigers. The presence and insertion of the muscle bands, as well as the arthrodial membrane are illustrated and discussed. The results obtained in this study show that the basal parts of the palps and ovigers have a similar internal structure. This is in agreement with the currently established 10-articled status for the ovigers (the basal element is not considered an article). Despite the currently established 10-articled status for the palps, our results suggest that the palp should be considered as being 9-articled.

The present contribution describes and discusses the observed morphological variability of pycnogonids in the genus Pallenopsis collected during the Italica XIX cruise to Victoria Land, Ross Sea, Antarctica (3 Feb-4 Mar, 2004). Four species are recognized in this collection and two of them are proposed as new species Pallenopsis gracilis n. sp. and Pallenopsis rotunda n. sp. The new species are described, illustrated, and compared morphologically with their closest congeners. The morphological diagnostic characters of the subgenus Bathypallenopsis are discussed and considered insufficient to justify a separation into subgenera. The observed morphological variability is instead proposed to be accommodated in one inclusive genus Pallenopsis. Finally, we propose an updated dichotomous key to all currently recognized Antarctic and Sub-Antarctic Pallenopsis species (adult form), and discuss some of the more disputable taxa, highlighting the lack of knowledge of the morphological and molecular characters of this genus.