Today's parasite is a strange worm found in a habitat that may shock and bewilder many readers on multiple levels. It's a worm that lives in the rectal gland of the spotted ratfish (Hydrolagus colliei). The spotted ratfish belongs to an enigmatic group known as the chimaera, an ancient group of cartilaginous fish that branched off from sharks almost 400 million years ago. The parasite itself is just as mysterious - it belongs to a group call Aspidogastrea (we have featured other species from this group on the blog before - Lobatostoma manteri and Aspidogaster conchicola), which made the evolutionary split with the far more diverse digenean trematodes probably also a few hundred million years ago (unfortunately, most parasites don't leave fossils). Rugogaster hydrolagi can grow up to 15 mm long (a bit over half an inch), and is so-called due its "rugae", which are the ridges on its body that give it a corrugated, accordion-like appearance. The life-cycle (like most aspecst of its ecology) is unknown, though like other aspidogastreans, it most likely features a mollusc intermediate host.
Photograph by Klaus Rohde
"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 23, 2011
March 17, 2011
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.