Most fish have an excellent sense of smell, and generally good vision. Hearing canals run down the length of their bodies and can detect slight vibrations in the water. Gills function to exchange gases betweeen a fish and the water. Most fish adjust their buoyancy using a gas-filled swim bladder . Fish physiologically regulate the salinity of their internal fluids.
Some fish are poisonous , while others are venomous.
Puffer fish are poisonous . The genus Cyclothone contains most of the bony fish that live in mesopelagic habitats, or deeper in the water column. Midwater fish are typically small. They have enormous mouths and sharp teeth. Some fish can unhinge their jaws to swallow proportionately large prey.