The depth of the ocean is much more than the highest landforms present on the surface of the Earth. On an average, the depth of an ocean is estimated to be about two miles. The ocean is divided into many layers or zones based on the amount of light that reaches a specific depth. The first layer is the sunlight or photic zone (from the ocean surface to 200 meters deep), below which lies the twilight or mesopelagic zone (200 meters to more than 1000 meters). The midnight zone is about 2000 meters below the ocean surface. In some oceans, the deepest layers, which are estimated to be about three times the average depth, are present. For example, the Marianas Trench.
There is no doubt that the marine biome represents the highest biomass on Earth. The type of life forms (microbes, plants, and fish) present in each ocean layer vary. To be precise, the deepest layers are inhabited by organisms that require minimum sunlight and have the ability to withstand high hydrostatic pressures and extremely cold conditions. Till now, humans have unraveled only up to the mesopelagic layer. Given below are brief descriptions of some deep ocean fish.
Viper fish (or Viperfish) is found in the mesopelagic zone at a depth of about 800 - 1500 meters. It has extraordinarily large eyes that serve the purpose of collecting light. Some types of this fish are totally black in color; they possess light-sensitive parts (photophores) in the body. The photophores on the dorsal fin are used for luring the prey. Some of them lack pigment and are translucent.
This is another species that is adapted to the mesopelagic layer of the ocean. One characteristic feature of this fish type is the hinged, rotatable skull adapted for swallowing large-sized preys. This fish also possesses a large mouth and stomach, which are adaptive features for feeding on larger preys (as big as its own size).
Abyssobrotula galatheae holds the record for the deepest ocean fish till now. It was found in the Puerto Rico Trench at a depth of about 8 km (5 miles). However, it was dead by the time it reached the ocean surface. Hence, more extensive studies could not be done regarding its adaptive characteristics.
This species is by far the deepest living fish ever discovered by scientists. It was found at a depth of 7.7 km (4.8 miles) in the Japan Trench, Pacific Ocean. It is about 30 cm long and uses vibration receptors (present in the snout) for locating food and navigating in the ocean.
This fish survives at a depth of about 7000 meters below the ocean surface. There is no significant information about this mysterious fish. However, marine scientists are of the opinion that it does not have eyes, rather it has other adaptive features that compensate its lack of vision.
Besides these, other species of fish residing deep in the ocean are the Flashlight fish, Angler fish, and Bristlemouth fish. Some characteristic features shared by these fish are big eyes, translucent bodies, and large, dagger-like teeth. Usually, they are bioluminescent and produce light on their own. Currently, various projects have been taken up in order to unravel the mysterious organisms of the deep oceans.