This is in the Atlantic ocean. Spade fish mostly live.
The spade fish tend to feed on the ocean floor. Since they're closer to the floor where the floor is a dirt color, it could allow adaptation.
The spade fish color is a silver color. But after years of feeding on the ocean floor some of them have begone to change color.
The silver spade fish tend to stay closer to the top and are easily spotted due to the silver color
The sharks will feed on the spade fish when they get the chance.
The brown color fish stays lower than the silver spade fish and becomes less noticeable
Since the sharks see the silver color way better it will attack the silver one way more often. And since the silver spade fish stay close to the surface it make also a easier target
Since the brown spade fish is the same color as the floor, it harder to see. And because of it habits of staying lower to the floor it adds on to shark inability to see them. It also uses it color to blind in with the floor to gather food more easier in most little time
Since the sharks can hunt the silver spade fish. It leads to a decrease in the fitness of the silver spade, and due to the adaptation to the spades color, it allow it to survive and increase the fitness of the brown spade fish