3D-microanatomy of the mesopsammic Pseudovermis salamandrops Marcus, 1953 from Brazil (Nudibranchia, Gastropoda)

Species of the nudibranch Pseudovermidae Thiele, 1931 are rare but conspicuous inhabitants of the marine mesopsammon. Their characteristic vermiform body with reduced\ud cerata and acorn-shaped head lacking appendages is well adapted to life in the interstices of sand grains. Traditionally, species descriptions are based mainly on external morphology\ud and radula characteristics; knowledge on their anatomy is scarce. Here we provide the first microanatomical redescription of a member of Pseudovermidae based on 3D-reconstruction\ud from histological semi-thin section series. The present study on Pseudovermis salamandrops Marcus, 1953 reveals several discrepancies to the original description especially within the complex triaulic genital system (i.e., absence of a connection between vas deferens and kidney, presence of a receptaculum seminis and a large muscular penial sheath gland).We also add microanatomical details such as the presence of gastroesophageal ganglia in the central nervous system, described for the\ud first time in Pseudovermidae. Concluding from the nematocysts found in the cnidosacs of P. salamandrops, this species is a cnidarivore which likely preys on various meiofaunal cnidarians. We show that microanatomical redescriptions of poorly known Pseudovermidae are needed to gather comparative data as a backbone to place these neglected meiofaunal slugs in a phylogeny and trace their evolutionary pathway into the mesopsammon. Traditional characters used for species delineation are insufficient to diagnose Pseudovermis and an integrative approach is needed to reliably address pseudovermid diversity in the future