Pseudovermis paradoxus 2.0—3D microanatomy and ultrastructure of a vermiform, meiofaunal nudibranch (Gastropoda, Heterobranchia)
2019
Flammensbeck, Christina K. | Haszprunar, Gerhard | Korshunova, Tatiana | Martynov, Alexander V. | Neusser, Timea P. | Jörger, Katharina M.
Pseudovermidae is the only clade of nudibranchs entirely comprised of mesopsammic species probably resulting from paedomorphosis. These minute slugs show a worldwide distribution with most lineages described from European waters. The present study redescribes the type species of Pseudovermidae, Pseudovermis paradoxus Pereyaslavtzeva, 1891, from the Black Sea with modern methodology. We provide computer-based 3D reconstructions of all organ systems and ultrastructural data on the digestive and renopericardial systems. Several aspects of the external morphology and the highly concentrated central nervous system in P. paradoxus are paedomorphic. The presence of a vestigial pericardium without a heart is interpreted as co-adaptation to the minute, vermiform body with a relatively large surface. The (partially) triaulic hermaphroditic genital system shows a small penis, suggesting true copulation as mode of sperm transfer. We provide a molecular barcode and a neotype in line with our detailed 3D microanatomy and ultrastructural data to establish a baseline for revision of Pseudovermidae. The current taxonomy of Pseudovermidae in European waters is likely artificially inflated, impeding a better understanding of distribution and diversification within the clade. Our study highlights the need for a taxonomic revision of European pseudovermid species based on molecular data, as traditional taxonomic characters mostly present a higher intraspecific rather than interspecific variation or might present artifacts (i.e., “denticulated” jaws).
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