RESUMO
Interactions of holoplanktonic mollusks with symbionts and parasites are poorly known. We investigated the ecology of infection (prevalence, intensity, and abundance) in Firoloida desmarestia, caught during two sampling campaign sessions in 2012, off the Baja California Peninsula, Mexico (IMECOCAL, 83 stations) and a coastal research center near La Sorpresa Beach, Baja California Sur, in the Gulf of California (14 stations). Only females of F. desmarestia were parasitized. Hemiuroidea parthenita rediae infected 1% of F. desmarestia population at IMECOCAL, whereas young unencysted metacercariae stages of Opechona pyriformis (Lepocreadiidae) parasitized 6.6% of the same host species at La Sorpresa. Overall, finding of rediae and metacercariae represent new geographical and host records and shows that F. desmarestia has a dual host function in the life cycle of trematodes. As first intermediate host, F. desmarestia harbors hemiuroid rediae, functioning as the source of infection to other zooplanktonic groups by dispersing successive cercariae. As second intermediate hosts, it harbors infective unencysted metacercariae stages of O. pyriformis, which parasitize nektonic predators (fish), most likely through trophic interaction. Our results suggest that some trematodes are able to spend their entire life cycle infecting only pelagic hosts. Parasite-F. desmarestia interaction is shown in a conceptual model, where we propose that transmission of trematodes may occur between individuals of F. desmarestia within the same swarm. Relevance of F. desmarestia as a potential host in which life cycle abbreviation of trematodes may take place is discussed. This is the first quantitative study of helminth interaction on F. desmarestia in the Eastern Pacific.