Morphogenesis of Contracaecum rudolphii (Nematoda: Ascaridoidea), a parasite of fish-eating birds, in its copepod precursor and fish intermediate hosts

Eggs of Contracaecum rudolphii obtained from female worms in double-crested cormorants (Phalacrocorax auritus) in Nova Scotia, Canada, hatched in 9-17 days in sea water at 15-20 degrees C. The newly-emerged, free-living (and presumably second-stage) larva is described in detail, as are larvae from experimentally-infected copepods (Tigriopus sp.), amphipods (Gammarus sp.), and fish (Lebistes reticulatus, Fundulus heteroclitus). Copepods are considered precursor hosts and morphogenesis of the parasite in hem primarily involved body size, the ventricular appendix, and the excretory system. Infection of amphipods and fish was much more successful when invading larvae were from copepods than when they were free living; amphipods served as paratenic hosts and fish as intermediates. In fish, second-stage larvae 20 or fewer days postinfection were within the intestinal wall. At 44 or more days postinfection, larvae were considered third stage; all were in the abdominal cavity, many within a closely adhering sleeve, of material resembling cuticle. The longest larvae (3.1-3.9 mm) in fish were associated with the oldest infection (152 d), suggesting that larvae continue to grow for a considerable period of time; growth was also found to be asynchronous, however.