Phenology of recruitment and infection patterns of Ascocotyle pachycystis, a digenean parasite in the sheepshead minnow, Cyprinodon variegatus

In northwest Florida, Cyprinodon variegatus are parasitized by Ascocotyle pachycystis, a digenean parasite that forms metacercarial cysts in the lumen of the bulbus arteriosus. Field experiments revealed that fish accumulated parasites at an uneven rate within the highly seasonal trematode recruitment period. Older (= larger) fish had higher rates of parasite recruitment and higher parasite prevalences and densities (numbers of metacercariae per individual fish) than did younger (= smaller) fish. Nearly all adults were parasitized (prevalence range 70-100%), and parasite densities ranged from zero to 6800 per fish. Parasite distributions were clumped (= aggregated) in fish of all age classes but were less heterogeneous in early juveniles and adults than they were in late juveniles. Parasites affected the population dynamics of sheepshead minnows by causing reduced winter survivorship, as evidenced by an increase in the average size of fish and a decrease in the average infection intensity over the winter.