Body design because of adaptation
Common fish like a clown fish, that are closer to the surface have a gas filled organ called a swim bladder. The swim bladder helps stabilize their body at different water depths. But most of the fish at deeper water depths, like the ones we are learning about don't have a swim bladder because the water pressure is so strong it would crush them if they had any gas-filled spaces in their body.
Many of the fish that live in some of the deeper zones had to adapt and now thrive in the harsh conditions. Many of them adapted by becoming energy efficient, having slow digestive rates, and just plainly moving slow. The also often have weak muscles and soft skin because there is not any predators to them because it's dark and the predators can't see them, so they don't need the muscles to swim fast.