The evolution of annelids reveals two adaptive routes to the interstitial realm.

Many animals permanently inhabit the marine interstitium, the space between sand grains [1, 2]. Different evolutionary scenarios may explain the existence of interstitial animals [3, 4]. These scenarios include (1) that the interstitial realm is the ancestral habitat of bilaterians [5, 6], (2) that interstitial taxa evolved from larger ancestors by miniaturization, or (3) progenesis [3]. The first view mirrors the former hypothesis that interstitial annelids, called archiannelids, were at the base of the annelid radiation [7]. Based on morphological data, however, progenesis is generally favored for interstitial annelids today [3, 4, 8]. Herein, our phylogenomic approach revealed that interstitial archiannelids are robustly placed into two groups nested within the annelid tree. Evolution of the first group comprising among others Dinophilidae is best explained by progenesis. In contrast, the second group comprising Protodrilida and Polygordiidae appears to have evolved by stepwise miniaturization adapting from coarser to finer sediments. Thus, in addition to progenesis [3, 4], miniaturization, thought to be too slow for an adaptation to the interstitium [3], is an important second route allowing adaptation to interstitial environments. Both progenesis and miniaturization should be considered when investigating evolution of interstitial taxa [1, 3].

Progenesis in Eunicida ("Polychaeta," Annelida)--separate evolutionary events? Evidence from molecular data.

The Eunicida are a well-defined annelid taxon currently comprising Dorvilleidae, Lumbrineridae, Eunicidae, Onuphidae, and Hartmanniellidae. Especially the Dorvilleidae include several species of small body size and apparently simple organisation, resembling larvae and juveniles of larger species. One hypothesis to explain the evolution of such species is progenesis. By this criterion the Dinophilidae were included in this group as well. Although their overall similarity suggests monophyly, parallel evolution of such progenetic taxa cannot be excluded. To infer the phylogeny of Eunicida and their presumed relationship to Dinophilidae and progenetic dorvilleids, 18S rDNA sequences from various Dinophilidae, Dorvilleidae, and members of other Eunicida were determined. The data matrix were extended by adding sequences from several annelid and molluscan species and analysed with maximum-parsimony, distance, and maximum-likelihood algorithms. Highly significant bootstrap-values support monophyly of the Dinophilidae, of a clade enclosing the Eunicidae and the Onuphidae, and of a dorvilleid group comprising six species including the presumed progenetic Parapodrilus psammophilus. The position of the Dinophilidae remains unresolved, so that their inclusion within the Eunicida can neither be corroborated nor rejected. However, a constraint monophyly of Dinophilidae and P. psammophilus is rejected by several tests, substantiating the hypothesis of separate progenetic evolutionary events. Furthermore, the molecular data indicate polyphyly of the Dorvilleidae.

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.