The mesopelagic (Greek μέσον, middle) (also known as the middle pelagic or twilight zone) is that part of the pelagic zone that extends from a depth of 200 to 1000 metres (650 to 3300 feet) below the ocean surface. It lies between the photic epipelagic above and the aphotic bathypelagic below, 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, and in warmer regions of the world the temperatures varies from over 20 °C (68 °F) at the top to around 4 °C (39 °F) 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.