Why do clupeiform fishes often have silvery sides and dark backs?

Clupeiform fishes often possess silvery sides and dark backs as a form of cryptic coloration that helps them survive in open-water environments. This specific pattern serves two primary protective functions:

• Concealment from below: The ventral part of these fishes typically forms a keel, which, combined with their glossy silver sides, helps the fish blend into the mirrorlike reflection of the water surface when viewed from below. This adaptation effectively removes the sharp shadow that would otherwise be created by top lighting if the fish were cylindrical.1
• Concealment from above: The dark coloring on the dorsal part of the fish allows it to blend in with the deeper, darker water when viewed from above.1

Together, these features act as a form of countershading, reducing the visibility of the fish's three-dimensional form and helping it avoid detection by predators.12