Evolution is a process that is always happening in the background and anything positive that comes out of is often kept and passed onto offspring.
In the deep sea there are a number of challenges that fish and other animals must overcome to be able to survive – there is no natural light, it is very cold, the weight of the water creates a crushing pressure and there is not a lot of food available. Fish in particular have a number of evolved traits that help them overcome these problems.
No natural light – some fish produce their own light source using a special bacteria that glows in the dark. They can use this to see and they can also use the light to attract both mates and prey to eat!
It is very cold – Humans are adapted to a water climate than the deep sea so our cells would be very unhappy at the cold temperatures of the water but deep sea fish have cells that have evolved to be happy at the low temperatures. Unlike us they would be very unhappy at warmer temperatures.
Weight of the water – Again the cells of the fish have changed to allow them to live under the pressure. Essentially the pressure inside their cells is higher to fight against the weight of the water. If they were to try and live in shallower waters where the weight is less their cell pressure would be too much and the cells would burst.
Not a lot of food – Some fish combat this problem by moving as little as possible so they don’t need to eat as much as fish that move around a lot. Other girls have huge stomachs so they can eat loads and loads of food. This allows them to enough food to last them weeks if not months!