Beschreibung:
<jats:title>Abstract</jats:title><jats:p>Spionidae, particularly polydorids, are common polychaete parasites of edible mollusks around the world. However, our understanding of the regulatory factors and population structure of these parasites is scant. In this study involving <jats:italic>Polydora bioccipitalis</jats:italic> and the surf clam <jats:italic>Mesodesma donacium</jats:italic> we evaluated (1) the environmental correlates of the prevalence and mean intensity of the infestation, (2) the relationship between the number of egg capsules and juvenile and adult parasites and the time elapsed since infestation, and (3) the spatial patterns of juveniles and adults within the host. Environmental factors showed no significant correlations with prevalence and mean intensity, suggesting that these factors do not act directly as regulators. Rather, storm surges seemingly induced clam stranding, which in turn affected both the prevalence and intensity of the infestation. The numbers of juveniles and egg capsules in blisters were significantly related to the time since infestation, suggesting mechanisms of use and expansion of the space within the host. Juvenile worms showed an aggregated distribution that was probably related to the episodic nature of infestation events, whereas adults exhibited uniform distributions that probably reflect territorial behavior and reproductive strategies.</jats:p><jats:p>Received June 7, 2010; accepted April 7, 2011</jats:p>