A Scanning Electron Microscopy Method to Visualise the Copulatory Organ Morphology of Microturbellarian Flatworms: <i>Trigonostomum</i> Schmidt, 1852 as a Case Study

Traditional methods for studying the morphology of microturbellarian flatworms rely on light microscopy, which often lacks the resolution necessary to capture fine structural details. Therefore, we present a protocol to improve the visualisation of structural morphological details in microturbellarians by means of scanning electron microscopy (SEM). We demonstrate this method by imaging the sclerotised copulatory organs of three species of Trigonostomum (Rhabdocoela, Trigonostomidae): T. venenosum, T. setigerum, and T. penicillatum. Additionally, we successfully visualise the bursal appendage of T. penicillatum. SEM imaging offered new morphological insights for the genus, and corrected earlier interpretations made with light microscopy. The method requires precision and careful handling, especially during the isolation of the hard parts. However, it is cost-effective and straightforward to carry out in any standard laboratory setting. Hence, our SEM protocol complements traditional light microscopy and opens new avenues for taxonomical research in microturbellarian taxa with hard parts.