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The Rat King: unnamed zygocercous cercaria

As a kind of follow-on from last week when I drew the larva of a parasitic barnacle infecting a crab ([link]), I decided to start a new series call "Nightmare Begins" depicting the infective stages of parasites that either most people have not seen or heard of, and explain how they infect their hosts.

This week, I decided to draw the cercaria (the free-swimming infective stage of flukes) from an as yet unnamed species of parasitic fluke from Heron Island, Great Barrier Reef. I have previously written about this species a few years ago on the Parasite of the Day blog ([link]) but I thought I'd go into more details here.

The cercariae of most species of flukes leave the snails as free-swimming individuals that head out independently to infect the next host in the life-cycle. But there are some flukes which instead of having cercariae that swim off on their own, they aggregate into a conjoined swarm. The term used to describe those species is "zygocercous" (this only describe their morphology and is not intended to reflect their evolutionary relationship - so flukes that have zygocercous cercaria are not necessarily all closely related).

They are also called "rat king cercaria" or Rattenkönig - so named because of their resemblance to the phenomenon whereby the tails of a number of rats become interwined [link](folklore). But whereas presumably it was not the intention of the rats to get themselves all tangled up, that is not the case for this species of parasite.

As mentioned above, instead of producing cercaria that leave the snail and leads an independent (and very short, if it they don't reach a host in time) life, the cercariae of this species rally in the mantle cavity of the snail. They then bind to each other via the lower half of their tail, and once there is a critical mass of them, they let go and leave the snail as a single unit composed of a few dozens or up to a few hundred individuals.

Once free in the tropical waters, they synchronise their movement so that the entire aggregate swim in a wheeling, corkscrew-like motion. The reason for all this gregarious cooperation is that while each individual cercaria is tiny and barely noticeable (on the left), as an aggregated mass, they resemble zooplankton or some other tiny creature that a small fish might like to eat. This effect is enhanced by the coloration of the tail, which was described as "heavily orange pigmented".

When a fish comes along and decides to nom on this wriggling mass of parasite, they immediate latch on to the inside of the fish's mouth. Sensing that something's not right, the fish might try to spit the wriggly mass out, but by then the parasites have already stuck on using their ventral pad and begin digging into the fish's mouth. The fish might succeed in spitting out the tails, but they've already served their purpose and the parasites readily discard the tail, leaving just the main cercaria body (on the right).

Imagine what a nightmarish experience that must be; it would be like eating a handful of popcorn only to find in mid-chew that the popcorns are drilling their way into your throat and you can't spit them out!

So now that the parasite is in the fish's mouth, how does something less than 0.05 mm long get through the lining of a fish's mouth and gills? Well the front of the parasite is equipped with a retractable, beak-like penetration organ (I have depicted it in its protruded form) with which it can use to rasp into the fish's epidermis. In addition, around the rim of the oral ring where the penetration organ is housed are a series of gland openings that can secrete digestive enzymes to help eat away at the epidermis. The parasite use the combination of both to dig their way into the fish's body.

It is currently unknown where this parasite goes once it gets inside the fish. However experimental infections conducted with a related species call Galactosomum bearupi (also from Heron Island) found that they first migrate to the connective tissue, then they become particular attracted to nervous tissues. Once they have moved into the nervous tissue, they migrate to the spinal cord and eventually end up in the optic lobes of the fish's brain. What the parasite does once it gets there is anyone's guess at the moment...

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 27:105-112.

Beuret, J., Scott, D. A. and Pearson, J. C. (2000) The cercaria of Galactosomum bearupi Pearson, 1973 (Trematoda: Heterophyidae) at Heron and Masthead Islands, Great Barrier Reef, Australia. Systematic Parasitology 46:69-77
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Zippo4k's avatar
Wait, this is real? I'm familiar with cercaria, and I even read Parasite of the Day, but I'm surprised that I some how missed this.
The-Episiarch's avatar
Oh yes, this is very real indeed - there's a screengrab of the SEM photo of a cercarial cluster from the paper in this post here:…
I wrote that post quite a while ago so it is understandable that you missed it.

As weird as it is, this species is not the only one to have evolved to have these "ratking cercariae" - this tendency for cercariae to band together into a writhing mass to attract the attention of the next host has actually independently evolved in a few different families that infect fish as their second host. 
Zippo4k's avatar
Also, by any chance would you know how the penetrating organ works? It looks like a raptorial beak, but I was previously under the impression that cercaria penetrate using some chemical action?
Heytomemeimhome's avatar
Why does something this horrible look so cute?
luxluna's avatar
WOW i like cercaria first time saw parasite art on DA.
aletia's avatar
What an amazing creature ! As a parasite enthusiast, parasite forms and lifecycles never cease to astound me. This one in particular is quite spectacular, thank you for sharing ( and thanks for adding the scientific references too!!!), cant wait to see what's coming up next :D !
The-Episiarch's avatar
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