Diurodrilus

Weigert, Anne & Bleidorn, Christoph, 2016, Current status of annelid phylogeny, Organisms Diversity & Evolution (New York, N. Y.) 16 (2), pp. 345-362 : 352

Diurodrilus and Lobatocerebrum

The genus Diurodrilus currently contains only six described species, and members are characterized as unsegmented, worm-like interstitial organisms, with a flattened body and minute size ( Worsaae and Kristensen 2005). Several characteristics support a placement among Annelida, e.g., the ultrastructure of the cuticle. However, Diurodrilus lacks typical annelid features such as segmentation, coelomic cavities, chaetae or nuchal organs and furthermore bear some similarities with Micrognathozoa (Lophotrochozoa, Gnathifera), such as the paired trunk ciliophores and the position of the protonephridia ( Kristensen and Niilonen 1982; Worsaae and Rouse 2008). Until recently, molecular data was limited and 18S and 28S rRNA data resulted in different phylogenetic positions either with Micrognathozoa or other lophotrochozoan taxa ( Worsaae and Rouse 2008). Mitochondrial genome data supported an inclusion within Annelida and placed them either with Orbiniidae or in the basal part of the tree ( Golombek et al. 2013). Due to the limited amount of molecular data and the unique morphological features their phylogenetic position could not be assessed, but showed a clear trend towards Annelida ( Worsaae and Kristensen 2005; Worsaae and Rouse 2008; Golombek et al. 2013). In the light of new molecular data and the incorporation of Diurodrilus within a large phylogenomic framework, the support for an annelid affiliation is substantiated ( Struck et al. 2015). As indicated with mitochondrial data, Diurodrilus groups within Sedentaria in close relationship with Orbiniidae together with Dinophilidae , Nerillidae , and Parergodrilidae in the newly composed clade Orbiniida ( Struck et al. 2015). A close relationship to orbiniids was also recovered in a phylogenomic analysis by Laumer et al. (2015). In summary, these new results settle the discussion and clearly support an annelid origin for Diurodrilus . As for other members of Orbiniida, an origin by progenetic evolution may explain the many reductions which have to be assumed for this taxon (e.g., loss of segmentation) ( Struck et al. 2015).

Another enigmatic taxon with uncertain affinities is Lobatocerebrum , which also lack segmentation and chaetae. Based on the structure of its organ systems, Lobatocerebrum had originally been described as an annelid lineage ( Rieger 1980, 1981). Recent investigations of the epidermis, nervous and muscular system of Lobatocerebrum riegeri using transmission electron and confocal microscopy are in line with this initial assessment ( Kerbl et al. 2015). As typical for annelids, a mid-ventral nerve could be found; even though as typical for interstitial groups the overall morphology was not only similar to other interstitial annelids, but also meiofaunal members of Platyhelminthes or Gnathostomulida. However, a placement within annelids was confirmed by a phylogenomic analysis, which groups them as sister taxon of Amphinomida + Sipuncula ( Laumer et al. 2015). Interestingly, they do not group with any of the other interstitial annelid taxa, but rather seem to represent a convergent case of inhabiting this environment.