The Pelagic Zone

The ocean is often viewed as being constantly teeming with life. But in reality, most of the ocean is a vast wilderness of blue, with nothing below but thousands of feet of water. It takes a well adapted organism to survive here, where food sources may be far and few between and where you never know if the next stretch of blue is safe, or if you’re not alone.

The pelagic zone is massive; it accounts for most of the ocean’s area. Therefore the open ocean pelagic zone is divided into several smaller zones based on depth. The first is the epipelagic zone. This also referred to as the surface zone. It extends from the surface 200 m down. The brilliant sunlight allows for a lot of phytoplankton growth; however the habitat is also fairly monotonous with only endless expanses of water. This means that there is very little diversity among species that live here. Despite this many interesting animals are still to be found here. These include large predatory fish like the bluefin tuna, many species of jellyfish that simply float with the current and smaller prey fish such as herring or anchovies. The largest epipelagic fish is the whale shark, a peaceful filter feeder. The epipelagic zone is the brightest in the warmest sublevel of the pelagic zone.

The next zone is the mesopelagic zone, which extends from 200 to 1000 m. Some sunlight penetrates the stone but not enough for phytoplankton to perform photosynthesis. This is why the mesopelagic zone is also called the twilight zone. Many creatures that dwell in the mesopelagic zone only dwell there during the day, rising at night to feed in the epipelagic zone. So these animals are squids, cuttlefish, and filter feeding fish. Some of the more interesting fish found this depth are the sabertooth fish, which has large teeth to prevent escape when it has caught its prey, and the lancetfish, which can grow up to 2 m long.

The next zone down is the bathypelagic zone, or the midnight zone. This zone extends from 1000 m to 4000 m. At this depth no sunlight penetrates and it is pitch black all the time. Organisms in the zone must be adapted to chilling temperatures, crushing pressure and of course, complete darkness. This combination of circumstances has resulted in some very strange looking fish. Many organisms in this zone display bioluminescence. One of these organisms is the anglerfish, which is a bioluminescent wooer projecting from its head to entice prey close to its jaws. Another animal that calls the bathypelagic zone home is the giant squid, which spends most of its life at this depth. It is also in this zone that they are hunted by the deep diving sperm whale.

The largest zone is the abyssopelagic zone, which extends from 4000 m to the bottom. Like the bathypelagic zone there is no light down here, to the extent that some animals have lost their eyes over the millennia. Some animals appear at this depth are the tripod fish, the black swallower, and the sea pig, a relative of the sea cucumber.

The final zone is the hadopelagic zone, the deep ocean trenches. Some of these zones reached depths of over 10,000 m. There is very little life this depth except around the geothermal vents, where diverse ecosystems thrive in the most unlikely of places.