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.