Unsuspecting workers at Galveston Island State Park, Texas' western Gulf Coast, discovered a fish that was quite unexpected.
The gaping mouth of the Atlantic croaker (Micropogoniasundulatus), where its tongue should be, was actually occupied by far too many legs to accommodate any fish or mouth. These legs also came with eyes.
The croaker's tongue was replaced by an isopod of the genus Cymothoa. This group of crustaceans' females migrate up from the fish's stomachs and pinch the tongue to drain the blood supply. The organ then falls off. The females attach themselves to the fleshy stub and continue to eat their captive's blood for many years.
The park workers shared the following on their Facebook page: "[It] happens that it] is the only known case in which a parasite functionally substitutes an organ of a host."
Mark Fisher, Texas Parks and Wildlife's science director, explained that parasites can be found in certain fish species like the spotted sitrout. He'd never seen one before in the Atlantic croaker (Micropogoniasundulatus), but a 2015 study revealed that they were quite fond of this host.
The parasite can grow up to 8mm (0.3 inches) in length and can cause changes in the feeding habits of certain fish, according to a 2013 study. However, it can reduce their growth.
Researchers suggested that this could be caused by respiratory stress. The parasites in the mouth prevent the fish from absorbing and filtering as much oxygen as possible. Others blame the parasite's loss of nutrients for this decrease.
These results were specific to C. borbonica, which infected the Trachinotus Botla fish. Although we don't know which species infested the mouth, the Atlantic croaker is believed to be infected by C. excisa.
Different fish species have mouth dwelling isopods. (Nico Smit, Int. J. Parasitol, 2014)
Unexpectedly, Rice University biologist Kory Evan also found another one of these isopods last year while studying the evolution of wrasse. These species are still very much unknown, despite the fact that tongue imposters continue to pop up.
A young Cymothoa attaches to its gills and feeds on a fish host. These protandrous hermaphrodites are isopods that can be developed as males and then transform into females to reproduce.
The males will be found on the gills before they migrate to the mouth. This might occur if there are more than one isopod infesting a fish's gill. The female who becomes pregnant can then be immured by the male.
The parasite dies after it completes its purpose of reproduction.
