Alison Wunderlich, Esli Emanoel Domingues Mosna, Marcelo Antonio Amaro Pinheiro

Host body size can influence the evolution of parasite body size in many host-parasite associations, a hypothesis called Harrison’s rule (HR). However, this pattern has not always been consistently observed, with some studies finding no association between host and parasite size. Moreover, other host-related factors (e.g., sex, immunity) and environmental changes can mediate this relationship. Here, we used a long-term dataset of cymothoid isopod-palaemonid prawn to test whether Harrison's rule applies to the relationship between host and parasite body traits (i.e., length, sex, weight), while accounting for the effect of seasons and years. Prawns were sampled seasonally over four years in a tropical stream in southeastern Brazil. We found a strong positive relationship between parasite and host body traits (i.e., length and weight), supporting Harrison’s rule. Our results further demonstrated that this relationship is independent of host sex and seasonal variation (i.e., dry and wet seasons) but is influenced by interannual variability. Host length and weight explained approximately 70% of the model variation, while interannual effects accounted for 30%. These highlight that the covariation between host and parasite body traits may be modulated by environmental changes, potentially altering Harrison's rule. Future research should consider interannual variability as a potentially critical factor driving host-parasite interactions and their evolutionary dynamics.