"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

July 28, 2010

July 28 - Unnamed zygocercous cercaria (trematode)

Today's parasite is a digenean trematode. And like most other digeneans, it multiplies asexually within its snail first intermediate host (in this case, a marine snail) producing larval stages call cercariae which are then released into the environment to infect the next host in the life-cycle. However, unlike most digeneans where individual cercaria swim off independently, each fending for itself, the cercariae of this species rally in the mantle cavity of the host snail where they hook the ends of their tails together before leaving via the snail's exhalant siphon as a collective. As they leave the snail, they synchronise their swimming motion so that the entire aggregate moves as one.

To an unsuspecting fish, the writhing mass of cercariae resembles a struggling little zooplankton which would make for a tasty mouthful. Instead, as the fish swallows the wriggling ball of cercariae, the parasites get tangled up in the fish's mouth and begin penetrating into the host tissue. Imagine what a nightmarish experience that must be! It would be like you eating a handful of popcorn only to find in mid-chew that the popcorns are drilling their way into your throat!

The scale bar in the photo is 0.5 mm in length and the photo is from this paper:

Beuret, J. and Pearson, J. C. (1994) Description of a new zygocercous cercaria (Opisthorchioidea: Heterophyidae) from prosobranch gastropods collected at Heron Island (Great Barrier Reef, Australia) and a review of zygocercariae. Systematic Parasitology 27:105-12.

Contributed by Tommy Leung.


  1. "It would be like you eating a handful of popcorn only to find in mid-chew that the popcorns are drilling their way into your throat!"

    Dr. Leung could always have a second career in writing horror stories.

  2. It seems like snails are part of so many different parasite life cycles. Are there any known benefits to the snails?

  3. @ cmassey: There's plenty of inspiration from parasites which makes most horror stories look tame!

    @ Andy: No benefits at all for poor little snail. The snail loses out completely and gets castrated. Most species of digenean trematodes use a gastropod as the first intermediate host where they asexually multiply (i.e: clone themselves), there a few families that use bivalves and even one that uses a polychaete, but most use a snail. Having that mollusc first intermediate host is a common trait to all digeneans, just like what lactating is for mammal. Considering there are an estimated 24000 extant species of trematodes, it's not surprising that snails show up so often as an intermediate host in a parasite (in this case digenean) life-cycle.

    Actually...you'll need a whole "Digenean of the Day blog" to run for about 60 years to feature all the digenean species currently in existence! How's that for biodiversity!