Isopod

Anilocra sp.

Once attached to a fish they remain there for life. They  attach themselves to the head/mouth area because they scavenge food particles as their host feeds. When two isopods are attached to a fish, one is the female and one is the male. However, if by chance a male should attach to a fish that already has a male attached, one of the isopods will change sex and become a female. The males are smaller and light in color and the larger, and dark individuals are probably brooding females. Isopods are crustaceans. In fact, they are one of the most morphologically diverse of the crustacean groups.  Some species of Isopods live inside the mouths of fishes. Some are as small as 1/2 inch others can grow as much as 11 inches and live at depths of 4,000 feet.  For more information Go Here (external link)