Challenging the Castrator Paradigm: Mating and Fertility in Rhizocephalan-Infected Male Shore Crabs (Carcinus maenas)

Parasitic castration by rhizocephalan barnacles is widely assumed to eliminate host reproductive capacity. In male Carcinus maenas infected by Sacculina carcini , the presence of the externa is widely regarded as causing functional reproductive suppression, both behaviourally and physiologically. However, the extent and reversibility of this suppression remain poorly documented. Here, we present the results of a series of behavioural and reproductive trials using C. maenas males naturally infected with S. carcini , collected from the Limfjorden, Denmark. In controlled laboratory experiments, we show that a majority of parasitised males, including some with visible adult externae, retain the ability to engage in mating behaviour, including courtship and pre-copulatory embrace of receptive, recently molted females. Mating success followed a predictable size hierarchy in competition trials, indicating that behavioural dominance is not disrupted by parasitism. Importantly, dissections revealed that females paired with parasitised males had received spermatophores containing morphologically intact spermatozoa in both examined cases—one from a male with a developing externa and the other from a male two months post-extirpation—providing evidence that fertility is likely retained despite infection. These findings suggest that reproductive suppression in S. carcini -infected C. maenas males may be incomplete and context-dependent. We discuss the implications for understanding host–parasite dynamics, potential recovery after parasite removal, and the broader ecological role of parasitised and previously parasitised males in natural crab populations.