When considering mechanisms of species coexistence, reproductive interference is often overlooked in favor of ecological interactions (e.g., competition and predation). In many taxa, however, closely related species share components of their mating biology leading to interspecific mating attempts. Such interactions are expected to impose strong costs on individuals involved that may have consequences for both species distributions and the evolution of mating traits. In this study, we used interbreeding experiments to assess the potential for reproductive interference in a freshwater amphipod species complex (species in the genus Hyalella). Using three undescribed species (Species A, B, & C) that co-occur in the littoral zone of small lakes, we performed pairwise mating trials and recorded precopulatory mate guarding, the presence of viable embryos, and female survival.

Results/Conclusions

We found that intraspecific pairs produced viable embryos in > 80% of trials, while none of the interspecific pairings produced viable embryos. Reproductive interference occurred among the three species, but it was not symmetrical. Species B males occasionally guarded females of the smaller Species A, and these mating interactions resulted in a 50% increase in female mortality relative to interactions with conspecific males. On the other hand, Species B males did not guard Species C females. This absence of interspecific mating interactions may explain why Species C females had high survival (85-90%) regardless of male species identity.

These results suggest that reproductive interference may contribute to the distribution of the three species in lakes. Species A and Species B have the most disjunct distributions; Species A is found in macrophyte beds well offshore whereas Species B is found primarily in shallow, near-shore habitats. Species C has a more continuous distribution along this gradient, although they are most common at mid-distances from shore. Future experiments that explore behavioral interactions that take place during interspecific encounters, and how traits (e.g., body size) mediate these interactions will shed light on the potential for reproductive interference to shape trait evolution in this species complex.