Microstomidae (Platyhelminthes: Macrostomorpha) diversity has been almost entirely ignored within recent years, likely due to inconsistent and often old taxonomic literature and a general rarity of sexually mature collected specimens. Herein, we reconstruct the phylogenetic relationships of the group using both previously published and new 18S and CO1 gene sequences. We present some taxonomic revisions of Microstomidae and further describe 8 new species of Microstomum based on both molecular and morphological evidence. Finally, we briefly review the morphological taxonomy of each species and provide a key to aid in future research and identification that is not dependent on reproductive morphology. Our goal is to clarify the taxonomy and facilitate future research into an otherwise very understudied group of tiny (but important) flatworms.

The meiofauna is an important part of the marine ecosystem, but its composition and distribution patterns are relatively unexplored. Here we assessed the biodiversity and community structure of meiofauna from five locations on the Swedish western and southern coasts using a high-throughput DNA sequencing (metabarcoding) approach. The mitochondrial cytochrome oxidase 1 (COI) mini-barcode and nuclear 18S small ribosomal subunit (18S) V1-V2 region were amplified and sequenced using Illumina MiSeq technology. Our analyses revealed a higher number of species than previously found in other areas: thirteen samples comprising 6.5 dm3 sediment revealed 708 COI and 1,639 18S metazoan OTUs. Across all sites, the majority of the metazoan biodiversity was assigned to Arthropoda, Nematoda and Platyhelminthes. Alpha and beta diversity measurements showed that community composition differed significantly amongst sites. OTUs initially assigned to Acoela, Gastrotricha and the two Platyhelminthes sub-groups Macrostomorpha and Rhabdocoela were further investigated and assigned to species using a phylogeny-based taxonomy approach. Our results demonstrate that there is great potential for discovery of new meiofauna species even in some of the most extensively studied locations.

Microstomum lineare is a common species of fresh and brackish waters found world-wide. Three genes (18S, CO1 and ITS) were sequenced from specimens of M. lineare collected from four countries, and the levels of cryptic diversity and genetic structuring were assessed. Results showed M. lineare has very wide haplotype distributions suggesting higher than expected dispersal capabilities. In addition, three new species were described on the basis of molecular taxonomy: Microstomum artoisi sp. nov., Microstomum tchaikovskyi sp. nov. and Microstomum zicklerorum sp. nov.