"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

November 19, 2010

November 19 - Asymphylodora tincae

Today's parasite is commonly found in freshwater fishes of the Palaearctic region. This particular worm has evolved to skip a few steps to the usual three-host life-cycle of most digeneans. Asymphylodora tincae uses a snails as a first intermediate host, where larval stages known as cercariae are produced through asexual multiplication - so far so usual for digeneans. However, instead of leaving the snail like most digeneans, the tailless cercariae stay inside the snail. Additionally, instead of then developing into the cyst-like metacercariae stage as a prelude to infecting the definitive host, this species has done away with that altogether. The cercariae of A. tincae can directly infect the fish definitive host, which occurs when the fish consumes an parasitised snail.

Reference and photo source:
Našincová, V. and Scholz, T. (1994) The life cycle of Asymphylodora tincae (Modeer 1790) (Trematoda: Monorchiidae): a unique development in monorchiid trematodes. Parasitology Research 80:192-197.

Contributed by Tommy Leung.


  1. hey tommy
    what is that circle in the middle?
    it looks like some kind of suction cup.

  2. Yes indeed, it is a ventral sucker. Most trematodes have this structure. At the larval cercaria stage, it is used by the cercaria to attach itself to the surface of the second intermediate host it is preparing to infect, while at the adult stage it is used by the worm to attach itself to the preferred infection site in host tissue, such as the intestinal epithelium.