Also known as Cymathoid Isopod, Parasitic Copepod, Tongue-eating Isopod, Tongue-eating Louse.
Found in pairs, permanently attached to fish heads and bodies, they then enter the fishes mouth via their gills, and then proceed over a period of time to replace the fish tongue, when the fish dies it will detach itself and start again, with a smoother unsuspecting fish!
They feed on fish blood and mucus.
Length - 0.3cm
Depth - ?m
Cymathoids are scavengers rather than parasites.
They will settle as a mating pair for life on the heads of fish, usually very close to the eyes (one either side) again causing no real damage to the fish except maybe some deformity where they are attached.
Some enter through the fish gills and attach to the tongues of fish including anemonefish, the tongue will atrophy and eventually be permanently replaced by the Isopod with no further damage to its host.
Cymothoids start life as tiny free swimming males. Once attached to a fish by their several pairs of hook-like legs they lose the ability to swim and so remain for life.
Cymothoid are hermaphrodites. Ref: https://en.wikipedia.org/wiki/Cymothoa_exigua