"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

March 17, 2011

Pygidiopsis macrostomum

Pygidiopsis macrostomum is a freshwater digenean from Brazil. Like most otherdigenean trematodes, it has a three-host life-cycle. It asexually multiplies in its first intermediate host, the snail Heleobia australis, producing cercariae (pictured) which are released into the surrounding water. The cercaria swims in an series of small, stepped leaps, and then spins rapidly on its own axis once it sinks to the substrate, almost like a tiny aquatic ballerina.

All this dance-like motion attracts the attention of guppies, the parasite's second intermediate hosts, which ingest the parasite and become infected. The parasites burrow into the mesentery tissue of the fish, where they form a cyst and await ingestion by the definitive host where the worm will mature into its adult stage. While the adult specimen of P. macrostomum were first described from a rat, a subsequent study have also found it in the piscivorous bat Noctilio leporinus which, given its diet, is more likely to be the parasite's usual definitive host.

Simões et al. (2009) The life history of Pygidiopsis macrostomum Travassos, 1928 (Digenea: Heterophyidae). Mem Inst Oswaldo Cruz 104:106-111.

Contributed by Tommy Leung.

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