Environmental variability influences the relationship between prawn host size and ectoparasite body size: insights from a long-term stream study

Larger host body size can lead to larger parasites, as more energy is available for parasite growth. However, this size-matching correlation can also be influenced by additional host and environmental factors. Here, we utilized a long-term dataset of cymothoid isopods parasitizing palaemonid prawns to investigate whether there is evidence supporting a relationship between host and parasite body traits (i.e., length, sex, weight), while accounting for the effects of season (i.e., dry and wet) and year. We also tested whether prawn body condition (i.e., Scaled Mass Index—SMI) is affected by parasitism. We compared parasitized and non-parasitized prawns by sex. Prawns were sampled seasonally over 4 years in a tropical stream in southeastern Brazil. Our analysis showed that isopod body length and weight were positively associated with host length and weight. This relationship was independent of host sex and seasonal variation, but was influenced by interannual variability. Host length and weight explained most of the variation, while environmental variability accounted for the remainder. We also showed that the SMI was reduced in parasitized individuals of both sexes, compared to non-parasitized prawns. Moreover, we found a significant difference in SMI between sexes and years, but not between seasons, when considering only parasitized hosts. The findings suggest that interannual variability and parasitism may jointly affect host energetics, shaping the dynamics of the ‘host-parasite-environment’ triad. Future research should consider interannual variability as a potentially key driver of host-parasite interactions in freshwater ecosystems.