Abstract
Host body size is often positively associated with parasite burden, but this pattern observed in naturally infected hosts can reflect cumulative parasite acquisition over time rather than size-dependent susceptibility to new infections. To disentangle these processes, we experimentally tested how host body size and species identity influence susceptibility to infection in two marine isopods, Paridotea ungulata and Isocladus armatus , maintained together with snails infected by the trematode Maritrema novaezealandense , for which both isopods serve as second intermediate hosts. Larger I. armatus acquired more metacercariae than smaller conspecifics, whereas parasite intensity in P. ungulata peaked in medium‑sized individuals. Contrary to predictions based on field patterns and body size alone, I. armatus acquired more parasites than P. ungulata , likely due to greater microhabitat overlap with infected snails during exposure. These findings demonstrate that size-dependent susceptibility does not necessarily mirror cumulative infection patterns and highlight the importance of host behaviour, microhabitat use, and ontogenetic changes in morphological traits in shaping parasite transmission.