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The Most Poisonous Creatures In The Sea

Updated September 23, 2021 45.6k views21 items

The open ocean is one of the most terrifying and unforgiving habitats on the planet. It is a brutal home, one that has shaped many of its inhabitants into formidable creatures. With so many dangerous animals lurking beneath the waves, a sort of biological arms race is in effect between predators and prey. After millions of years of evolution, there are now dozens of species of poisonous fish, sea snakes, and other ocean life that could seriously ruin a scuba diving vacation. 

The most poisonous sea animals in the world are as diverse as they are deadly. Every animal on this list is poisonous, not to be confused with all the venomous marine life in the ocean. The key difference is the delivery method, as venomous creatures actively inject their toxins while poisonous animals transfer their poisons only when touched or eaten. It's a small but important distinction, one that can be the difference between life and death in the wild. These are all of the most poisonous animals in the sea, and they make the ocean a more dangerous place to visit. 

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  • Photo: Alexander R. Jenner / Wikimedia Commons / CC BY-SA 3.0

    Nudibranchs are some of the most beautifully bizarre animals in the ocean. There are roughly 2,000 different species, all of which have their own color scheme. Often referred to as sea slugs, these flashy critters can pack a powerful punch when threatened.

    Nudibranchs absorb toxins from the food they eat and store them inside their bodies, making them poisonous to most predators. Scientists aren't completely sure how the nudibranchs survive after ingesting so many toxins, but the amazing skill has proven to be an effective defense mechanism.

  • Photo: Florida Keys National Marine Sanctuary / Wikimedia Commons / Public Domain

    Barracudas are notorious for their frightening appearance and murderous appetite, but it's not just their teeth you should fear. Their flesh is rich with a nasty toxin known as ciguat­oxin, which can induce ciguatera fish poisoning in humans. Barracudas are so poisonous, the Center for Disease Control and Prevention places them on top of their list of predatory fish to avoid. 

    While non-fatal, barracuda poisoning will wreak havoc on your digestive system and induce extreme nausea within 24 hours of consumption. Barracudas have such high toxin levels because the fish they eat consume a type of algae that produce ciguatoxin and the toxins build up in their bodies over time. 

  • Photo: Scubagirl85 / Wikimedia Commons / CC-BY-SA 3.0

    Striped Pyjama Squid

    There are not many poisonous cephalopods, but the striped pyjama squid is an exception to that rule. Its striped skin is believed to be a warning that these guys are not safe to eat. The have specialized glands on their bellies that secret a toxic slime if they are assaulted by a predator, rendering them unpalatable.

    These squid-like creatures are actually a species of cuttlefish and not true squids.

  • Photo: Sylke Rohrlach / Wikimedia Commons / CC BY-SA 3.0

    Smooth Toadfish

    The toadfish are another group of animals that are closely related to puffers, and they too are incredibly dangerous. Unlike other members of tetraodontidae, smooth toadfish lack the spines that can be found covering most pufferfish skin. The spines in a smooth toadfish are embedded in the skin, creating the smooth appearance from which the fish gets its name.

    That doesn't mean they aren't deadly, as they are full of toxins and should never be eaten.