Impact on reproductive performance and body condition in a small limpet parasitized by a large castrator pea crab

Parasitic castrators utilize the energy reserves that the host allocates on reproduction resulting in sterilization of the host. However, whether other processes (e.g., growth) are also impaired depends on the balance between the castrator's energy requirements and the available resources that the castrated host does not use in reproduction. We investigated whether the castrator pea crab Calytraeotheres garthi alters body condition, reproductive performance, and occurrence of copulation in its limpet host Bostrycapulus odites. For this purpose, we examined the anatomy of the ovary, the seminal receptacles, and the body condition of parasitized and unparasitized limpets. The histology of the female gonad of parasitized limpets revealed the ovarian tubules are smaller and characterized by a greater proportion of intertubular and intratubular free space compared with non-parasitized individuals. The body condition of female limpets from all sizes (during summer) and those larger than ~16 mm (during spring and autumn) were impacted by the pea crab. These results are in contrast to that previously reported in the comparatively larger limpet species Crepidula cachimilla, in which the same pea crab species does not alter or even increase the host body weight. We concluded this pea crab species could drastically impair its host's reproduction and body condition although deleterious effects are species-specific and likely depend on limpet body size. The histology of seminal receptacles revealed an uncommon disposition of spermatozoa (i.e., excessive debris and acrosomes detached from epithelium) in seminal receptacles of some parasitized limpets. However, this analysis failed to determine whether sperm derived from present matings or previous pea crab infection. Further studies are needed to address whether pea crabs interfere with the mating behavior of limpets and if the alterations in sperm disposition are a consequence of castration.