Major Revisions in Pancrustacean Phylogeny and Evidence of Sensitivity to Taxon Sampling.

The clade Pancrustacea, comprising crustaceans and hexapods, is the most diverse group of animals on earth, containing over 80% of animal species and half of animal biomass. It has been the subject of several recent phylogenomic analyses, yet relationships within Pancrustacea show a notable lack of stability. Here, the phylogeny is estimated with expanded taxon sampling, particularly of malacostracans. We show small changes in taxon sampling have large impacts on phylogenetic estimation. By analyzing identical orthologs between two slightly different taxon sets, we show that the differences in the resulting topologies are due primarily to the effects of taxon sampling on the phylogenetic reconstruction method. We compare trees resulting from our phylogenomic analyses with those from the literature to explore the large tree space of pancrustacean phylogenetic hypotheses and find that statistical topology tests reject the previously published trees in favor of the maximum likelihood trees produced here. Our results reject several clades including Caridoida, Eucarida, Multicrustacea, Vericrustacea, and Syncarida. Notably, we find Copepoda nested within Allotriocarida with high support and recover a novel relationship between decapods, euphausiids, and syncarids that we refer to as the Syneucarida. With denser taxon sampling, we find Stomatopoda sister to this latter clade, which we collectively name Stomatocarida, dividing Malacostraca into three clades: Leptostraca, Peracarida, and Stomatocarida. A new Bayesian divergence time estimation is conducted using 13 vetted fossils. We review our results in the context of other pancrustacean phylogenetic hypotheses and highlight 15 key taxa to sample in future studies.

Adding pieces to the puzzle: insights into diversity and distribution patterns of Cumacea (Crustacea: Peracarida) from the deep North Atlantic to the Arctic Ocean.

The Nordic Seas have one of the highest water-mass diversities in the world, yet large knowledge gaps exist in biodiversity structure and biogeographical distribution patterns of the deep macrobenthic fauna. This study focuses on the marine bottom-dwelling peracarid crustacean taxon Cumacea from northern waters, using a combined approach of morphological and molecular techniques to present one of the first insights into genetic variability of this taxon. In total, 947 specimens were assigned to 77 morphologically differing species, representing all seven known families from the North Atlantic. A total of 131 specimens were studied genetically (16S rRNA) and divided into 53 putative species by species delimitation methods (GMYC and ABGD). In most cases, morphological and molecular-genetic delimitation was fully congruent, highlighting the overall success and high quality of both approaches. Differences were due to eight instances resulting in either ecologically driven morphological diversification of species or morphologically cryptic species, uncovering hidden diversity. An interspecific genetic distance of at least 8% was observed with a clear barcoding gap for molecular delimitation of cumacean species. Combining these findings with data from public databases and specimens collected during different international expeditions revealed a change in the composition of taxa from a Northern Atlantic-boreal to an Arctic community. The Greenland-Iceland-Scotland-Ridge (GIS-Ridge) acts as a geographical barrier and/or predominate water masses correspond well with cumacean taxa dominance. A closer investigation on species level revealed occurrences across multiple ecoregions or patchy distributions within defined ecoregions.

"Unicorn from Hades", a new genus of Mysidae (Malacostraca: Mysida) from the Mariana Trench, with a systematic analysis of the deep-sea mysids.

Since the 19th century, oceanic explorations have confirmed that the hadal zone (water depth > 6000 m) is not lifeless, but contains many fascinating organisms. Amongst them are the Mysida, which is a group of crustaceans found in many deep-sea trenches. Based on morphological observations and molecular phylogenetic analyses of an undescribed taxon within the subfamily Erythropinae, a new genus of deep-sea mysids, Xenomysis gen. n., is described from the Challenger Deep in the Mariana Trench. The new genus is not assigned to any of the tribes within the Erythropinae, as our analyses do not support the current classification scheme of Erythropinae. The result of a molecular-clock analysis with fossil calibration reveals that several groups of Mysida have independently colonized deep water habitats in different geological periods, from Triassic to Cretaceous. In addition, ancestral state reconstruction analyses indicate the degenerate eyes in both Mysidae and Petalophthalmidae is a result of parallel evolution, and the reduction of compound eyes to both "single fused eyeplate" and "two separate eyeplates" occurred multiple times independently in the evolution of Erythropinae.

Phylogenetic evidence that both ancient vicariance and dispersal have contributed to the biogeographic patterns of anchialine cave shrimps.

Cave shrimps from the genera Typhlatya, Stygiocaris and Typhlopatsa (Atyidae) are restricted to specialised coastal subterranean habitats or nearby freshwaters and have a highly disconnected distribution (Eastern Pacific, Caribbean, Atlantic, Mediterranean, Madagascar, Australia). The combination of a wide distribution and a limited dispersal potential suggests a large-scale process has generated this geographic pattern. Tectonic plates that fragment ancestral ranges (vicariance) has often been assumed to cause this process, with the biota as passive passengers on continental blocks. The ancestors of these cave shrimps are believed to have inhabited the ancient Tethys Sea, with three particular geological events hypothesised to have led to their isolation and divergence; (1) the opening of the Atlantic Ocean, (2) the breakup of Gondwana, and (3) the closure of the Tethys Seaway. We test the relative contribution of vicariance and dispersal in the evolutionary history of this group using mitochondrial genomes to reconstruct phylogenetic and biogeographic scenarios with fossil-based calibrations. Given that the Australia/Madagascar shrimp divergence postdates the Gondwanan breakup, our results suggest both vicariance (the Atlantic opening) and dispersal. The Tethys closure appears not to have been influential, however we hypothesise that changing marine currents had an important early influence on their biogeography.

Taxonomic review of the orders Mysida and Stygiomysida (crustacea, peracarida).

The order Mysida (2 families, 178 genera, 1132 species) contains species across a broad range of habitats, such as subterranean, fresh, brackish, coastal, and surface to deep-sea habitats. The Stygiomysida (2 families, 2 genera, 16 species), however, are found primarily in subterranean waters, but always in waters with a marine influence. The Mysida and Stygiomysida body is divided into three main regions: cephalon, thorax, and abdomen. They are shrimp-like in appearance, containing morphological features earlier referred to as defining a "caridoid facies". The shrimp-like morphology was to some extent diagnostic for the historic Decapod taxon Schizopoda, containing the Nebalia, Mysida, Lophogastrida, and Euphausiacea. In 1904 the concept of Schizopoda was abandoned, and the Mysidacea (Mysida and Lophogastrida) along with Cumacea, Amphipoda, Isopoda, and Tanaidacea were placed in a new taxon, the Peracarida. Later discoveries of groundwater mysids led to the establishment of Stygiomysida, but placement to either Lophogastrida or Mysida remained unclear. The presence of oostegites and absence of podobranchiae, coupled with non-statocyst bearing uropods have been used to classify the Stygiomysida as a primitive Mysida family, comparable to Petalophthalmidae. On the other hand, equally suggestive characters, but for a Lophogastrida affiliation, was suggested for the archaic foregut characters and again, non-statocyst bearing uropods. With the inclusion of DNA sequence data of ribosomal genes, sister group relationships between Stygiomysida, Lophogastrida, and Mictacea within the Peracarida are observed, which supports a classification of the Stygiomysida as a separate order removed from the Mysida.

A taxonomical review of the Gnathophausia (Crustacea, Lophogastrida), with new records from the northern mid-Atlantic ridge.

Taxonomy of 11 species contained within the Lophogastrida genus Gnathophausia is presented. We report new records of G. affinis, G. gigas, G. ingens, and G. zoea from the Atlantic Ocean between Iceland and the Azores. We also describe a new species, G. bergstadi that resembles G. zoea, but differs in having a heart-shaped apex on the telson and having the lower later keels of the carapace not fusing with the upper lateral keels. Gnathophausia is found to be most abundant at bathyal depths. G. zoea, G. gigas, and G. ingens are reported distributed throughout the world's oceans. G. affinis and G. bergstadi are confined to Atlantic water masses, whereas G. childressi, G. longispina, and G. elegans, are found in the Pacific. A more limited distribution in tropical regions of the world's oceans is observed in G gracilis, and also G. fagei and G. scapularis that are confined to the South China Sea and Indian Ocean, respectively. We include an identification key and known distribution of all accepted Gnathophausia species.

The disunity of "Mysidacea" (Crustacea).

New studies on malacostracan relationships have drawn attention to issues concerning monophyly of the order Mysidacea, manifested in recent crustacean classifications that treat the taxon as two separate orders, Lophogastrida and Mysida. We present molecular phylogenies of these orders based on complete sequences of nuclear small-subunit ribosomal DNA (18S rRNA), and morphological evidence is used to revise the classification of the order Mysida to better reflect evolutionary history. A secondary structure model for 18S rRNA was constructed and used to assign putative stem and loop regions to two groups of partitions for phylogenetic analyses. Phylogenies were estimated by maximum-likelihood, Bayesian inference, and maximum-parsimony. The analyses gave strong support for three independently derived lineages, represented by three monophyletic groups, Lophogastrida, Stygiomysida, and Mysida. The family Petalophthalmidae is considered as sister group to the family Mysidae, and Boreomysinae and Rhopalophthalminae are the most early derived of the Mysidae. The tribes contained in the current classification of the subfamily Mysinae are not well-supported by either molecular data or morphology.