Morphology of parenchymally implanted foreign bodies indicates copulatory wounding in a planarian

Traumatic mating, that is, copulation that involves wounding the partner's body, is a widespread phenomenon that is particularly prevalent in hermaphroditic animals. Traumatic mating is generally a collateral side effect of diverse strategies (from physical anchorage to injection of substances to manipulate the partner), but the trauma could also be adaptive by itself if it delays remating by the injured partner. In the Tricladida (the clade of planarian flatworms), reciprocal sperm transfer is often assumed to occur by means of a 'regular' nontraumatic copulation, that is, insertion of the penis through the partner's gonopore and deposition of the ejaculate into its genital atrium, with subsequent sperm migration to the oviducts. However, while studying the anatomy of Brazilian land planarians for taxonomic purposes, we found foreign bodies, reminiscent of spermatophores, implanted within the parenchyma of Choeradoplana albonigra (Riester, 1938). Herein, we describe and illustrate several lines of morphological evidence indicating that these foreign bodies likely represent a novel case of intragenital copulatory wounding (e.g., structural and histochemical similarity to land planarians spermatophores; implantation at the level of the gonopore; vestiges of rupture of the genital atrium's wall), corroborating that traumatic mating is pervasive and underreported in Metazoa. We also propose two different hypotheses to explain such copulatory wounding, viz., that it concerns (1) a regular mating strategy or (2) an accidental effect of the copulation. In any event, this land planarian may prove useful as a novel, noninsect terrestrial model organism to investigate the evolution of traumatic mating.