Aggregations in the “Parasite–Host” System: Formation and Adaptive Value
Severtsov Institute of Ecology and Evolution, RAS, Moscow, Russian Federation
E-mail: vicnikmik@gmail.com
Diversity of the life cycles of parasites, their living in a wide range of habitats, and interactions with different hosts create many ecological and behavioral situations in which it is difficult to act alone. Among individual and group adaptations that determine the success of parasite transmission and, ultimately, their fitness, aggregation in the external environment and in host populations plays a crucial role. A review and analysis of the behavioral and ecological characteristics of monoxenous (ectoparasitic crustaceans Branchiura, Monogenea) and heteroxenous (Trematoda) parasites allows us to consider aggregation as one of the main factors determining the success of parasite transmission in the “host space”. The success of parasite dispersal and transmission, besides the large number of dispersal larvae produced, is facilitated by a variety of behavioral tactics: active search for hosts; various types of the “sit-and-wait” tactic; aggregation into mono- or polymorphic colonies which increase the availability of the parasite to the next host (predator); manipulation of the intermediate host’s phenotype that reduces/increases its availability to the predator. Aggregation affects transmission processes at all stages of the life cycle of both mono- and heteroxenous parasites. Along with parasite aggregations, host aggregations play a significant role in the parasite-host system. Host aggregations are not only the result of their self-organization and emergent properties of groups, but also a result of manipulation of their behavior by parasites. Aggregation of both parasites and their hosts serves as one of the most important elements of behavioral interactions in the parasite–host system. In parasites, it affects the success of transmission from one host to another; in hosts, it helps them avoid areas with a high risk of infection and maintain a significant portion of their population free of parasites. Aggregations increase stability of the parasite–host system and fitness of its members, helping parasites’ reproduction and transmission; in hosts, such as fish, aggregation provides protection from enemies (predators and parasites) and facilitates foraging and orientation.
