Encystment patterns and metacercarial size of an opecoelid trematode in two polychaete hosts

The growth, and thus to some extent the fitness, of parasites may vary among individuals based on a range of factors including which host species they infect or how many other parasites share the host with them. Here, we investigate the determinants of the size attained by metacercariae of an opecoelid trematode within two species of intertidal polychaetes serving as second intermediate host, Heteromastus filiformis and Abarenicola affinis. Metacercarial cysts were found predominantly in the anterior region of H. filiformis, particularly in segments 3-6, whereas no apparent pattern of encystment was observed in A. affinis. No relationship was found between the volume achieved by an individual metacercarial cyst and either host body volume, total number of cysts per host, the segment number in which a cyst was located, or the number of other cysts sharing a distinct segment; this was true for parasites in either polychaete species. However, cysts were found to be significantly larger, on average, within A. affinis than within H. filiformis. It remains to be determined whether the polychaete species in which larger sizes are achieved is also the one in which transmission to fish hosts is maximized.