Molecular phylogenetic position of a rare and enigmatic meiofaunal flatworm from the Pacific Ocean: Retronectes hyacinthe sp. nov. (Platyhelminthes: Catenulida)

Catenulids comprise the earliest diverging major lineage of flatworms. Although the majority of catenulid species live in fresh water, a small number of taxa have been documented from marine interstitial environments and most of these belong to the genera Retronectes and Paracatenula within the family Retronectidae. Representatives of Retronectes are extremely uncommon and almost only found in detritus-rich sand, with the last formal description of a species of Retronectes dating back to 1977. Little is known about the biology of the seven known species in this genus despite the fact that a unique combination of characters makes them relatively straightforward to recognize. Moreover, previous molecular phylogenetic analyses have so far been unable to include any representatives of Retronectes, so the phylogenetic position of these rarely encountered marine catenulids remains unclear. Here we describe a new species of Retronectes, namely R. hyacinthe sp. nov., from subtidal seagrass meadows in British Columbia (Canada) and present an updated phylogeny inferred from 18S and 28S rDNA sequences, including data from the new species of Retronectes and a selection of other catenulids. Our molecular phylogenetic trees suggest that Retronectidae sensu Sterrer & Rieger, 1974 is not monophyletic, implying that the current taxonomic classification of the Catenulida and the importance of certain morphological characters on which this classification is based are in need of revision.

Automated summary

Catenulids are the earliest diverging major lineage of flatworms, with most species inhabiting freshwater environments. However, a few species have been found in marine interstitial habitats, particularly in the genera Retronectes and Paracatenula. We describe a new species of Retronectes, R. hyacinthe, from subtidal seagrass meadows in British Columbia, Canada, and provide an updated phylogeny based on molecular sequencing. Our findings suggest that the current taxonomic classification of Catenulida and the importance of certain morphological characters need to be revised.