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Asexual reproduction by means of splitting, also called fissiparity, is a common feature in some asterozoan groups, especially in ophiactid brittle stars. Most fissiparous brittle stars show six instead of the usual five rays, live as epibionts on host organisms, and use clonal fragmentation to rapidly colonize secluded habitats and effectively expand the margins of their distribution area. While the biology and ecology of clonal fragmentation are comparatively well understood, virtually nothing is known about the evolution and geological history of that phenomenon. Here, we describe an exceptional fossil of an articulated six-armed brittle star from the Late Jurassic of Germany, showing one body half in the process of regeneration, and assign it to the new species Ophiactis hex sp. nov. Phylogenetic inference shows that the fossil represents the oldest member of the extant family Ophiactidae. Because the Ophiactis hex specimen shows an original six-fold symmetry combined with a morphology typically found in epizoic ophiuroids, in line with recent fissiparous ophiactid relatives, we assume that the regenerating body half is an indication for fissiparity. Ophiactis hex thus shows that fissiparity was established as a means of asexual reproduction in asterozoan echinoderms by the Late Jurassic. [ABSTRACT FROM AUTHOR]

Ophiuroidea is the most speciose of all classes of Echinoderma. It is an important component in benthic ecosystems, occurring in almost all ecological niches of modern seas. To date, the phylogeny and complete evolutionary history of the ophiuroids have not yet been fully resolved. In this study, we sequenced the complete mitochondrial genomes (mitogenomes) of Ophiothrix (Ophiothrix) exigua and two deep‐sea species Histampica sp. CS049 and Ophioplinthaca sp. M5261. These two deep‐sea ophiuroids displayed reversed strand‐compositional bias and rearranged gene orders. Thirteen distinct patterns of mitochondrial gene order among ophiuroid mitogenomes were detected, with two gene order newly found in Ophiuroidea. Our data supported the gene order found in all sampled Ophiuridae as the most likely ancestral order of all Ophiuroidea. To improve phylogenetic accuracy based on nucleotide differences, two different criteria were used for the analyses: (i) nucleotide sequence from all codon positions (PCG123); (ii) the NTE method ("Neutral Transitions Excluded") for ameliorating the misleading effects of a reverse strand bias in the data. The two methods confirmed the polyphyly of the orders Ophiacanthida and Amphilepidia. At family and genus level, Ophiuridae, Ophionotus and Ophioplinthus were not monophyletic. The most notable exception was that the NTE phylogeny showed low variation of branch length. NTE dataset generated younger age for most lower‐level nodes than that from PCG123 dataset. All analyses suggested that the ophiuroids radiation occurred around the Permian–Triassic mass extinction event, and the divergence time of the deep‐sea lineages was during the Cretaceous. [ABSTRACT FROM AUTHOR]

A proposed new Ordovician asterozoan genus and species, Cantabrigiaster fezouataensis , has been recently described and assigned to the class Somasteroidea. An accompanying phylogenetic analysis treated twenty-two asterozoans and sixteen early non-asterozoans, one of the latter providing the outgroup. The phylogenetic hypothesis focused on derivation of the subphylum Asterozoa and emphasized the Crinoidea; Cantabrigiaster fezouataensis was interpreted as documenting a critical transition stage in the origin of the Asterozoa. The diagnosis for the proposed new generic name posits absence of ambital framework ossicles as the single character expression that distinguishes Cantabrigiaster among somasteroid genera. Ambital framework ossicular presence is documented here in the holotype and other members of type suite; Cantabrigiaster is synonymized with Villebrunaster Spencer and V. fezouataensis nov. comb. is redescribed. The data matrix for the former Cantabrigiaster phylogenetic analysis relies on an earlier study treating overall echinoderm construction. Expressions outlined in the earlier study are not amenable to transfer to a data matrix without evaluation of level of universality of each cited expression; such evaluations were not provided thereby leading to flawed phylogenetic conclusions that are rejected. An alternative hypothesis for the early history of the Asterozoa supported by aspects of the fossil record, that subphylum diversification preceded the origin of readily preserved skeletons, and therefore potentially no tenable pre-asterozoan outgroup candidate survives in the fossil record, has not been falsified. [ABSTRACT FROM AUTHOR]

Mitochondrial genomes provide an additional data source for nuclear phylogenomics. Recently, the taxonomic classification of Ophiuroidea was changed dramatically based on a molecular phylogenetic analysis that utilized a huge nuclear dataset of transcriptome and target-capture approaches. However, the mitochondrial genome analysis of Ophiuroidea was not conducted sufficiently in depth to allow comparison with current phylogenetic relationships, especially for the Ophintegrida. In this study, eight mitogenomes were newly reported in Amphilepidida and Ophiacanthida. Phylogenetic analyses were undertaken based on the nucleotide sequences of 13 protein coding genes (PCGs), 22 tRNA, and two rRNA, and the amino acid translated sequences of 13 PCGs. The results of the phylogenetic analysis suggested that the amino acid translated sequences were more useful than the nucleotide sequences for the analysis of higher phylogenetic relationships. The mitochondrial gene order of 13 PCGs and two rRNA in Amphilepidida and Ophiacanthida was relatively conserved. However, more complex gene orders occurred in the phylogeny of Amphilepidida and Ophiacanthida, including 22 tRNA. The results of our study suggest that Ophiuroidea has undergone more complex gene rearrangements than other classes of echinoderms. [ABSTRACT FROM AUTHOR]

The peristomial plates are skeletal components of the interbrachial frame (or mouth frame), which is located below the true mouth of ophiuroids. Whilst the peristomial plates were extensively described and used as diagnostic characters by some early workers, for the past 100 years they have been largely neglected as a taxonomic resource. In this investigation the peristomial plates of 48 species representing 21 families were examined directly, and information on a further 61 species, including representatives of another eight families, was obtained from the published literature. Observations were made with regard to fragmentation state, relative size and orientation of the peristomial plates. Although fragmentation state showed little consistency at any taxonomic level, relative size and orientation segregated a group of families comprising species with relatively small, inclined peristomial plates, viz. Ophiotrichidae, Ophiopholidae, Ophiactidae, Amphiuridae and Ophiocomidae, together with a single hemieuryalid species–Ophioplocus januarii. The distribution of peristomial plate traits was strongly correlated with that of several other character states pertaining to the interbrachial frame. This supported the proposition that two major types of interbrachial frame are present in ophiuroids (designated ‘A’ and ‘B’). Current phylogenies inferred from both morphological and molecular data imply that type B is derived and has evolved independently at least twice in the orders Amphilepidida and Ophiacanthida. This represents a remarkable example of evolutionary convergence. An analysis of the distribution of all interbrachial frame character states suggested that within the Amphilepidida paedomorphosis was probably responsible for the complete reversion of the interbrachial frame to the ancestral type A condition in two families (Ophiothamnidae and Amphilepididae) of suborder Gnathophiurina and possibly responsible for varying degrees of trait reversal in the four families of suborder Ophionereidina. Such paedomorphic events may have been associated with a secondary return to the deep-sea from shallow-sea environments. [ABSTRACT FROM AUTHOR]

The first phylogenetic and phylogeographic studies of the cosmopolitan deep-sea species, Asteronyx loveni Müller and Troschel, 1842, were conducted, based on specimens collected from off the Pacific coast of Japan. Partial sequences of mitochondrial 16S (68 specimens) and COI (24 specimens) genes were analyzed. Phylogenetic trees and network analyses revealed three lineages and five sub-lineages of A. loveni . Genetic distance and AMOVA (analysis of molecular variance) analyses suggested that the lineages should be considered three cryptic species. Skin on the aboral disc is partly thickened and forms a mesh-like pattern, which has never been reported in previous descriptions of Asteronyx ; this is provided as a new taxonomic character. Based on absence/presence of mesh-like skin, one of the three lineages was assigned to A. loveni and another was described as Asteronyx reticulata new species. This new species can be distinguished from congeners by having mesh-like skin and small genital slits at the innermost position on the interradial oral disc. [ABSTRACT FROM AUTHOR]

The power and throughput of next-generation sequencing is instigating a major transformation in our understanding of evolution and classification of life on our planet. The new trees of life are robust and comprehensive. Here we provide a landmark phylogeny of the living ophiuroids and use it as the basis for a major revision of the higher classification of this class of marine invertebrates. We used an exon-capture system to generate a 1484 exon (273 kbp) data-matrix from DNA extracted from ethanol-preserved museum samples. We successfully obtained an average of 90% of our target sequence from 576 species spread across the known taxonomic diversity. The topology of the major lineages was robust to taxon sampling, exon-sampling, models and methods. However, estimates of node age were much less precise, varying by about a quarter of mean age. We used a combination of phylogenetic distinctiveness and temporal-banding to guide our revision of the family-level classification. Empirically, we determined that limiting family crown age to 110 ± 10 Ma (mid Cretaceous) selected phylogenetically distinct nodes while minimising disruption to the existing taxonomy. The resulting scheme of 32 families and six orders considerably expands the number of higher taxa. The families are generally longitudinally widespread across the world’s oceans, although 17 are largely confined to temperate and equatorial latitudes and six to relatively shallow water (less than 1000 m depth). [ABSTRACT FROM AUTHOR]

A Lochkovian (Early Devonian) ophiuroid (Echinodermata), Ophiocantabria elegans n. gen. and sp., is based on a single small, well-preserved specimen collected from a shale-rich horizon of the Furada Formation, Asturias, Spain. Sedimentologic and palaeontologic data suggest its occurrence was in a near-shore setting subjected to frequent storms. Ophiocantabria is assigned to the Encrinasteridae based on the morphology of individual skeletal elements, although overall form of the new species is similar to that of approximately coeval members of the asteroid family Xenasteridae. Such homoplasy, especially among earlier members of asterozoan class-level clades, is an important but not well understood aspect of subphylum evolution.

Several details of the anatomy of the axial complex in brittle stars remain unknown, and there are many mismatching descriptions of its structure in different sources. The comparison of the ophiuroid axial complex with other classes of echinoderms is important for understanding of the phylogenetic relations in Ophiuroidea. We describe the organization of the axial complex of Ophiura robusta, compare it with other Ophiuroidea and analyse how the specific structure of the brittle star axial complex could appear in evolution. The standard technique of dehydration of material in alcohols of increasing concentration was used, followed by embedding material in paraplast and dissection. In the main components, the axial complex of Ophiuroidea fits other Asterozoa. But there are some important differences. The stone canal connects with the ambulacral ring from the outside, not from the inside. The somatocoelomic perihaemal ring is closer to the mouth than the axocoelomic ring. The axial complex lies between the genital coelom and the digestive tract. The gastric haemal ring is located on the outer side of the axial complex. The 'pericardial' part of the axial organ is shifted to the oral side, but all its anatomical connections are retained: with the genital haemal ring, with the haemocoel of the body wall, with the gastric haemal ring and the vessels of the axial part of the axial organ. All these features could be explained as a result of shifting of madreporic plate along interradius CD from the aboral side to the oral side. [ABSTRACT FROM AUTHOR]