The mesopelagic (Greek μέσον, middle) (also known as the middle pelagic or twilight zone) is a pelagic zone extending from 200 m (650 ft.) down to around 1000 m (3280 ft.) below sea level. It is located between the photic epipelagic and the aphotic bathypelagic, where there is no light at all. Although some light penetrates this deep, it is insufficient for photosynthesis. Though the temperature varies less at any one height than the epipelagic, the mesopelagic is the location of the thermocline, with temperatures varying from over 20°C at the top to around 4°C at the boundary with the bathyal zone. Animals such as swordfish, squids, wolf eels, cuttlefish, and other semi-deepsea creatures live here. Also, there is enough sunlight for animals, such as the chain catshark, to be fluorescent.
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