| Summary: | 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 <i>Urodasys bifidostylis</i> 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 <i>Urodasys</i> 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 <i>Urodasys</i> and the closely related genus <i>Macrodasys</i>. 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 <i>Urodasys</i> is the ability of multiple and consecutive copulations and sperm transfers and, differently than <i>Macrodasys</i>, the transfer of sperms unlikely occurs simultaneously between the two hermaphroditic partners. These findings provide new insights into the reproductive strategies of <i>Urodasys</i> and are expected to advance future studies on the evolution of reproductive strategies and the rise of interspecific reproductive barriers in interstitial meiofauna.
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