Probably started at first as a way to protect their backs from solar radiation, but proved to have the added benefit of camouflage, because looking down at a shark would make it harder to see in the sun, and looking up would make it harder to see in the light, therefore, the sharks with this trait survived better and weren’t selected against, while sharks without this were not as good at survival. Over time, this trait became the norm.
If you’re at the bottom of the sea looking up, what you see is lighter water, cos sunlight is reaching it. If you’re at the top of the sea looking down, what you see is darker water, cos sunlight doesn’t penetrate that far. If you’re looking up, you see the bottom of a shark. If you’re looking down, you see the top of it.
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