9 Dangerous Coral Reef Creatures

A green, pink, yellow, and orange coral reef teaming with fish and colors.

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It's a pleasure to swim in clear, warm tropical waters and take in all the color and life along coral reefs and coastlines. But these areas can be just as dangerous as swimming in the open ocean. Many people think sharks are the main creatures to worry about, but the real dangers lie in undersea life you might not suspect, like snails, jellyfish, and certain camouflaged fish.

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Blue-Ringed Octopus

blue ringed octopus on the ocean floor

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This small, colorful octopus can be found in tide pools and coral reefs in the Pacific and Indian oceans. It is also one of the most deadly marine animals in the world. The blue-ringed octopus, which grows to only about 5 to 8 inches, is armed with a venom powerful enough to kill 26 humans within a few minutes, and there is no anti-toxin for it. This octopus is especially dangerous because the bite is often not extremely painful, so victims don’t always realize they have been bitten until symptoms, including paralysis and respiratory and cardiac arrest, occur.

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Box Jellyfish

A box jellyfish underwater with a scuba diver in the background

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Box jellyfish are considered the world’s most venomous creatures; their stings have caused 60 deaths in the past 100 years. They are found throughout warm coastal waters, but the most lethal of box jellyfish are in the Indo-Pacific region and northern Australia. The deadly varieties of box jellyfish have tentacles covered in what are essentially minuscule poison darts. A person stung by the most lethal box jellyfish may experience symptoms such as paralysis, cardiac arrest, and potentially death within a few minutes of being stung. 

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Irukandji Jellyfish

Irukandji jellyfish in a small sealed vial held by a human hand between two fingers

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This may be one of the smallest species of jelly in the world, but it's also one of the most powerful. The venom of an Irukandji causes symptoms of severe muscle cramps, back and kidney pain, profuse sweating, nausea and vomiting, headaches, and even psychological effects that are collectively known as Irukandji syndrome. Even small doses of the Irukandji’s venom can cause the syndrome, and symptoms, which require the victim to be hospitalized, can last from a few hours to several days. Irukandji jellyfish are primarily found around Australia, but the syndrome can be caused by other jellyfish as well, including box jellyfish species found in Hawaii, Florida, Puerto Rico, and Guam.

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lionfish next to a colorful coral reef with blue and yellow coral

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They may be a favorite for aquariums, but lionfish are a top predator along coral reefs. Lionfish eat practically anything to satisfy their voracious appetite, and have hardly any predators thanks to their frilly defense mechanism that includes as many as 18 dorsal fins with venomous spikes. The sting from a lionfish is extremely painful and can cause nausea, breathing difficulties, convulsions, and sweating. Lionfish stings are rarely fatal in humans, but can cause heart failure in some victims.

Lionfish are one of the few fish species that have established new populations in open water after being introduced to an area. They are native to the Indo-Pacific but have been introduced to and become invasive in the Atlantic and Caribbean.

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Moray Eels

The head of a moray eel peeking out of coral reef with its mouth open

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There are around 200 species of moray eels, and though many, such as the giant moray, look menacing, none of them are inherently dangerous to humans. The risk comes when humans provoke eels or try to feed them. Eels will bite, so the best way to stay safe around moray eels is to avoid disturbing them in their burrows. Luckily, the only way you could actually be killed by a moray eel is not if it eats you, but if you eat it. They accumulate ciguatoxin through eating toxic algae or fish that have eaten the algae, and can potentially poison humans who consume them.

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needlefish swimming through blue-green water with its mouth open

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Needlefish are not dangerous because they are aggressive, venomous or poisonous, or pack a mean bite. They're dangerous mostly because of their shape, their needle-like teeth, and their ability to become airborne. The dagger-shaped fish usually swim just a few inches below the water's surface, but they can launch themselves out of the water at speeds of up to 37 miles per hour. They have been known to cause injury and sometimes death in people who happen to be in their way.

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Sea Snakes

a blue sea snake with black stripes and a yellow face on a coral reef

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While sea snakes aren't particularly dangerous, most species have extremely powerful venom. Because the amount of venom is relatively low, there are few fatalities due to sea snakes. Fishermen, who catch sea snakes in their nets, are at the greatest risk of being bitten. The most deadly of sea snakes are two species that live in the waters off of Asia and Australia.

If a person is bitten, the bite itself is usually small and may be painless and unnoticed. However, from 30 minutes to a few hours after the bite, symptoms may set in that include headache, thirst, vomiting, muscle aches, and later paralysis, renal failure, and cardiac arrest.

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a stonefish siting on a pile of small pebbles on the bottom of the water is well-camouflaged as a rock

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Stonefish may look like an innocuous rock, but actually are one of, if not the most, venomous fish in the world. And because they look just like a stone, swimmers can find themselves in uncomfortably close proximity to one without even realizing it. The stonefish species have neurotoxins in spines running along their dorsal fin, which stand up when the fish feels threatened. Depending on the amount of venom it injects, a stonefish can cause death in an adult human in less than one hour. The venom causes extreme pain, swelling, temporary paralysis, shock, and possibly death if not immediately treated with anti-venom.

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Cone Snails

Textile cone snail, with its white and rust patterned shell on the bottom of the sea

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Cone snails use an elongated barbed tooth as a harpoon to inject venom that paralyzes prey before they eat it. For humans, many species of cone snails have stings that feel a bit like a bee sting, but the geography cone, striated cone, and textile cone, or "cloth of gold cone," all have powerful venom. Symptoms of a sting include localized pain, swelling, vomiting, and in extreme cases, paralysis and respiratory failure. The effects can start immediately or be delayed for as long as days after the sting.

The powerful venom also has potential for medical uses. Researchers at the University of Utah are studying the effects of insulin in cone snail venom as a fast-acting insulin treatment for patients with diabetes.