Midwater fish are typically small. They have enormous mouths and sharp teeth. Some fish can unhinge their jaws to swallow proportionately large prey. Some midwater fish are bioluminescent. These adaptations allow midwater fish to exploit a meal whenever one is handy.
Angler fish have particularly unusual adaptations and life cycles.
The Chondrichthyes first apeared in the Late Devonian, about 370 My ago. Fish in Chondrichthyes have skeletons made of cartilage. They have an accute sense of smell.