Demersal zone facts for kids

The demersal zone is a special part of the sea or ocean, or even a deep lake. It's the area of water that is very close to the seabed (the bottom of the sea). Think of it as the water layer just above the ocean floor, where the bottom of the sea really affects the water around it.

This zone is just above the benthic zone (the actual seafloor). The demersal zone can be different depths. Sometimes, it's shallow enough for sunlight to reach, which is called the photic zone. In these sunny areas, plants and tiny organisms can grow. Other times, it's very deep and dark, where no light ever goes. This deep, dark part is called the aphotic zone, and it starts at depths of about 200 to 1,000 meters (650 to 3,300 feet).

Amazing Fish of the Demersal Zone

Demersal vs. Pelagic Fish

Fish that live in the demersal zone are called demersal fish. They often stay near the seabed. But sometimes, it's hard to tell them apart from pelagic fish, which live in the open water far from the bottom.

For example, the Atlantic cod is a typical demersal fish, meaning it usually lives near the bottom. But you might also find it swimming in the open water. On the other hand, the Atlantic herring mostly lives in the open ocean. However, when it's time to lay eggs, it gathers in huge groups near the seabed.

How Fish Stay in Place

There are two main types of fish in the demersal zone. Some fish are heavier than water and simply rest on the seabed. Others have what's called "neutral buoyancy." This means they can float without sinking or rising, staying just above the bottom.

Many fish use a gas-filled bag called a swim bladder to control their buoyancy. They can add or remove gas to float higher or lower. The tricky part is that water pressure changes as a fish swims up or down, so they have to keep adjusting their swim bladder.

Another way fish float is by using fats, or lipids, which are lighter than water. For example, sharks have large livers filled with a special lipid called squalene, which helps them float. The velvet belly lanternshark, which lives near the bottom, has a liver that makes up 17% of its body weight, and 70% of that liver is lipids!

Rays and skates, which also live on the seabed, have smaller livers with less fat. This makes them heavier than water, so they often rest on the bottom instead of swimming all the time. Some fish don't have buoyancy aids at all. They use their pectoral fins (the ones on their sides) to create lift as they swim. The downside is that if they stop swimming, they will sink, and they can't hover or swim backward.

What Demersal Fish Eat

Demersal fish have many different ways of finding food. Many eat tiny animals called zooplankton that drift in the water, or they feed on organisms and algae found on the seabed. Some eat epifauna, which are invertebrates that live on top of the seafloor. Others specialize in eating infauna, which are invertebrates that burrow under the seafloor.

Some demersal fish are scavengers, meaning they eat the dead remains of plants or animals. And many are predators, hunting other creatures for food.

Amazing Invertebrates of the Demersal Zone

Invertebrates are animals without backbones. Many of them also live in the demersal zone.

Zooplankton

Zooplankton are tiny animals that mostly drift with the ocean currents. However, many can move a little bit and control how deep they float. They might use gas-filled sacs or store light substances to help them float. Some also have special body parts that slow down their sinking. For adult creatures that live only at certain depths, their young (larvae) need to find the right kind of seabed to settle on and grow.

Cuttlefish and Nautilus

Cuttlefish are amazing invertebrates that can change their buoyancy using a special internal shell called a cuttlebone. This cuttlebone has lightweight, hard parts with gas-filled spaces inside, which helps them float. This allows them to swim easily at different depths.

Another interesting invertebrate that lives and feeds on the seabed is the nautilus. It stores gas in the different chambers of its shell. It can adjust its buoyancy by using a process called osmosis, which means it can pump water in and out of these chambers to control how much it floats.

See also

In Spanish: Zona demersal para niños