When people think of dangerous animals, they tend to think big. Megalodon-size sharks, giant squid capable of sinking ships — that sort of thing. But the deadliest creatures in the sea, pound for pound, are so small, you probably wouldn’t think twice. The secret weapon? Venom.

Blue-ringed Octopus

These octopodes don’t make their own venom but rely on bacteria in their salivary glands. Tobias Friedrich

For instance, a small, often painless, bite from the beautiful blue-ringed octopus can contain tetrodotoxin, which it uses to hunt. A 25-gram octopus carries enough of this powerful toxin to fatally paralyze 10 adult humans. Cone snails use their barbed tooth like a venom-laced harpoon to inject a cocktail that can contain hundreds of different toxins to instantly paralyze fast-moving fish.

Cone Snail

The geographic cone snail is nicknamed the cigarette snail, because once stung, you have just enough time to smoke your last cigarette before you keel over. BIOSPHOTO/ALAMY

Research suggests that the snails can combine these components on the fly, adapting their venom to the situation at hand. Some of these substances are being researched as potential pharmaceutical products — one compound was found to have pain-relieving properties 10,000 times more potent than morphine.

Beaked sea snake This aggressive snake accounts for 90 percent of human deaths from sea-snake bites. Terrence Ross/Tom Stack & Associates

Beaked sea snakes can deliver enough venom in a single bite to kill five people, but fortunately, of the few bites that do occur, most are “dry bites,” in which no venom is injected. These three venomous creatures have the power to kill humans, but unless you threaten them, they’d just as soon live and let live.

Animals that rely on venom for protection generally aren’t able to “attack” others. Take the flower urchin, for instance — this 6-inch echinoderm’s lovely petallike appendages each end in three venom-wielding claws. It won’t be chasing anyone down, but come too close and you’ll wish you hadn’t.

Flower urchin

These urchins cover themselves in rocks and debris to hide from predators. Christian Skauge

The stonefish is another infamous example of extreme defensive measures. In fact, the stonefish cannot eject its own venom; rather, the pressure from a predator’s jaws clamping down or the weight of an unfortunate foot are what drive the fish’s spines into the victim and contract the attached venom sacs, delivering the nasty stuff deep into the wound.

Stonefish

These masters of disguise don’t have scales but secrete a fluid that promotes plant growth on their skin. Wayne MacWilliams

Fortunately, if you keep your hands to yourself and watch where you put your fins, you should be able to prevent an uncomfortable close encounter. The Australian box jellyfish is widely considered to be the most venomous marine animal, and one of the most dangerous ones on the planet. Just 6 feet worth of tentacle can carry enough venom to kill a human in two minutes.

Box Jellies

Box jellies are complex compared with other jellies. They have 24 clusters of eyes — some of which have a sophisticated lens, retina, iris and cornea — that are arranged to give them 360-degree vision. They are thought to be active predators and can swim as fast as 4.5 mph. While Australian box jellyfish are considered the most dangerous, there are a number of different species of box jellies, including the South African box jelly, pictured here, and the Irukandji jellyfish, which is infamous for Irukandji syndrome. While not deadly, the excruciating side effects of its sting include not just pain and vomiting but a feeling of impending doom. Research has suggested that it acts on the adrenal system and creates a prolonged fight-or-flight response. Geo Cloete