Starting from the edge of the continental shelf, it can be said that it enters the great depths, characterized by the practically lack of light. It is the bathyal zone, which includes the continental slope, followed by the abysmal plain area with more than 3.000 meters, and the hadal area, typical of the Mariana trenches.
These deep areas have a impoverished marine ichthyofauna due to lack of food, the low oxygen concentration, the very high pressures and a very low constant temperature.
In the Mediterranean, the decline is especially notable, Beyond 1000 meters depth only 36 species are recorded de peces. The extremely harsh situation of environmental factors is aggravated in this sea by the shallow depth of the Strait of Gibraltar, which prevents the passage of deep species from the Atlantic.
All the mesopelagic and bathypelagic species, between 200 and 1000 m, they have more or less strange shapes that deviate a lot from what is the ideal conception of a typical fish. A good example of this rare morphology is the ax fish. The majority de peces At these great depths they are equipped with luminous organs or photophores, which can serve both for intraspecific recognition and to attract possible prey.
In general, they are small in size. There are, however, notable exceptions, for example, 'regalecus glesne'it can reach and exceed even three meters in length. Typical mesoplegic families are, among others, the mytophids, scrawny, gonostomatids, sharp and gifts.
Many of these fish show significant vertical migrations, and move at night to epipelagic levels. Some go as far as 1000 meters at night, vertical nocturnal migrations of between 300 and 700 meters are relatively normal in many myctophids.
All the betonic species of this area are usually located above 1000 m of depth. Between 2.000 and 3.000 m of depth the total number of species is reduced to only seven in the Mediterranean.