Egg fibrils and transmission in the acanthocephalan Acanthocephalus dirus

Acanthocephalans have multi-host life cycles that include arthropods as intermediate hosts and vertebrates as definitive hosts. Eggs are dispersed into the habitat from definitive hosts and in some species eggs possess fibrils, which have been proposed to facilitate transmission to intermediate hosts. We examined the potential role of fibrils in transmission of the acanthocephalan Acanthocephalus dirus to its intermediate host Caecidotea intermedius, a stream-dwelling isopod. We identify three properties of fibrils that could favor transmission. First, there was a slow rate of fibril release, which was dependent on the actions of stream microorganisms. Second, eggs with fibrils were more likely to adhere to the substrate than those without fibrils. Third, in feeding trials, isopods exposed to eggs with fibrils had a higher infection prevalence than isopods exposed to eggs without fibrils. These properties could favor transmission by increasing the likelihood that eggs sink to the sediment occupied by their target hosts before adhering to items on the substrate (e.g., leaves) and by increasing recruitment after the eggs have been consumed.