In the waters of the east Pacific lives a creature so strange and unsettling it has inspired a horror film called The Bay in which mutant parasites eat people’s tongues and take control of their minds.
Welcome to the world of the tongue-biting louse Cymothoa exigua, which has made it to the shortlist of Guardian’s invertebrate of the year competition and whose life cycle is stranger than fiction. South African Russell Yong, who nominated it for the competition, says: “A lot of people find these isopods disgusting but, like all parasites, theirs is a unique and important niche in the world, and with their interesting biology, sexual politics and iconic status, they’d be a great candidate for invertebrate species of the year.”
In the vastness of the ocean, you would be forgiven for hardly noticing tiny Cymothoa exigua as they float around looking for a host to prevent them from starving to death. These tiny crustaceans, which look a bit like woodlice, are on the hunt for a tongue.
When they come across a fish – such as a snapper fish or a salmon – they make a beeline for the gills and burrow their way in, clinging to the base of the tongue. This is where the sinister transformation starts.
Rather than simply killing the fish, they suck blood from the tissue of the tongue so it withers away. Unlike human tongues, which are flexible and muscular, fish tongues are predominantly bone surrounded by soft tissue. Their job is helping fish funnel down food and get water through their gills.
No job is too big for the inventive Cymothoa exigua, which takes over the job of the tongue, clinging on to the withered stump, keeping the fish alive by allowing it to feed. Once ensconced, it taps into the fish’s blood and mucus and can consume what the fish is eating, so it keeps itself and the fish alive (and kind of healthy). It is the only known parasite to do this.
It is believed they can survive like this for years, with the parasite growing along with the fish. Cymothoa exigua can grow up to an inch in length, about the size of a cockroach.
All these tongue-biting lice start out as males. The first to get inside the host fish changes sex to become a female. The females grow inside the mouth, eventually curling around the tongue and using strong hook-like legs to cling on.
She mates with any smaller males floating around in the gills and then spews out tiny new parasites, which seek another host to begin the wonderfully grim process again.
New arrivals are prevented from becoming females, possibly by the female excreting hormones that prevent the ability to change sex. It is believed the creatures may be able to survive up to three years and have several broods. If the female dies, the largest male will morph into a female.
There are lots of species of tongue parasites with different fish hosts. They damage the tongue in different ways, but all are incentivised to keep their host alive, as most can no longer swim once they have grown inside the mouth.
The lifecycle of Cymothoa exigua is a reminder that the natural world is more gruesome than most of us could imagine. This parasite is the most wonderful and devious of them all, which is why it deserves to be crowned invertebrate of the year.
Between 24 March and 2 April, we will be profiling a shortlist of 10 of the invertebrates chosen by readers and selected by our wildlife writers from more than 2,500 nominations. The voting for our 2025 invertebrate of the year will run from midday on Wednesday 2 April until midday on Friday 4 April.
