Buoyancy
Arrowtooth eels, Synaphobranchus kaupi - many fish living deep in the ocean are long and thin, like eels.
How do some fish and other sea creatures stay buoyant in the water while they are hunting for food?
To stay buoyant in the water, many fish have very light bones and flesh. These fish often look like eels – long and thin, often with very long tails. Some also have gas bladders that stop them sinking. This allows them to swim around, moving up and down in the water, without using up too much energy, as there is very little food available in the deep sea.
Heavier species sink if they stop swimming and so they spend much of their time resting on the sea floor or searching it for food.