the kinky bunga bunga of porn worms

Talk about kinky sex: flatworms would put your basic porn stars to shame! Biologist Lukas Schärer of the University of Basel, Switzerland says “One has to be specific about what one means by sex” In the specific case, Schärer is discussing hermaphrodite worms doing it in the 69 position — in other words, copulating flatworms. Schärer, the lead author of a new study on the intercourse habits and sperm design of flatworms, is an authority on the subject, having studied the behavior of a particular species, Macrostomum lignano, over the past decade. This species was discovered only in 1995 and so far found only on Mediterranean beaches. To mate, partners insert their male genitalia into each other’s female opening and inject sperm. Both worms then double over and appear to attempt to suck their partner’s sperm back out. Schärer says there are several possible explanations for this, all of which require an understanding of what sex means for an organism that is both male and female.

Schärer, L., Littlewood, D. T. J., Waeschenbach, A., Yoshida, W., & Vizoso, D. B. (2010). Mating behavior and the evolution of sperm design. PNAS, 108(4), 1490-1495.

Abstract – Sperm are the most diverse of all animal cell types, and much of the diversity in sperm design is thought to reflect adaptations to the highly variable conditions under which sperm function and compete to achieve fertilization. Recent work has shown that these conditions often evolve rapidly as a consequence of multiple mating, suggesting a role for sexual selection and sexual conflict in the evolution of sperm design. However, very little of the striking diversity in sperm design is understood functionally, particularly in internally fertilizing organisms. We use phylogenetic comparative analyses covering 16 species of the hermaphroditic flatworm genus Macrostomum to show that a complex sperm design is associated with reciprocal mating and that this complexity is lost secondarily when hypodermic insemination—sperm injection through the epidermis—evolves. Specifically, the complex sperm design, which includes stiff lateral bristles, is likely a male persistence trait associated with sexual conflicts over the fate of received ejaculates and linked to female resistance traits, namely an intriguing postcopulatory sucking behavior and a thickened epithelium of the sperm-receiving organ. Our results suggest that the interactions between sperm donor, sperm, and sperm recipient can change drastically when hypodermic insemination evolves, involving convergent evolution of a needle-like copulatory organ, a simpler sperm design, and a simpler female genital morphology. Our study documents that a shift in the mating behavior may alter fundamentally the conditions under which sperm compete and thereby lead to a drastic change in sperm design.

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