Phylogenetic relationships of a new schizorhynch genus refute the monophyly of Schizochilus Boaden, 1963 (Platyhelminthes, Rhabdocoela)

Abstract

Rhabdocoel flatworms constitute one of the most ecologically diverse and speciose groups of microturbellarians (Platyhelminthes). Within this clade, Schizorhynchia, a monophyletic group of nearly 200 species worldwide, remains taxonomically challenging, with a poorly resolved phylogeny and a classification that does not accurately reflect the taxon’s evolutionary history. In this contribution, we describe a new schizorhynch genus from Cuba and the southeastern coast of the United States, integrating morphological data with new molecular phylogenetic evidence. Species of this new genus exhibit a conjuncta-duplex type male copulatory organ, which terminates in a sclerotised cap and comprises a prostate vesicle, cirrus, and stylet. The female system features an elaborate bursa that proximally opens to the digestive parenchyma through an internal pore. The three new species described here are primarily distinguished by differences in bursa morphology and stylet length. Morphologically, members of this new genus most closely resemble species of Schizochilus Boaden, 1963, yet molecular evidence clearly separates them in the phylogeny. Our analyses further demonstrate that the current concept of Schizochilus is non-monophyletic, splitting it into two distinct lineages. To resolve this, we here propose to transfer two previously described species to the new genus and hypothesize that several additional species of Schizochilus will ultimately require reassignment once molecular data become available.