Confocal Laser Scanning Microscopy Applied to a New Species Helps Understand the Functioning of the Reproductive Apparatus in Stylet-Bearing Urodasys (Gastrotricha: Macrodasyida)

Gastrotrichs are highly diverse and abundant in all aquatic ecosystems; however, they are often overlooked. During a biodiversity survey in Sardinia (Italy), a new species of gastrotrich herein described was discovered. Specimens of Urodasys bifidostylis sp. nov. were found in sandy sediments from two submarine caves. Using an integrative approach of traditional light (DIC) and high-resolution (CLSM) microscopies, we herein reveal, for the first time, the fine structure and function of the reproductive organ in an Urodasys representative. This is particularly relevant considering the complex reproductive organs and strategies of this group. Results allow comparisons between the reproductive apparatus and sperm transfer modalities in Urodasys and the closely related genus Macrodasys. One similarity is that both groups transfer male gametes in packets, suggesting the production of spermatophores to be a common phenomenon in Gastrotricha. Unique to Urodasys is the ability of multiple and consecutive copulations and sperm transfers and, differently than Macrodasys, the transfer of sperms unlikely occurs simultaneously between the two hermaphroditic partners. These findings provide new insights into the reproductive strategies of Urodasys and are expected to advance future studies on the evolution of reproductive strategies and the rise of interspecific reproductive barriers in interstitial meiofauna.