All things creepy: parasitism pt 3, the tongue biters

Cat got your tongue? Not in this case. Something else does.

Meet the “tongue biters,” Cymothoa exigua, a species of parasitic isopods in the family Cymothoidae. These things range in size from 0.3-1.1 inches in length for the females, and 0.3-0.6 inches in length for males. What they lack in size, they make up for in weirdness.

As a juvenile, the isopod will swim against the clock, so to speak, searching for a suitable host before it starves to death. If the creature is fortunate, it will come upon a fish and attach to the gills. Once mature, it finds its way to the fish’s mouth, the sought-after prize being the fish’s tongue (the basihyal). Here it severs the tongue’s blood vessels and like a crustacean version of Count Dracula, drinks up. This eventually causes the tongue to wither away. The sneaky parasite will then attach itself to the tongue stub and remain here, feeding on fish mucus or blood. This is actually the only known instance of a parasite replacing an organ of its host.

This isopod is a protandric hermaphrodite, meaning it has a reproductive system where individuals mature as males, but reproduce as females later. So, this isopod starts life out as a male all the way to when it attaches to the gills. When another isopod shows up, this triggers the first one to change to a female. She enters the fish to start her blood sucking quest, and the fish’s mouth becomes quite a hot bed of isopod activity after this. The other male finds his way through the gill chamber to mate with the female, meaning the fish has two of these things inside its mouth. Then the female births a whole brood of males, and never has to leave the fish’s mouth during any of this!

The fish will use the isopod much how it would use its old tongue, had the whole thing remained. Some biologists note that the isopod does not completely decimate the basihyal, because without it, the fish dies. Not a good situation for the little crustacean who lacks the swimming prowess it had as a juvenile. With the fish host dead, the isopod can’t swim off to find a new host, and will also die.

How the isopod affects the host fish differs, and is not consistent among the fish species targeted by this parasite. Some fish don’t seem to be affected much at all. Others however are anemic, struggle to breathe or obtain sufficient nutrition, and others just die. For some fish, once the isopod breeds, her role in life is over and she either gets swallowed or drifts off to eventually die. Bad news for the fish now without any tongue at all, even if it was a creepy parasite tongue.

Some biologists have suggested that infected fish may even use the isopod as a tool of sorts, assisting it with better food accrual. From an evolutionary stand point, this would seem to benefit both host and parasite. A healthier fish means a potentially longer life, and more time for that sneaky lady crustacean to breed and send her babes into the world.

Read all of our All Things Creepy Halloween blogs!

Part 1: mermithids and earwigs

Part 2: the corpse lily

Part 4: mite pockets

Part 5: crypt keepers

SOURCES

“Cymothoa Exigua: Meet the Sex-Changing, Tongue-Eating Parasite.” Futurism

Simon, Matt. “Absurd Creature of the Week: This Parasite Eats a Fish’s Tongue – and Takes Its Place.” Wired, November 22, 2013.

Thomas, Pete. “Recent catch ‘spooky’ because of what’s living in its mouth.” USA Today, October 21, 2021.

Wu, Katherine. “The Mystery at the Base of One of Biology’s Strangest Relationships” The Atlantic, July 14, 2021.