Ocean animals have evolved all kinds of different ways to protect themselves from threats. Some use camouflage to blend into their surroundings. Others have hard shells that keep the animals (relatively) safe inside. And many others rely on toxins to dissuade potential predators from bothering them.

Today we’re sharing some of our ocean’s most notorious venomous residents. Before we get started, a quick reminder: venomous animals are ones that produce specific toxic substances that can be injected into their foes. Venomous is different than poisonous: venom is injected, but poison is ingested. Learn more about the difference between venomous and poisonous.

There are more venomous animals in the ocean than you might think—although most pose no harm to humans as long as you give them their space! All octopuses and cuttlefish, as well as some squid, are venomous, and mostly use their venom to hunt and kill their prey. Jellyfish can also be venomous, and use a harpoon-like stinging cells called a nematocysts to inject their prey. There are also over 1,200 venomous fish in the ocean.

Here are five venomous ocean animals to know:

Lionfish

Distinguished by their bold stripes and spines, lionfish are an invasive species in the Western Atlantic, Gulf of Mexico and Caribbean. Their spines contain a strong neurotoxin that can cause extreme pain and swelling if injected.

Blue-ringed octopus

The blue-ringed octopus’ venom is 1,000 times more powerful than cyanide. This golf-ball sized powerhouse packs enough venom to kill 26 humans within minutes. It’s no surprise that it’s recognized as one of the most dangerous animals in the ocean.

Australian box jellyfish

The tiny Australian box jellyfish is considered the most venomous animal in the sea—their sting can cause cardiac arrest, paralysis or death in humans in just a few minutes. Let’s just say you want to avoid it.

Sea snake

Sea snakes are more toxic than many venomous snakes found on land. Fortunately, they typically do not bite unless provoked and their short fangs make it difficult to bite through thick fabrics.

Stonefish

The stonefish holds the title of most venomous fish in the sea. Stonefish have 13 spines lining its back that release venom under pressure. Stings result in terrible pain, swelling, necrosis (tissue death) and even death.

These are just a few of the many venomous critters in our ocean. Looking to learn about more toxic ocean animals? Learn about the poisonous AND venomous striped pyjama squid and the poisonous sea pig.