Imagine yourself as a Common Jack Mackerel, swimming peacefully along the coast of New South Wales. Then, out of nowhere, an alien-looking crustacean enters your mouth through your gills, gets hold of your tongue and won’t let go. Slowly it sucks away the blood until it eventually becomes your tongue. You can’t remove it so you’re stuck with it – forever!

For some fish, this is a nightmare that just got real. Those alien creatures exist, and they’re called tongue biters – parasites that live in the mouths of fishes.

Two tongue biters in the mouth of a Bonito
Two tongue biters in the mouth of a Bonito caught on hook and line by G. Atkinson, at Cabbage Tree Island, east of Port Stephens, New South Wales, May 2004. Image: David and Leanne Atkinson
© David and Leanne Atkinson

Tongue biters in Australia

I first became interested in tongue biters in my early years as a marine biologist when I took a course in marine parasitology. It was in this class that I saw my first tongue biter, and it was love at first sight.

I was lucky enough to be awarded a scholarship from the Malaysian government to do a four-year research degree at the National Centre for Marine Conservation and Resource Sustainability and Museum of Tropical Queensland. My research aims to review the taxonomy (classification, or defining organisms according to shared characteristics) of tongue biters in Australia.

Tongue biters are isopods (a type of crustacean) belonging to the family Cymothoidae. This whole family consists of fish parasites, some of which attach to the fish’s mouth, others to the gills and fins, while others burrow into the flesh. They’re found on hundreds of species of marine and freshwater fishes throughout the world’s tropical and temperate regions.

Two of the more common mouth-attaching genera are Ceratothoa and Cymothoa. According to the Zoological Catalogue of Australia, six out of 29 Ceratothoa species and 13 out of 47 Cymothoa species are known from Australian waters on a wide range of fish hosts.

During my recent Geddes Postgraduate Award working with the Australian Museum’s collections I’ve discovered four Ceratothoa species and probably five Cymothoa species new to Australia. This increases the known diversity of the Australian region and gives us a better resolution of host-preference and distribution patterns. So my time here has certainly been worthwhile.

Cymothoa epimerica, Parasitic Isopod
Cymothoa epimerica - Parasitic Isopod Image: Ben Diggles
© DigFish Services Pty Ltd

The modified alien

These alien-looking parasites are adapted to a life of what an Australian might call bludging. After entering through the gills, the tongue biter hooks onto the fish’s tongue. Its seven pairs of strongly hooked legs cut off the blood supply, causing the tongue to degenerate, while anchoring the parasite against the currents. A thickened, calcified cuticle protects its slender, tapered (streamlined) body from abrasion. And there it stays, serving as a mechanical replacement for the tongue. Some species in genus Cymothoa have lost functional eyes, since there’s no need to ‘look around’ once they are attached.

It’s complicated

If their appearance seems strange, their life history is downright complicated. Cymothoids are protandrous hermaphrodites; in other words, all juveniles are males before developing into adult females. The female releases up to a hundred eggs at a time into a brood pouch on the underside of its abdomen. The eggs hatch and undergo two or more moults to form juveniles which are released into the water to seek out a suitable host.

The first male to parasitise a fish will change into a female whereas subsequent males attaching to the same fish remain male, possibly from a pheromone released by the female. Depending on where you live, you might never come across a cymothoid parasite, though they can be a nuisance in some fish farms and often turn up in recreational catches.

And while a parasitised fish mouth may not look very appetising, the fish is safe to eat. Just remove the parasite before cooking and try not to think about it.

Melissa Beata Martin, Geddes Postgraduate Fellow.

First published in Expore 35(1), pp 16-17 (2013)

Melissa is enrolled at the National Centre for Marine Conservation and Resource Sustainability, University of Tasmania and is based at the Museum of Tropical Queensland, Townsville. She is supervised by Dr Niel Bruce and Prof Barbara Nowak.

Further reading
Brusca, RC & Gilligan, MR, 1983. Tongue replacement in a marine fish (Lutjanus guttatus) by a parasitic isopod (Crustacea: Isopoda). Copeia, 3, 813–16.