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London, England (CNN) -- Octopuses are so smart they tip-toe around awkwardly on the ocean floor hoarding coconut shells to later build themselves a fort to defend themselves from predators, a new Australian study has found.
"Amazing behavior," said Australian biologist Julian Finn of Museum Victoria in Melbourne, who first observed the octopuses' smart moves while leading a team in Indonesia.
"We were blown away," teammate Mark Norman, head of science at the Museum Victoria, said about the discovery. "It was hard not to laugh underwater and flood your (scuba) mask."
"This is an octopus that runs around collecting cups, stacking them, running along with them underneath its body, and then assembling them as perfect armour if a predator comes along," Norman said. "Planned future use."
It's that forward planning -- and use of tools -- that surprisingly makes the veined octopus one of the smartest animals on Earth.
"Using tools, something we think is very special about humans, exists in other animal groups we've never considered before," said Norman in an interview with Australian television. "A low-life form, a relative of a snail, these octopuses, they're not simple animals."
Finn and Norman were in Indonesia's north Sulawesi region looking for the mimic octopus, when they chanced across the veined octopus doing its thing.
The coconut-carrying -- and subsequent hiding -- makes the veined octopus the newest member of the animal tool-using club, scientists say, an extremely elite group in the animal kingdom.
And it's the first member missing a backbone -- an invertebrate.
The researchers, who spent more than 500 hours underwater observing and filming the veined octopus's habits, noticed the animals were frequently using their long tentacles to lug around coconut shells bigger than their own bodies.
The octopus would dig up the two halves of a coconut shell, discarded by man, then later use them as protection when either stopping or resting on the ocean floor.
Then, astonishingly, they would arrange the shells neatly underneath their bodies and walk around with them suctioned to their bellies -- for future use.
To maneuver with the oversized shells, the octopus has to stick its arms out to envelope them, giving the appearance of walking around on stilts, the scientists said. That awkward walking makes it more vulnerable to predators, they said. Their findings are being published in the journal, Current Biology.
The scientists think the octopus probably first started out using clam shells, but graduated to coconuts discarded by humans when the coconut shells became plentiful.
Cephalopods -- octopuses, squid and their relatives -- have the most complex brains of any invertebrates, according to Science magazine. An octopus brain has 50 to 75 lobes and at least as many neurons as a mouse brain. Octopus also have smaller "brains" in each arm and even smaller "brains" associated with their suckers, according to Science.
Cephalopods used to rule the seas 500 million years ago. But the diversification of life on Earth that included the origin of fish meant the ocean became full of predators.
It was that evolution that led to their developing intelligence, scientists believe, a process that could teach much about brain evolution.
Octopuses of many species are already known for their smarts. They've been known to master mazes, open jars and even remember past events.
Now they've also got tool-use to add to their repertoire.