Recent postglacial population expansions may explain a surprising lack of lineage splitting in Arctic meiobenthic flatworms

Meiobenthic metazoans often showcase a puzzling combination of poor dispersal capabilities and wide geographical distributions. Because meiofauna ‘should’ be prone to endemism, the surprising observation of widely distributed morphospecies has spurred substantial interest and explanatory attempts. We place the searchlight on two small flatworm species – Itaspiella helgolandica and Notocaryoplana arctica – which are particularly restricted in habitat preferences and dispersal capabilities but which occur across the Arctic. Two hypotheses which could account for their wide distributions are explored: (1) populations are genetically divergent but showcase morphological stasis in an instance of cryptic speciation and (2) postglacial population expansion occurred recently enough that genetic drift and local adaptation has not produced divergent allopatric clades yet. We observed consistent gross and copulatory stylet morphology across local and global scales, confirming the general lack of morphological differences noted by other authors. Our phylogenetic analyses and species delimitation test (bPTP) of partial ribosomal gene sequences (28 S and 18 S rDNA) support the monophyly of each species, with no apparent signs of nascent allopatric lineage splitting. Likewise, haplotype networks do not indicate geography-derived clustering of populations but rather point toward interconnectivity across global and local scales. While our sample sizes and the genetic markers included limit the interpretations of the study, the star-like shapes of the haplotype networks together with negative Tajima’s D and significant, negative Fu’s Fs test values strongly point toward a history of recent population expansions in the studied flatworms. We find strong support for postglacial population expansions as the most parsimonious explanation for the observed morphological similarities and a weakening of competing hypotheses, e.g. that of widespread cryptic speciation. We note that both species may still be dispersing over long distances e.g. through drifting of egg capsules, and our findings carry important implications for phylogeographical studies of meiobenthic invertebrates and the associated ‘meiofauna paradox’.