5 Poisonous Ocean Animals to Stay Away From

These poisonous animals use toxins to protect themselves

Swimming beneath the waves is a sea of creatures even more unique and wild than you could ever imagine—fish that look like horses, whales that are 100 feet long and sharks living to more than 400 years old.  But, what about those ocean animals that have adapted to use poison to defend themselves against predators?

Before we begin today’s exploration, let’s lay some ground work. Venom is injected, but poison is ingested. Venomous animals are ones that produce specific toxic substances that can be injected into their foes. Poisonous animals, on the other hand, coat part or all of their body in a toxic substance, so they’re harmful to touch or eat.

Today, we are going to explore these five poisonous ocean animals. Dive in with us …


One pufferfish carries enough poison to kill 30 adult humans! Almost all pufferfish contain tetrodotoxin, a substance that makes them bad-tasting and often deadly to fish. Tetrodotoxin is also deadly to humans and is up to 1,200 times more toxic than cyanide.

Puffer fish


Commonly known as sea slugs, nudibranchs are a group of shell-less marine mollusks. They live in shallow tropical ocean waters. There are more than 3,000 species of nudibranch. Nudibranchs absorb and store toxins from their food, making themselves toxic to potential predators. Scientists aren’t certain how nudibranchs survive after ingesting so many toxins, but one thing is sure: they have a great defense mechanism.

Nudibranch in the ocean


The boxfish is related to the pufferfish—although not as poisonous. The boxfish uses its poison to help defend itself against predators—it would rather not become the predator’s next meal. The boxfish excretes a toxin into the water, poisoning the water and any marine life in the area.

Boxfish among corals

Flamboyant Cuttlefish

The flamboyant cuttlefish is a member of the class Cephalopoda (which also includes octopuses). These flamboyant cuttlefish are native to sandy habitats in the Indo-Pacific Ocean and are relatively small in size, reaching only three inches in length. These colorful, captivating cephalopods will steal your heart, but beware, they are also poisonous. Their colorful and bold exteriors may be a warning sign to predators. According to the Monterey Bay Aquarium, research by Mark Norman with the Museum Victoria in Queensland, Australia, has shown the flamboyant cuttlefish’s toxin to be as lethal as that of the  blue-ringed octopus. That’s quite impressive, considering the blue-ringed octopus is one of the most toxic animals in the ocean.

Cuttlefish in the ocean

Blue Ringed Octopus

Speaking of… the blue ringed octopus is an absolute overachiever—it is both poisonous and venomous! If an animal somehow avoids being injected with venom when it attacks and tries to eat the blue ringed octopus, the animal will learn the hard way that the meat is poisonous. Just another reason to leave the blue-ringed octopus alone!

Blue Ringed Octopus in the ocean

Poisonous versus Venomous

Now that you know the difference between poisonous and venomous, here is a fun game you can play. Can you spot inaccuracies as you are doing research, web surfing or reading news articles? It happens more than you know. Many, many times the terms are used interchangeably, but they are not the same. A lionfish isn’t poisonous, and a pufferfish isn’t venomous.

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