Lost in a taxonomic Bermuda Triangle: comparative 3D-microanatomy of cryptic mesopsammic Solenogastres (Mollusca)

Solenogastres (Mollusca) have a quite uniform bodyplan and an evolutionary history with few shifts out of their deep-water habitat and beyond their epibenthic lifestyle. Consequently, few clades inhabit the shallow subtidal mesopsammon; only Meiomeniidae (order Pholidoskepia) is entirely restricted to this habitat. What was initially designed as a comparative microanatomical redescription of Meiomeniidae to explore the diversity of this clade with its unique evolution, developed into a taxonomic nightmare of cryptic, co-occurring lineages: three out of four valid species of Meiomeniidae co-occur in coarse sands in the Bermuda archipelago and were re-collected at the respective type localities. We analyzed the material combining three-dimensional (3D) reconstructions from histological serial sections and ultrastructural data, providing novel insights into meiomeniid anatomy and discussing potential phylogenetic implications. However, not all collected material could be unambiguously assigned to known lineages of mesopsammic Solenogastres. In addition to meiomeniids, we discovered another co-occurring, externally highly cryptic but anatomically distinguishable lineage. It is provisionally placed within Dondersiidae, but its taxonomic assignment remains problematic due to an exclusive character mosaic and a unique foregut gland complex. Our study reveals the risk of creating chimeric taxa in small-bodied Solenogastres, as morphological characters needed for species delineation cannot be extracted from single individuals, while conspecifity based on external features is risky to assume with cryptic species co-occurring. Molecular markers will be needed to reliably retrieve Meiomeniidae from their current Bermuda Triangle of taxonomy and to proceed in solenogaster taxonomy confronted with a wealth of poorly known lineages especially in meiofaunal forms.