Phylogenomic Resolution of Sea Spider Diversification through Integration of Multiple Data Classes.

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 3 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.

Two new species and new findings in the genus Pallenopsis (Pycnogonida: Pallenopsidae) with an updated identification key to Antarctic and Sub-Antarctic species.

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.