Morphology and phylogeny of Itaspiella helgolandica, an interstitial marine flatworm with circumpolar distribution

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Flatworms in the family Otoplanidae Hallez, 1892, are ubiquitous inhabitants of the swash zone of marine sandy beaches worldwide. We present new insights into the morphology and phylogenetic position of the otoplanid Itaspiella helgolandica (Meixner, 1938) based on specimens sampled in northern Norway. While the species was originally described from the German coast, it has since been reported in the High Arctic as well as in Greenland, Canada and Alaska, indicating that it has a circumpolar distribution. Apart from these reports, I. helgolandica has received marginal scientific attention beyond the original species description, and apart from drawings, there is not much detailed data available of its morphology. We employ antibody staining and confocal laser microscopy to provide detailed morphological information on the ciliation, myoanatomy and genital organs of specimens of I. helgolandica and compare their morphology with other accounts of the species from across the Arctic. We investigate the phylogenetic relationships of Itaspiella Ax, 1955 through a Bayesian/RAxML analysis of 28S sequences and confirm the morphologically underpinned notion that this genus sits within the subfamily Otoplaninae Hallez, 1910, as the sister taxon to Xenotoplana Ax, Weidemann & Ehlers, 1978. In light of our findings, we explore the question of the apparent circumpolar distribution of Itaspiella in the context of the larger “meiofauna paradox” and touch upon various evolutionary phenomena that may have contributed to phenotypic stability despite its limited dispersal capabilities and the vicariance imposed by the complex recent geological history of the Arctic.