Colony discrimination and competition in the eusocial trematode, Himasthla rhigedana

Abstract

Abstract The California horn snail (Cerithideopsis californica) hosts a diverse community of trematode parasite species, yet these species rarely co-occur in the same host. Some trematodes in this community competitively exclude conspecifics and heterospecifics using a soldier caste. How these trematodes can distinguish colonymates from competitors is unknown. Here we examine patterns of colony discrimination in Himasthla rhigedana, a marsh-dwelling species of parasitic trematode that possesses a soldier caste in their intermediate snail hosts. Aggression assays pairing colonies against multiple opponents demonstrate that H. rhigedana distinguish between conspecific colonies, consistently directing more attacks towards colonies collected from a distant marsh. We demonstrate that conspecific interactions between colonies are predominantly symmetrical (both colonies attack during encounters), and that the likelihood of aggression is the same whether the attacker soldier is “sterile” (soldier redia with no germinal balls) or an “intermediate” (soldier redia with developing germinal balls). Recognizing heterospecific or conspecific threats is a necessary function for the evolution of soldier castes, which almost exclusively occur within eusocial insects. By finding parallels in disparate phyla, our results in H. rhigedana provide a foundation for understanding the evolution of colony discrimination generally, as well as in trematode species and other parasite taxa with similar competitive interactions.