due to the high pressure surroundings, and due to lack of sunlight (making it hard for underwater plants to photosynthesize). and i guess due to cold temperature?
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Barometric pressure
These animals have evolved to survive the extreme pressure of the sub-photic zones. The pressure increases by about one atmosphere every ten meters. To cope with the pressure, many fish are rather small, usually not exceeding 25 cm in length. Also, scientists have discovered that the deeper these creatures live, the more gelatinous their flesh and more minimal their skeletal structure. These creatures have also eliminated all excess cavities that would collapse under the pressure, such as swim bladders.
Lack of light
The lack of light requires creatures to have special adaptations to find food, avoid predators, and find mates. Most animals have very large eyes with retinas constructed only of cones, which increases sensitivity. Many animals have also developed large feelers to replace peripheral vision. To be able to reproduce, many of these fish have evolved to be hermaphroditic, eliminating the need to find a mate. Many creatures have also developed very strong senses of smell to detect the chemicals released by mates. And it means
Lack of Resources
At this depth, there is not enough light for photosynthesis to occur and not enough oxygen to support a fish living at higher levels. To survive, these creatures have much slower metabolisms and therefore can survive using little oxygen. They can also go months without food. Most food comes from either organic material that falls from above or from eating other creatures that have derived their food through the process of chemosynthesis (the process of changing chemical energy into food energy). Because of the sparse distributions of creatures, there is always at least some oxygen and food. Also, instead of using energy to search for food, these creatures use particular adaptations to ambush prey.
These animals have evolved to survive the extreme pressure of the sub-photic zones. The pressure increases by about one atmosphere every ten meters. To cope with the pressure, many fish are rather small, usually not exceeding 25 cm in length. Also, scientists have discovered that the deeper these creatures live, the more gelatinous their flesh and more minimal their skeletal structure. These creatures have also eliminated all excess cavities that would collapse under the pressure, such as swim bladders.
Lack of light
The lack of light requires creatures to have special adaptations to find food, avoid predators, and find mates. Most animals have very large eyes with retinas constructed only of cones, which increases sensitivity. Many animals have also developed large feelers to replace peripheral vision. To be able to reproduce, many of these fish have evolved to be hermaphroditic, eliminating the need to find a mate. Many creatures have also developed very strong senses of smell to detect the chemicals released by mates. And it means
Lack of Resources
At this depth, there is not enough light for photosynthesis to occur and not enough oxygen to support a fish living at higher levels. To survive, these creatures have much slower metabolisms and therefore can survive using little oxygen. They can also go months without food. Most food comes from either organic material that falls from above or from eating other creatures that have derived their food through the process of chemosynthesis (the process of changing chemical energy into food energy). Because of the sparse distributions of creatures, there is always at least some oxygen and food. Also, instead of using energy to search for food, these creatures use particular adaptations to ambush prey.