There are many kinds of marine habitats. Source: Peter Liu Photography via Compfight cc

Because oceans cover more than seventy percent of the Earth’s surface, marine habitats are large and very diverse ! From the warmest waters to the coldest oceans, and from dark underwater trenches to colorful coral reefs, all sorts of plants and animals live underwater. Almost half of all species on earth depend on the ocean.

The largest animal in the world, the blue whale, calls the ocean its home – it can reach 100 feet long and weigh 150 tons (that’s more than 15 school buses!). But the smallest animal in the world also lives in the ocean – it’s the tiny tardigrade, or “water bear.” Tardigrades are so small that you usually can’t see them without a microscope.


Coral reefs are homes for many animals. Source: Wikimedia

Coral reefs are the cities of the underwater world. Small animals called coral polyps build hard limestone skeletons outside their bodies that act like apartment buildings for many other sea creatures. Moray eels hide in crevices right near octopuses, which peek out and watch colorful clownfish swim by. Crabs, lobsters, sharks, fish, anemone, turtles, sea sponges and many other creatures live side-by-side in these bustling coral reefs.



A fish’s gills. Source: Creative Recovery via Compfight cc

If you put an oak tree or a kangaroo in the ocean, it wouldn’t survive for very long. That’s because plants and animals that live on land don’t have adaptations that help them survive in the water. All the species that live in our oceans have evolved ways to eat, sleep, and breathe underwater.

Almost every animal on earth needs oxygen. Our lungs help us breathe oxygen from the air, but they only work where it’s dry. How do animals get oxygen in the ocean?

Most fish breathe with an organ called gills . As a fish swims, water moves across its gills. The gills separate out oxygen that’s mixed in with the water, which lets the fish breathe.

Ocean-dwellers that do breathe with lungs, including mammals like whales and dolphins, have special adaptations that help them hold their breath for a long time while they are underwater. When they dive down deep, their heartbeat slows so that they use less oxygen and don’t have to come to the surface to breathe as often. Elephant seals can stay underwater for two hours! How long can you hold your breath?



Frogfish are experts at camouflaging themselves. Source: alfonsator via Compfight cc

Many predators and prey live in the ocean. Predators are always trying to catch their prey, and prey are constantly working hard to avoid being caught. One particularly strange underwater predator is called a frogfish. It may not be the prettiest creature in the ocean – it looks more like a rock than a fish. But this predator has a clever trick up its fin.

When it’s time for a meal, the frogfish wiggles the fleshy bump between its eyes like a fishing lure to attract its prey. The prey approaches the wiggling lure, thinking it’s a tasty snack. Because the frogfish looks so much like a rock, the unsuspecting prey doesn’t know that a predator is quietly waiting for them!

Suddenly, the frogfish’s lightning-fast jaws open and the prey is sucked into its enormous mouth. Using a combination of camouflage and trickery, the frogfish is a very successful predator.


One of the biggest challenges facing the earth’s oceans is humans! Each year, we throw away lots of trash, and about 10 million tons of it end up in the sea. Ocean currents carry our trash far and wide, and it ends up on beaches, reefs, and the sea floor. In the Pacific Ocean, there’s even a giant floating patch of garbage that’s the size of Texas!


Litter on the sea floor. Source: jdj150 via Compfight cc

Not only is this trash ugly, but it also is very dangerous for species that live in the ocean. Animals like birds, fish, and turtles often eat small pieces of plastic because they look like food. It’s also easy for animals to get tangled in plastic bags and fishing line.


A seagull carries plastic trash. Source: peretzp via Compfight cc

So what can we do? Scientists are trying to figure out a solution, like inventing biodegradable plastic that can dissolve in water. But the best way to keep the oceans clean is to make sure trash doesn’t get in there in the first place. We need to recycle more, throw away less, and be very careful not to litter. And if you see trash on the beach, pitch in and clean it up!

Explore Another Ecosystem