There is no clear definition for the deep sea. However, it generally refers to the sea area with a depth of 200 meters or more. The deep sea is a very harsh world. At a depth of 6500 meters, the pressure is about 651 atm, and for every 1 m. The water temperature is about 1000m deep, next to 2-4 degrees °C, and is almost constant even in the deep sea. Moreover, the sun's rays do not reach the deep sea. At a depth of around 1000m, the amount of sunlight that reaches is about one-hundredth of the sea level. Beyond that is a complete "dark world" that no living things can detect.
The sea is divided into surface, mesopelagic, bathyal, deep-sea, and hadal zones. This division is called the pelagic zone. In general, fish that have a major habitat promotion in the waters from the mesopelagic zone are called deep-sea fish.