From the Super Mario Wiki
Gleamin' Breams are bulky yellow fish from Donkey Kong Country 3: Dixie Kong's Double Trouble! These obese breams appear only in one level, Floodlit Fish. Unlike their red lookalikes, called Bounty Basses, they do not carry any items and they cannot be defeated. However, Gleamin' Breams have a unique ability: they are bioluminescent, which means they are able to produce and emit light. If the player manages to attack one, using Enguarde the Swordfish's stab, the Gleamin' Bream makes a shocked face and illuminates the area. This light fades in a short while, though.
Although they can hurt Enguarde and the Kongs, Gleamin' Breams are not real enemies. Indeed, they unwillingly help Dixie and Kiddy Kong by lighting the dark waters of the Razor Ridge lakes. Their bioluminescence proves very useful in "Floodlit Fish", though in the Game Boy Advance remake of the game, it is possible to get through the level without hitting a Gleamin' Bream even once, due to the console's increased brightness. According to the game's credits, Gleamin' Breams belong to the "Aquatic Attackers" enemy class.
Gleamin' Breams seem to be the inspiration for a species of bioluminescent fish that appear in Donkey Kong Country: Tropical Freeze, only in Amiss Abyss. They are small and encountered in schools. Scattering a school of such fish with a corkscrew move makes them produce light for a short time and illuminate the dark area of the stage.
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