The thermocline is a layer of water in the twilight zone of the ocean where the water temperature drops really quickly. The sun’s light and warmth keep the sunlight zone pretty warm, and the wind and waves mix that warmth to a certain depth, around 330 feet (100 meters). But below the sunlight zone the water temperature starts dropping quickly. The temperature difference from the top of the thermocline to the bottom can be as great as almost 60 degrees Fahrenheit (about 20 degrees Celsius). Below about 1,000 feet (300 meters), the ocean temperatures stay pretty much the same.
It is brightest at the top of the zone and almost completely black at the deepest point. Th e twilight zone is the ocean layer with the biggest difference in temperature, which can change as much as 60 degrees Fahrenheit (15 degrees Celsius) from the top of the zone to the bottom.
Food is scarce in the twilight zone because not enough light penetrates this zone for plants to grow. Creatures that live here are in stiff competition for survival, and have to have very powerful senses to stay alive and to hunt for food. Animals in the twilight zone have adapted so that they are not easily seen by predators and can capture whatever food comes their way.
Creatures in the twilight zone have really good eyesight—most of them have eyes that are very big for the size of their body, and many have adapted so that their eyes are on the top of their heads. That way they can see the shape of creatures above them reflected against the light of the higher zone.
Many animals in the twilight zone are transparent so that their predators look right through them rather than at them. Others are silvery so they blend in better with the dim light. But the most interesting adaptation of animals in the twilight zone is bioluminescence. Animals that are bioluminescent can make their own light.
They use their bioluminescence to lure smaller prey toward them, and to confuse predators who want to eat them. It is a pretty effective adaptation, and more than 90 percent of the animals that live in the twilight zone are bioluminescent in some way.
Twilight zone animals that aren’t bioluminescent have other ways to trick predators. Some have strange body shapes. Hatchet fish are completely flat, so they seem to disappear when predators look at them head-on. Th ese fish are also covered in reflective silver scales that act like mirrors, bouncing back any light that might hit them. Other animals can change their shape, so they confuse predators. For example, when pout eels are threatened, they put their tails in their mouths and fl oat motionless in the water. Th is makes them look like jellyfish, so other fish avoid eating them. Because very little food sinks down from the sunlight zone to the twilight zone, lots of creatures who live in the twilight zone will move up to the sunlight zone to hunt at night.
Just how many? Millions. So many millions, in fact, that when sailors were trying to use sonar at night to map the ocean bottom in the 1950s, they kept getting false bottom readings. They finally figured out that the false bottom they were measuring was a layer of millions of fish, jellyfish, squid, and other deep-water creatures that travel up to the shallower layers of the ocean to find food.