The association of Zygocotyle lunata and Echinostoma trivolvis with Chaetogaster limnaei, an ectosymbiont of Helisoma trivolvis.

Helisoma trivolvis snails collected from a lake in Warren County, New Jersey, in June 2007 possessed the ectosymbiont Chaetogaster limnaei (Annelida). Some of these snails were also infected with larval stages of Zygocotyle lunata and Echinostoma trivolvis. Chaetogaster limnaei associated with the infected snails fed on the cercariae of both digeneans. Zygocotylae lunata cercariae were observed in the stomach of C. limnaei and whole cercariae were loosely attached to the ventral surface of the chaetogasters. Cercariae in the stomachs were digested within 48 hr and probably served as a source of nutrient for the annelids. Whole cercariae and 1 nonviable metacercaria of E. trivolvis were seen in the stomachs of the chaetogasters. The protective action of the chaetogasters on the transmission of the cercariae of E. trivolvis and Z. lunata to second intermediate hosts in the wild awaits further study.