Answer: Their flat bodies and silver sides.
Explanation:
Hatchet fish have line of photophores on their silver side that produce light or bioluminescence. This property helps the fish to swim in water where sunlight do not penetrate. By producing their own light with the same intensity as that of the sunlight , the hatchetfish can make themselves invisible to the predators. This is called as counter-illumination. Also the large flat bodies of these fishes keep them hidden from predators.
