Point of View The rearticulation of annelids

Annelid Diversity: Historical Overview and Future Perspectives

Annelida is a ubiquitous, common and diverse group of organisms, found in terrestrial, fresh waters and marine environments. Despite the large efforts put into resolving the evolutionary relationships of these and other Lophotrochozoa, and the delineation of the basal nodes within the group, these are still unanswered. Annelida holds an enormous diversity of forms and biological strategies alongside a large number of species, following Arthropoda, Mollusca, Vertebrata and perhaps Platyhelminthes, among the species most rich in phyla within Metazoa. The number of currently accepted annelid species changes rapidly when taxonomic groups are revised due to synonymies and descriptions of a new species. The group is also experiencing a recent increase in species numbers as a consequence of the use of molecular taxonomy methods, which allows the delineation of the entities within species complexes. This review aims at succinctly reviewing the state-of-the-art of annelid diversity and summarizing the main systematic revisions carried out in the group. Moreover, it should be considered as the introduction to the papers that form this Special Issue on Systematics and Biodiversity of Annelids.

Are Echiura derived from a segmented ancestor? Immunohistochemical analysis of the nervous system in developmental stages of Bonellia viridis

Despite several unquestionably homologous characters with Annelida, the Echiura have generally been considered to form a distinct taxon due to apparent lack of segmentation: neither in the body cavity nor in any other structures of the adult animals have clear signs of a metameric organization been observed. However, it must be considered that this lack of segmentation could represent a secondary condition and that Echiura are derived from segmented ancestors. An immunohistochemical analysis visualized with confocal laser-scanning microscopy (cLSM) shows the development of serially repeated units in the nervous system of Bonellia viridis. This organization corresponds to the metameric ganglia typical of Annelida. Antibodies against the neurotransmitters serotonin (5-hydroxytryptamine) and FMRFamide label distinct subsets of neurons. Their perikarya are arranged in discrete repetitive units in the ventral nerve cord of different larval stages. Labeling of neurotubuli using antibodies against different tubulin isoforms provides additional support for the metameric character of the nervous system. Contrary to previous descriptions, the peripheral nerves extending from the ventral nerve cord into the body wall musculature are paired and are evenly distributed; their arrangement corresponds to that of the serotoninergic perikarya. Morphological and neurochemical differentiation of the nervous system clearly proceeds from anterior to posterior, indicating the occurrence of a posterior growth zone. The serial ganglionic organization of the nervous system of Articulata is generally regarded as having evolved in conjunction with repetitive muscular units, in particular with the formation of typical annelid segments. Therefore, these results are interpreted as an indication that Echiura are derived from segmented ancestors and thus support the systematic inclusion of the Echiura within the Annelida.

Molecular phylogeny of the Annelida

Traditionally, the Annelida has been classified as a group comprising the Polychaeta and the Clitellata. Recent phylogenetic analyses have led to profound changes in the view that the Annelida, as traditionally formulated, is a natural, monophyletic group. Both molecular and morphological analyses support placement of the Siboglinidae (formerly the Pogonophora) as a derived group within the Annelida; there is also evidence, based on molecular analysis of the nuclear gene elongation factor-1α, that the unsegmented echiurids are derived annelids. While monophyly of the Clitellata is well-supported by both molecular and morphological analyses, there is no molecular evidence to support monophyly of the polychaete annelids; the Clitellata fall within a paraphyletic polychaete grade. Relationships among groups of polychaete annelids have not yet been resolved by molecular analysis. Within the Clitellata, paraphyly of the Oligochaeta was indicated in a phylogenetic analysis of cytochrome c oxidase I, which supported a sister relationship between the leeches, including an acanthobdellid and a branchiobdellid, and two of the four oligochaetes in the analysis. There is some evidence from analysis of 18S rRNA sequences for a sister-group relationship between the clitellates and the taxon Aeolosoma. There is no agreement regarding the body form of the basal annelid, and while molecular analyses provide strong support for the Eutrochozoa, the identity of sister-group to the Annelida among the Eutrochozoa remains enigmatic. It is recommended that future investigations include additional conserved gene sequences and expanded taxon sampling. It is likely that the most productive approach to resolving annelid phylogeny, and thus increasing our understanding of annelid evolution, will come from combined analyses of several gene sequences.