Long-held ideas regarding the evolutionary relationships among animals have recently been upended by sometimes controversial hypotheses based largely on insights from molecular data. These new hypotheses include a clade of moulting animals (Ecdysozoa) and the close relationship of the lophophorates to molluscs and annelids (Lophotrochozoa). Many relationships remain disputed, including those that are required to polarize key features of character evolution, and support for deep nodes is often low. Phylogenomic approaches, which use data from many genes, have shown promise for resolving deep animal relationships, but are hindered by a lack of data from many important groups. Here we report a total of 39.9 Mb of expressed sequence tags from 29 animals belonging to 21 phyla, including 11 phyla previously lacking genomic or expressed-sequence-tag data. Analysed in combination with existing sequences, our data reinforce several previously identified clades that split deeply in the animal tree (including Protostomia, Ecdysozoa and Lophotrochozoa), unambiguously resolve multiple long-standing issues for which there was strong conflicting support in earlier studies with less data (such as velvet worms rather than tardigrades as the sister group of arthropods), and provide molecular support for the monophyly of molluscs, a group long recognized by morphologists. In addition, we find strong support for several new hypotheses. These include a clade that unites annelids (including sipunculans and echiurans) with nemerteans, phoronids and brachiopods, molluscs as sister to that assemblage, and the placement of ctenophores as the earliest diverging extant multicellular animals. A single origin of spiral cleavage (with subsequent losses) is inferred from well-supported nodes. Many relationships between a stable subset of taxa find strong support, and a diminishing number of lineages remain recalcitrant to placement on the tree.

Currently, between one-third and two-thirds of marine species may be undescribed, and previous estimates of there being well over one million marine species appear highly unlikely. More species than ever before are being described annually by an increasing number of authors. If the current trend continues, most species will be discovered this century.

Metazoa comprises 35-40 phyla that include some 1.3 million described species. Phylogenetic analyses of metazoan interrelationships have progressed in the past two decades from those based on morphology and/or targeted-gene approaches using single and then multiple loci to the more recent phylogenomic approaches that use hundreds or thousands of genes from genome and transcriptome sequencing projects. A stable core of the tree for bilaterian animals is now at hand, and instability and conflict are becoming restricted to a key set of important but contentious relationships. Acoelomorph flatworms (Acoela + Nemertodermatida) and Xenoturbella are sister groups. The position of this clade remains controversial, with different analyses supporting either a sister-group relation to other bilaterians (=Nephrozoa, composed of Protostomia and Deuterostomia) or membership in Deuterostomia. The main clades of deuterostomes (Ambulacraria and Chordata) and protostomes (Ecdysozoa and Spiralia) are recovered in numerous analyses based on varied molecular samples, and also receive anatomical and developmental support. Outstanding issues in protostome phylogenetics are the position of Chaetognatha within the protostome clade, and the monophyly of a group of spiralians collectively named Platyzoa. In contrast to the broad consensus over key questions in bilaterian phylogeny, the relationships of the five main metazoan lineages-Porifera, Ctenophora, Placozoa, Cnidaria and Bilateriaremain subject to conflicting topologies according to different taxonomic samples and analytical approaches. Whether deep bilaterian divergences such as the split Electronic supplementary material The online version of this article (between protostome and deuterostome clades date to the Cryogenian or Ediacaran (and, thus, the extent to which the pre-Cambrian fossil record is incomplete) is sensitive to dating methodology.

BackgroundSeveral unicellular organisms (prokaryotes and protozoa) can live under permanently anoxic conditions. Although a few metazoans can survive temporarily in the absence of oxygen, it is believed that multi-cellular organisms cannot spend their entire life cycle without free oxygen. Deep seas include some of the most extreme ecosystems on Earth, such as the deep hypersaline anoxic basins of the Mediterranean Sea. These are permanently anoxic systems inhabited by a huge and partly unexplored microbial biodiversity.ResultsDuring the last ten years three oceanographic expeditions were conducted to search for the presence of living fauna in the sediments of the deep anoxic hypersaline L'Atalante basin (Mediterranean Sea). We report here that the sediments of the L'Atalante basin are inhabited by three species of the animal phylum Loricifera (Spinoloricus nov. sp., Rugiloricus nov. sp. and Pliciloricus nov. sp.) new to science. Using radioactive tracers, biochemical analyses, quantitative X-ray microanalysis and infrared spectroscopy, scanning and transmission electron microscopy observations on ultra-sections, we provide evidence that these organisms are metabolically active and show specific adaptations to the extreme conditions of the deep basin, such as the lack of mitochondria, and a large number of hydrogenosome-like organelles, associated with endosymbiotic prokaryotes.ConclusionsThis is the first evidence of a metazoan life cycle that is spent entirely in permanently anoxic sediments. Our findings allow us also to conclude that these metazoans live under anoxic conditions through an obligate anaerobic metabolism that is similar to that demonstrated so far only for unicellular eukaryotes. The discovery of these life forms opens new perspectives for the study of metazoan life in habitats lacking molecular oxygen.