We’re not inclined to think well of parasites, but not all organisms that parasitize others are created equal. For example, some parasites steal food that others have gathered, some force other animals to raise their babies, and some use other organisms just for locomotion. However, some parasites kill slowly, leech life force, or even control the minds and actions of their hosts. However, it’s very unusual for a parasite to ingest a body part of its host and then replace it, but hey, this is a big, weird world, and evolution has dabbled in a little bit of everything.
Take the parasitic isopod, for example Cymothoa exigua, which parasitizes fish. This isopod, which is a crustacean like a shrimp or lobster (it looks a bit like a roll poly, or a sow bug, which is a terrestrial crustacean), lives in the ocean and earns its living from a few different species in the perch family — mostly snappers and drums. The living they earn may seem a little a lot of (read: Boschian horror show) for our refined human tastes, however Cymothoa exigua makes an honest living by attaching itself to a fish’s tongue, sucking blood from it until it falls off, and then replacing it by clinging to the tongue stump and acting as a prosthetic tongue for the rest of its host’s life.
“It’s now able to consume what the fish eats or its blood and tissues,” says Regina Wetzer, curator and director of the Marine Biodiversity Center at the Natural History Museum of Los Angeles County.
Although there are other mouth infesting woodlice out there that feed on other fish species like barramundi or mahi-mahi, Cymothoa exigua is the only one known to science that eats and then replaces the tongue.
How protandrous hermaphrodites work
“Isopods in the family Cymothoidae are parasites of fish, and as juveniles, all cymothoids must find and attach to their host,” says Wetzer Cymothoa exigua – the tongue-attached kind – males enter the body via the gills, mature, mate, and females move to the tongue.”
How, you might ask, do the females enter the fish when only male hatchlings enter the fish’s gill slits? Cymothoa exigua is a protandrous hermaphrodite, meaning he has the ability to transform from male to female once adult. In other words, they are all born males, and when a baby enters a fish’s gills, it continues to mature as a male, but as soon as another male hatchling emerges, the first one gets the signal that it’s time to move in to turn into a female. After this matter is settled, the female crawls into the mouth of the fish host and begins to feed on his tongue. Once she’s replaced the tongue, she can mate with any of the males hanging around in the fish’s gill chamber and raise babies in a nice, safe den.
How to replace someone’s tongue
Cymothoa exigua is a stout little crustacean with seven pairs of legs tipped with spikes that help it anchor itself in the fish’s mouth. The first step in this process, however, is to use their five pairs of jaws, modified with a variety of ice pick-like tubes, to pierce the fish’s tongue and suck out the fish’s blood. Incidentally, this process is not perceived as very pleasant for the fish.
As the isopod empties the blood from the fish tongue, the muscle itself atrophies and atrophies. At this point, it grabs what’s left of the tongue stub with three or four of its spiny leg inserts and burrows in, functionally completely replacing the tongue.
As unpleasant as that is, these isopods generally do not kill their host. However, Cymothoa exigua does not survive well without a host.
“Without their adult host, adult isopods would not survive well because they are an obligate parasite,” says Wetzer. “She has lousy swimming skills, and pregnant females — females with eggs and young in her pouch — are particularly immobile. This is in contrast to some species [of the same family of isopods] which are free-living and can be found in such large numbers that they can completely flesh off a fish or body.”
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