Marine BiOLogy > What is Marine Biology?

Layer by layer, millions of small living corals and other reef-building animals create giant limestone structures known as coral reefs . Algae grows on the surface of the reef. And a densely packed community of animals lives in the reefs’ nooks and crannies.

These unusual trees thrive in seawater along tropical coastlines. They trap mud in their roots, protecting the land from gradually eroding away. Mangroves  also protect young fish, shrimp, and other organisms. Hundreds of species depend on mangroves.

Near the shoreline grow giant forests of giant kelp , a type of brown algae. This seaweed can grow over 30 meters (100 feet tall)! Countless species, such as crabs, sea urchins, and snails, live here.

Estuaries are places where the river meets the sea. Tides carry salt water upstream, where it mixes with fresh river water. The mixing churns up nutrients, making estuaries a great place for all kinds of organisms to find food.

The icy waters of the Arctic and Antarctic may seem too cold for comfort. But a variety of marine life—from microscopic algae to enormous whales—make their home here.

This part of the seafloor is pitch dark and mostly flat. Pretty much the only food here is sea snow: dead animals and other organic matter that drift down from above. You might think life would be sparse here, but more than 200,000 species have already been found.

Almost every day, a new hydrothermal vent  springs up, spewing plumes of hot, mineral-rich water from cracks in the ocean floor. Some of Earth's oddest creatures live here, thriving on the chemicals released by the vents.

Humpback whales  communicate with one another with beautiful underwater “songs” that travel long distances through water.

Most jellyfish can swim by squeezing the bell-shaped parts of their bodies, pushing water out. Lots of animals eat them, including humans. To protect themselves from predators, many jellyfish can sting.

Sea sponges live in clear salt water. The water flows through holes in their bodies, bringing them food and oxygen and taking away waste. These invertebrate animals don’t have hearts or brains!

Like their relatives the seahorses, seadragons are a kind of marine fish that live in coastal waters. To help them hide, they have small fronds on their bodies that look like seaweed. Seadragon dads take care of the eggs before they hatch.

Marlins are big, fast fish that can weigh more than a cow. They use their long, spear-like snouts to kill prey, smaller fish including mackerel and tuna.

Way down in the dark, deep ocean live the hatchetfish. Their bodies are so thin, they resemble the blade of an ax. Their eyes point upward to help them see food in the water above them. Hatchetfish are also bioluminescent: they emit light from different body parts to confuse their predators or to attract mates.

Seaweed is a type of algae that grows in saltwater, often attached to rocks near the coasts. Like plants, seaweed gets energy from sunlight. Many kinds of seaweed use air pockets to float, keeping them near the sunlit surface.

Often called “ starfish ,” sea stars aren’t fish at all. They’re invertebrates: animals that have no backbone. Sea stars have the ability to regenerate. If a fish bites off a sea star’s arm, it can grow a new one!

Sea cucumbers live on the ocean floor around the world. They are scavengers: they scoop up sand mixed with dead and discarded matter and poop out the clean sand. Like living filters, they help clean up their surroundings.

These marine mammals live in warm coastal waters and rivers, where they forage for plants and algae. Manatees  may look like seals, but they’re more closely related to elephants.

In 1938, fishermen off the east coast of Africa pulled up a strange fish in their nets. It turned out to be a coelacanth  (SEE-la-canth), an animal that scientists thought had been extinct for almost 80 million years!

With their wide, winglike fins, eagle rays seem to fly through the water. Electrosensory organs on their snouts help them find shellfish and other food buried in sand or mud.

Hawksbill sea turtles spend most of their lives in shallow coastal waters, near their favorite food, sea sponges. But when it’s time to lay their eggs, they can migrate vast distances. Because people hunt them for their beautiful shells and disturb the beaches where they lay their eggs, hawksbill sea turtles are critically endangered.

Monk seals are endangered mammals that live in warm, coastal waters. Their slippery, streamlined bodies and flippers make them good swimmers.

With their long legs, spider crabs look like the spiders they’re named after. To make up for their poor eyesight, they can sense their environment with organs at the ends of their legs!

These large snails can sometimes grow more than 30 centimeters (12 inches) long! The insides of their shells are glossy pink or orange. Queen conches can wander for miles across the sea floor, eating algae, sea grasses, and dead material.

Tube anemones may look like a plant that lives on the ocean floor, but they’re actually carnivorous animals. They have hundreds of tentacles that they use to grab prey, such as small crustaceans that swim near.

This submersible is equipped with technology to explore the deep ocean. Its thick acrylic dome can withstand intense underwater pressure, allowing marine biologists to dive deep. Specialized lights and cameras illuminate and capture images of marine organisms. A mechanical claw with foam-padded fingers (inset) can wrap gently around delicate specimens.

To photograph the glow of biofluorescent organisms, scientists dive with specially designed, high-resolution underwater cameras.

Scientists use newly designed tracking tags that attach better to large marine animals. These tags can record data, such as sounds, pictures, and location information.

To study ocean currents, scientists use small devices called mini-autonomous underwater explorers (m-AUEs) that can drift with the current.

REMUS, an autonomous underwater vehicle, is being lowered into the ocean. It uses sonar to investigate plankton and the animals that eat them.