How does the ventral keel of many clupeiform fishes help conceal them from predators?

The ventral keel in many clupeiform fishes serves as a specialized adaptation for camouflage in open-water environments. Because these fish often live near the surface, they are vulnerable to predators from below. If the fish were cylindrical, top lighting would create a sharp, visible shadow beneath them. The ventral keel helps eliminate this shadow, allowing the fish to blend into the mirrorlike reflection of the water surface when viewed from below.1

This concealment is further enhanced by the fish's glossy silver sides, which help it disappear against the water's surface reflection. Conversely, when viewed from above, the fish is protected by dark, cryptic coloration on its dorsal side, which mimics the appearance of deep water. Additionally, the schooling behaviour of these fish can deceive predators by making the group appear as a single, larger organism.1