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Go diving in the tidal waters of the Great Barrier Reef along Australia’s northeast coast and you might come across a tiny octopus, no bigger than the size of your hand. If he perceives you as a threat, the dazzling blue rings patterned all over his body are likely to flash. Take this as a sign to stay away.

The blue-ringed octopus’s warning system doesn’t seem to work on many people, as many have looked too closely and been horribly bitten by its poisonous saliva. Although not particularly aggressive, he does not like to be turned around or touched.

The venom of this pint-sized creature (it averages 5 to 8 inches long) is strong enough to paralyze a human. Although it is rare for someone to die from its bite, it has happened, as there is still no known antidote. There is debate as to how many human victims this octopus has caused, but the consensus in a textbook on venomous marine animals published in 1996 was that at least eleven had succumbed in that time.

The blue-ringed octopus “incubates” its eggs until they hatch

A 1973 study in Marine Biology observed the life history of captive blue-ringed octopuses, or Hapalochlaena maculosathat were bred in a laboratory. He found octopuses to have a strange characteristic. Females didn’t lay eggs in dens or under rocks like other octopuses—instead, expectant mothers carried them until they hatched.

Assuming an egg-laying posture, a female would carry her eggs in groups (like a bag) or even singly, moving them from one hand to the other as needed. To put things into perspective – she can lay up to 50 eggs per clutch, making this an impressive feat.

Scientists believe the species will evolve for maximum efficiency as it reproduces quickly and thrives on a simple diet of crabs. It penetrates these crustaceans, injects poisonous saliva to pre-digest the flesh, and proceeds to absorb the tissues.

As well as helping to prepare its meals, its venom also keeps predators such as porpoises, dolphins and larger fish at bay.

The coconut octopus walks on two tentacles like a human

This avid collector of discarded coconut husks is a widespread inhabitant of seas around the world. It is usually tiny, sometimes even smaller than the blue-ringed octopus. It likes to make its home in barks and often encloses itself in two barks in the presence of a predator.

The coconut octopus, or Amphioctopus marginatususes marine debris to subtly cover its “bed”. This cephalopod maintains such a low profile that it will quickly slide into bottles, cans and other collectibles when approached or disturbed.

This “tool” use is probably the first evidence of tool use in invertebrates, and is not the only human behavior of its kind. It has also been seen to walk on two or four tentacles, using its remaining tentacles to carry shells or mimic the appearance of one.

The reproductive system of this species is robust, allowing females to lay up to 100,000 eggs per clutch.

The female octopus is 10,000 times larger than the male

Blanket octopuses are extremely rare, having only been seen a handful of times in the past decade. It is not a single species, but a group of four, all of which appear as iridescent, coiled creatures with a long trail. Many divers, scientists and others who have seen this creature believe it to be the most beautiful species of octopus.

If threatened, they will not hesitate to pull out their long, web-like blankets of flesh to entangle a predator, allowing time for a quick escape. It also has a strange hunting technique – it will tear off the tentacles of jellyfish to fish for food.

The blanket octopus has the largest sex-size difference of any animal. Females average about 6-7 feet in length, while males are just 1.5 inches. The latter, of course, is not equipped with the blanket.

The mating ritual of this group is just as strange as their hunting habits. The female tears off the male’s arm and stores it in the mantle cavity until she is ready to use it for fertilization. As expected, the male dies soon after.

The world’s oceans are home to up to 300 species of octopus, each more remarkable than the last. From polar waters to tropical sea grasses, they have evolved to survive in a range of habitats. They possess some of the highest levels of intelligence and camouflage of any animal.

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