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Pygmy seahorse

Reef stonefish

Spotfin lionfish

Potato cod

Clownfish

Nudibranch

Humphead wrasse

Green turtle

Giant oceanic manta ray

Giant clam

Dugong

Cuttlefish

Crown-of-thorns starfish

Box jellyfish

Barrel sponge

This minute master of camouflage survives by adapting its body to closely resemble sea fans, the soft coral it calls home. It's just one of the many creatures that could disappear along with the Great Barrier Reef because of global warming. 

Published: / Caption: / Photo: Reinhard Dirscherl/Getty Images
1
of 15

Pygmy seahorse

Reef stonefish

Spotfin lionfish

Potato cod

Clownfish

Nudibranch

Humphead wrasse

Green turtle

Giant oceanic manta ray

Giant clam

Dugong

Cuttlefish

Crown-of-thorns starfish

Box jellyfish

Barrel sponge

The stonefish hides itself among the coral using its crusty exterior as camouflage. It's earned the title of most venomous fish in the world, thanks to two sacs of poison on each of its 13 spines. The spines have pierced the soles of shoes, so look out!

Published: / Caption: / Photo: Reinhard Dirscherl/ullstein bild/Getty Images
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of 15

Pygmy seahorse

Reef stonefish

Spotfin lionfish

Potato cod

Clownfish

Nudibranch

Humphead wrasse

Green turtle

Giant oceanic manta ray

Giant clam

Dugong

Cuttlefish

Crown-of-thorns starfish

Box jellyfish

Barrel sponge

Don't touch those pretty dorsal fins -- they're venomous. Stealthy during the daylight hours, the spotfin lionfish is active at night, feeding on small fish and crustaceans.

Published: / Caption: / Photo: Reinhard Dirscherl/ullstein bild/Getty Images
3
of 15

Pygmy seahorse

Reef stonefish

Spotfin lionfish

Potato cod

Clownfish

Nudibranch

Humphead wrasse

Green turtle

Giant oceanic manta ray

Giant clam

Dugong

Cuttlefish

Crown-of-thorns starfish

Box jellyfish

Barrel sponge

These prehistoric-looking fish are large, curious beings, reaching lengths of nearly 9 feet. They're fond of reefs off northeast Queensland. 

Published: / Caption: / Photo: Auscape/UIG/Getty Images
4
of 15

Pygmy seahorse

Reef stonefish

Spotfin lionfish

Potato cod

Clownfish

Nudibranch

Humphead wrasse

Green turtle

Giant oceanic manta ray

Giant clam

Dugong

Cuttlefish

Crown-of-thorns starfish

Box jellyfish

Barrel sponge

Pixar's 2003 "Finding Nemo" brought these guys to the big screen, acquainting us with their symbiotic relationship with sea anemones. They start life as males and later develop into females, a process known as sequential hermaphroditism.

Published: / Caption: / Photo: Barcroft Media/Getty Images
5
of 15

Pygmy seahorse

Reef stonefish

Spotfin lionfish

Potato cod

Clownfish

Nudibranch

Humphead wrasse

Green turtle

Giant oceanic manta ray

Giant clam

Dugong

Cuttlefish

Crown-of-thorns starfish

Box jellyfish

Barrel sponge

Vividly vibrant, shell-less and often referred to as "sea slugs", these gastropods are known to come in 2,300 individual species. 

Published: / Caption: / Photo: The Ocean Agency
6
of 15

Pygmy seahorse

Reef stonefish

Spotfin lionfish

Potato cod

Clownfish

Nudibranch

Humphead wrasse

Green turtle

Giant oceanic manta ray

Giant clam

Dugong

Cuttlefish

Crown-of-thorns starfish

Box jellyfish

Barrel sponge

This predator of the coral-munching crown-of-thorns starfish has been classified as an endangered species by the International Union for Conservation of Nature.

Published: / Caption: / Photo: Auscape/UIG/Getty Images
7
of 15

Pygmy seahorse

Reef stonefish

Spotfin lionfish

Potato cod

Clownfish

Nudibranch

Humphead wrasse

Green turtle

Giant oceanic manta ray

Giant clam

Dugong

Cuttlefish

Crown-of-thorns starfish

Box jellyfish

Barrel sponge

These endangered turtles can live to be up to 80 years old. Nearly all of that time is spent underwater. They routinely dive for four to five minutes, before surfacing for a gulp of air.

Published: / Caption: / Photo: Craig Parry/Barcroft Media/Getty Images
8
of 15

Pygmy seahorse

Reef stonefish

Spotfin lionfish

Potato cod

Clownfish

Nudibranch

Humphead wrasse

Green turtle

Giant oceanic manta ray

Giant clam

Dugong

Cuttlefish

Crown-of-thorns starfish

Box jellyfish

Barrel sponge

As the name suggests, these rays are, well, giant. The largest type of ray in the world, they can reach up to 7 meters in width.

Published: / Caption: / Photo: The Ocean Agency
9
of 15

Pygmy seahorse

Reef stonefish

Spotfin lionfish

Potato cod

Clownfish

Nudibranch

Humphead wrasse

Green turtle

Giant oceanic manta ray

Giant clam

Dugong

Cuttlefish

Crown-of-thorns starfish

Box jellyfish

Barrel sponge

Listed as "vulnerable" by the IUCN, these bottom feeders can live more than a century and have been recorded weighing more than 200 kilograms.

Published: / Caption: / Photo: Reinhard Dirscherl/ullstein bild/Getty Images
10
of 15

Pygmy seahorse

Reef stonefish

Spotfin lionfish

Potato cod

Clownfish

Nudibranch

Humphead wrasse

Green turtle

Giant oceanic manta ray

Giant clam

Dugong

Cuttlefish

Crown-of-thorns starfish

Box jellyfish

Barrel sponge

Affectionately dubbed the "sea cow", these creatures normally eat seagrass, which grows abundantly at the Great Barrier Reef. They can live as long as seven decades.

Published: / Caption: / Photo: Reinhard Dirscherl/Getty Images
11
of 15

Pygmy seahorse

Reef stonefish

Spotfin lionfish

Potato cod

Clownfish

Nudibranch

Humphead wrasse

Green turtle

Giant oceanic manta ray

Giant clam

Dugong

Cuttlefish

Crown-of-thorns starfish

Box jellyfish

Barrel sponge

Despite the name, these alien-looking creatures are actually molluscs. They're also among the most intelligent of invertebrates, with a huge brain-to-body ratio.

Published: / Caption: / Photo: The Ocean Agency
12
of 15

Pygmy seahorse

Reef stonefish

Spotfin lionfish

Potato cod

Clownfish

Nudibranch

Humphead wrasse

Green turtle

Giant oceanic manta ray

Giant clam

Dugong

Cuttlefish

Crown-of-thorns starfish

Box jellyfish

Barrel sponge

These spiky, venomous invertebrates are one of the Great Barrier Reef coral's chief nemeses. Scientists estimate these creatures are responsible for nearly a quarter of coral destruction over the past three decades.

Published: / Caption: / Photo: Education Images/UIG/Getty Images
13
of 15

Pygmy seahorse

Reef stonefish

Spotfin lionfish

Potato cod

Clownfish

Nudibranch

Humphead wrasse

Green turtle

Giant oceanic manta ray

Giant clam

Dugong

Cuttlefish

Crown-of-thorns starfish

Box jellyfish

Barrel sponge

Extremely venomous and nearly invisible, the box jellyfish can extend its translucent tentacles up to 3 meters. Good luck, swimmers.

Published: / Caption: / Photo: Auscape/UIG/Getty Images
14
of 15

Pygmy seahorse

Reef stonefish

Spotfin lionfish

Potato cod

Clownfish

Nudibranch

Humphead wrasse

Green turtle

Giant oceanic manta ray

Giant clam

Dugong

Cuttlefish

Crown-of-thorns starfish

Box jellyfish

Barrel sponge

Sponges filter and clean the ocean, as well as serve as homes for smaller invertebrates. Similar to coral, these animals are also suffering because of changes in the ocean's temperature.

Published: / Caption: / Photo: Reinhard Dirscherl/ullstein bild/Getty Images
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Inside the Sea Simulator mimicking the Great Barrier Reef