Cymothoid isopods are parasitic and they occur
in the mouth of a large number of host fish. As the isopod attaches to the tongue we see
tongue degeneration in the host and this can also have an effect on the sizing of the fish. The isopods are attached to the tongue based on their pereopods which are their hook like legs. When they mature you often see a pair of male and female inside the mouth cavity with the female being the larger one. When they reproduce in the mouth cavity the female
rear the young in a marsupium, or pouch, on the underside of the body and then these young
will be released into the environment as free-living juveniles.