Deep-sea fish are fish that live at the bottom of the ocean, below the photic zone. The photic zone is the top 200 meters of ocean. It consists of the area that sunlight reaches and the human eye can see. The ocean is so deep that humans are unable to see around 90% of it. The deep sea is a hostile environment that many are unable to survive. Pressures of anywhere from twenty to thousand atmospheres, temperatures from three to ten degrees Celsius and a lack of oxygen make it unliveable for many.

The fish that live in this area are very strange and completely unsuitable for life in an aquarium. It is simply too difficult for humans to simulate their natural environment. Sadly, due to this we know little about them. In most cases the best scientists can do is examine dead specimens. Observing them in their natural state is not always an option either, as deep-sea exploration is expensive and difficult. Most deep-sea fish are only known in the scientific community and therefore are still called by their scientific name.

Of the deep-sea fish, the Lantern fish is the most common. An estimated 65% of deep-sea fish are Lantern fish. They are believed to be so abundant that they have a worldwide mass of between 550-650 metric tonnes. However, despite being so plentiful, there are very few commercial Lantern fish fisheries. Viperfish, dragon fish, oarfish and anglerfish are also well-known deep-sea fish.

As there is no natural illumination below the photic zone, the fish that live there have extra large eyes well adapted to seeing in the dark. They are also bioluminescent and many have long feelers to help them locate prey and for mating. Most deep-sea fish are smaller with larger mouths and stomachs and many have minimal bones and are very jelly-like. Deep-sea fish are far slower and less agile than those who swim in the light. Most deep-sea fish give birth in the deeps but a few come to the surface to have their young. The young slowly sink to the ocean depths as they grow older.

Currently there is not a lot of information known about these very unusual fish but with advances in technology, hopefully one day soon we will be able to observe them in their natural environment and learn and be amazed by them. Truly these fish of the deep are among some of the most unique and elusive animals on Earth. What are deep-sea fish? Hopefully one day soon we will be able to answer that question even better.