"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

January 24, 2016

Artystone trysibia

The tongue-biter Cymothoa exigua is arguable one of the most (in)famous fish parasite in the world. It was famous enough to get a mention on the Colbert Report, and while the world recoil in collective horror at the sight of a fish which had its tongue replaced by a parasite, among its fellow parasitic crustaceans, tongue-biter's modus operandi is actually rather quaint. It can easily be upstaged by other parasitic isopods in the horror department, and today's post is about one those species.

The parasite we are featuring today is Artystone trysibia - it is in the same taxonomic group as the tongue-biter (Cymothoidae), and it parasitises a number of freshwater fish in the Amazonian basin. But unlike its more famous cousin which is content with merely living in the host's mouth, A. trysibia cranks the nightmare fuel up to eleven and lives inside a fleshy capsule in the host's body cavity.
Photo from Figure 2 of this paper
This parasite and others like it are actually relatively common. This parasite has been documented from a range of freshwater fish from South America, and has also been reported from aquarium ornamental fish. They're so common that A. trysibia has gained a common name - "ghili" - among the Kichwa people.

Female A. trysibia (right) and Female A. trysibia with larvae
Photo from Figure 3 of this paper
This study documented its presence in the Bristlemouth Armoured Catfish (Chaetostoma dermorhynchum). Despite its armouring, this fish has no protection against A. trysibia. Usually, the only sign of the parasite's presence is a small, gaping hole on their belly or their flank. But that hole serves as a window for the parasite within. For this study, these catfish were sampled from three pristine sites at the Tena River in the Amazonian region of Ecuador.

These catfish are fairly small fish, and most of them are about 15 - 20 cm long (6-8 inches), but A. trysibia can grow to 1.5 - 3 cm (0.6-1.2 inches) long and takes up quite a lot of space within the catfish. For comparison, it would be the equivalent of having something the size of a pet rabbit living in your torso. As mentioned above, the only contact the parasite has with the outside world is with a tiny hole through which they breath and release their offspring, and they can reproduce in prodigious numbers - one female isopod was recorded to be carrying 828 larvae. Each catfish was found to (thankfully?) only ever have a single A. trysibia, and it seems that the bigger the host, the bigger the parasite, possibly because a larger host would give the parasite more room to grow.

Artystone trysibia is not alone in its style of parasitism, there are other similar species which are found in both freshwater and marine fish around the world (for example, see here). So the next fish which you come across might not just have a tongue-biter in its mouth, it might also have a ghili in its belly.

Junoy, J. (2016). Parasitism of the isopod Artystone trysibia in the fish Chaetostoma dermorhynchum from the Tena River (Amazonian region, Ecuador). Acta Tropica 153, 36-45.

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