Underwater Excursion

Take a swim through some of the most beautiful waters from around the world

Saunders Reef, Great Barrier Reef, Australia

Coral reefs are diverse ecosystems found in tropical waters. They provide food and habitat for thousands of marine species. The reef also protects the mainland from strong waves created by storms. The Great Barrier Reef is one of the largest living structures on Earth.

[Tap and drag to look around the reef]

Visible from space, the Great Barrier Reef provides habitat for thousands of species of invertebrates, fish, reptiles and mammals.


The sun provides an essential source of energy for the coral reef ecosystem. Producers include various species of single celled algae, called zooxanthellae, which live within the coral providing it with energy and nutrients.  In return the coral provide shelter and other nutrients.

In return the coral provide shelter and other nutrients. The algae also contributes to the color of the coral. Without it the coral would appear white.


Corals are composed of colonies of animals related to sea anemones. Their rocklike appearance comes from the calcium carbonate they secrete. This substance becomes the foundation on which coral reefs are built.

Corals will filter the water for nutrients provided by algae and other microscopic organisms.


This angelfish is one of many species that lives around the Great Barrier Reef. The bar across the eye of this angelfish makes it difficult for other fish to determine which end of the fish is the head. 

Komodo Island, Indonesia

Komodo Island is known for being the home of the Komodo dragon. Located within the “Coral Triangle” in Indonesia it is surrounded by waters rich in marine biodiversity. 


Rays are related to sharks. Their skeleton is made of cartilage, not bone, and they have grinding dental plates that can crush through the exoskeletons of crabs and other crustaceans.

Brain coral

Can you guess why brain coral was given its name? This slow growing coral is an important foundation of coral reefs.

Sea squirts

Can you spot some hidden amongst the corals? Even though they are usually found fixed to a surface, sea squirts are classified as animals. They have two openings - one for taking in water, oxygen and nutrients and the other for expelling waste. 

Isla Contoy, Mexico

The waters surrounding coral reefs contain much more than what can be easily seen. If you were to examine the water with a microscope you would discover it is filled with cells and organisms such as phytoplankton, zooplankton, coral larvae and fish eggs.

This “plankton” can be carried away from the reef on ocean currents and filtered from the water by organisms that use it as a valuable source of energy. 

Whale Shark

Whale sharks are not actually whales - they are the largest species of fish in the world. They reach their great size by filtering water for plankton and small fish. Adults can reach up to 12 meters. This is about equal to the length of a school bus. 

Manta Ray

This manta ray is also a large filter feeder. It must swim continuously in order to keep water flowing over its gills for oxygen. 


Remoras have a flat disk on the top of their head. This disk is designed for sucking  and they use it to grip onto sharks and ships. 

Heron Island, Great Barrier Reef, Australia

Although part of the Great Barrier Reef, the reefs outside Heron Island look very different from Saunders Reef. The “spur and groove” pattern in the reef is created by erosion found on the windward shores of islands. 

Green Sea Turtle

The green sea turtle can reach up to 5 feet in length and weigh as much as 500 pounds. It is an herbivore. 

Plate Coral

The large surface area of plate coral maximizes the amount of sunlight that can reach the algae living within the coral.

Yellow Longnose Butterflyfish

A fish commonly found in saltwater aquariums, the yellow longnose butterflyfish feeds on small crustaceans and the tube feet of echinoderms.

Galapagos Islands, Ecuador

The Galapagos Islands are a group of volcanic islands located off the coast of Ecuador. Geographically isolated from the mainland they are home to many biologically unique species. 

Sea Lions

These playful sea lions are the largest animal species located in the Galapagos. They hunt fish in the waters surrounding the islands. 

Club Urchins

Urchins are related to sea stars. The clublike spines of club urchins are thicker and stronger than most urchins. 

Benwood Wreck, Florida Keys, United States

It’s a ship! At least it used to be. Because many marine invertebrates such as corals, sponges and oysters need hard surfaces on which to attach there are many locations around the world where sunken structures have become the foundation for a coral reef ecosystem. 

Wood decay

Aquatic worms and other decomposers will feed on decomposable parts of the ship leaving behind steel frames and beams on which coral larvae can attach.

Sergeant Majors

Sergeant Majors is a species of fish named because of the stripes resembling those in the military rank. 


Grunts, also known as porkfish, are named after the “grunting” sound they make by rubbing their teeth together. Schools of fish can be seen swimming through the water at night. 

Dive into more Underwater Earth stories, videos and images in our collection.

Credits: Story

AXA XL, The University of Queensland, Google, UNESCO, Charles Darwin Foundation, Galapagos National Park, Office of National Marine Sanctuaries, Florida Keys National Marine Sanctuary, Florida International University, Aquarius Reef Base, Panedia, Fourth Element

Credits: All media
The story featured may in some cases have been created by an independent third party and may not always represent the views of the institutions, listed below, who have supplied the content.
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