Octopuses, octopi, octopodes

Chances are, you are familiar with the overall structure with these 8-legged sea mollusks; a hard parrot-like beak, practically no internal skeleton, and suction-cup arms make these animals pretty bad ass to begin with. 

The picture above depicts the only octopus with a venom deadly enough to kill humans, the Blue-ringed Octopus, but all are venomous.

You may have also heard that octopuses are smart, and generally they are accepted as the smartest invertebrates around. In fact, many nations deem these animals "honorary vertebrates" and as such are protected by their vertebrate animal rights laws.  However, this is just a general concept... how smart are these creatures? Studies have shown that they posses long- and short- term memory thru maze and problem-solving tests. They also have been known to easily escape aquarium tanks and move to feeder tanks to feast on fish. Some octopuses have boarded fishing boats, opened crab holds, and nom'd on fisherman goodies before jumping back in the ocean.
They also posses some pretty awesome defense mechanisms. They hide with camouflage by changing color with their chromatophore cells, and eject ink to make a cloud to lose predators. The mimic octopus has a remarkable ability to confuse predators by changing structure, movement, and color to look like more dangerous fish.

These octopuses have been known to imitate 15 other fish.
Unfortunately, octopuses have a short life span, which limits their intellectual capacity. Some live only 6 months, while others can live for 5 years. But, males and females die shortly after reproduction. The male implants its sperm packet into the female and will die shortly after, whereas the female, when her eggs have matured, tends to her egg nest for months. She blows air on the eggs to keep enough oxygen in their  environment and guards the nest for the entirety of their development which can take months. After the babies, around 60,000 of them, have hatched, she is very weak and is sometimes attacked and consumed by predators, dies from starvation, or dies from an endocrine-signaled programmed death.