Oxygen dissolves in water - one of the components of a fishtank is one that bubbles oxygen into the water to keep it oxygenated, but it happens naturally too via different processes. It is this oxygen that fish breathe.
Only the top 'layer' of the ocean is particularly oxygen rich, below this, it is colder and poor in oxygen. However water from the top level can become cold and salty under some circumstances, the increased density makes it sink. This is how oxygenated water is brought downwards for bottom-dwelling animals.
many animals that live in the very depths of the oceans where there is no light such as the angler fish
As blue as the oceans depths
angelfish
yes
Yes, crabs have gills that allow them to extract oxygen from water. They absorb dissolved oxygen in the water through their gills, enabling them to breathe underwater.
it's deep depths and tempretures
A bathymetrist is a person who carries out bathymetry - the measurement of the depths of oceans or seas.
The deepest parts of oceans are dark all the time because natural light does not penetrate to the ocean depths. Little is known about life in the depths of the ocean. More is known about the moon than the depths of the oceans. Water pressure is great, the temperature is constant.
inhabit tropical/temperate ocean waters. They are mostly shallow-water animals although they are known to go to depths of about 600 metres (2,000 ft)
1900,000 feet below sea level
Jellyfish have been seen at extreme depths in oceans.
anglerfish lives in the dark depths of the Atlantic and Antarctic oceans.