An unaware worker at Galveston Island State Park on Texas' western Gulf Coast found a fish bearing a surprise.

How Isopods Replace Atlantic Croaker's Tongue

Where the tongue is supposed to be in the Atlantic croaker's gaping mouth, there were too many legs for any mouth or fish. These legs also come with eyes.

The croaker's tongue had been replaced with an isopod from the species Cymothoa. Females of this crustacean group move up the gills of the fish, pinching and funneling off the blood supply to the tongue until it withers and comes off. They then attach themselves to the residual fleshy stub and feast for years on the blood of their victim.

The park officials said on their Facebook page, "[It] happens to be the only known case where a parasite functionally replaces a host's organ."

These parasites are abundant in some fish species, such as the spotted seatrout, according to Mark Fisher, research director at Texas Parks and Wildlife.

While he'd never seen one in an Atlantic croaker (Micropogonias undulatus), a 2015 research confirmed the isopods prefer of this species.

The parasite, which may grow to be 8 mm long (0.3 inches), does not appear to modify the eating patterns of any of the fish it infects, it does cause them to have a slow growth, according to a 2013 research.

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Isopods 

Since the huge parasite plugging the mouth prevents the fish from taking up and filtering as much oxygen from the water, the researchers believe this is due to respiratory stress caused by obtaining less oxygen over time. Others link the decrease to the parasite's nutritional depletion.

The C. borbonica species infecting the Trachinotus botla fish was the focus of these observations. Although C. excisa is assumed to have great host specificity for this fish, researchers don't know which species has invaded the mouth of the Atlantic croaker.

While scanning wrasse to research the fish's evolution last year, Rice University biologist Kory Evans unintentionally discovered another of these isopods. While the tongue imposters continue to appear, there is still a lot that is yet to be known about them.

When a baby Cymothoa finds a suitable fish host, it feeds by attaching itself to its gills, then matures into a male. These isopods are protandrous hermaphrodites, which means they can develop as males before changing into females and reproducing.

How to Get Rid of Isopods

Here, males are normally seen on the gills before moving to the mouth. This change may only occur if the fish's gills are infested by more than one isopod, allowing the female to be impregnated by the male.

If the parasite hasn't killed the fish before it completes its life's goal - reproduction - it slips away, leaving the unfortunate fish suffering without a tongue.

Quarantine is the first line of defense (with close observation of new fishes), mechanical removal with forceps is sometimes possible, osmotic shock (freshwater or saltwater dips) is sometimes effective, organophosphate-based water treatment is occasionally necessary, and diflubenzuron is one option too.

When contemplating isopod infection treatments, keep in mind that certain isopods may detach from their hosts and and thrive independently for long periods of time.

Related Article: Giant Isopod in Japan has Stopped Eating for over Four Years

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