The first cephalopods ("head-foot") appeared 500 million years ago. They were shelled, just like their relatives, snails and clams. But later, in Devonian, Carboniferous, Triassic, and Tertiary Epochs, with the evolution of the fish, marine reptiles, and later whales, to survive, the shelled cephalopods had to retreat to the depths or turn more mobile.

Indeed, today, the Nautilus, with several species, is the only shelled cephalopod. The rest have lost their shells, partially (squids and cuttlefish) or completely (octopus).

1. The first octopuses evolved towards the end of the Cretaceous (the last dinosaur era). They were very small (8 cm long). Today there are 289 species of octopuses, forming about one third of the all cephalopod species.

2.The world's largest octopus is the North Pacific Giant Octopus (Octopus dofleini): on average 3 m (10 ft) in length and weighing15 kg (33.3 pounds). Still, in 1957 an individual of almost 10 m (30 ft) and 272 kg (604 pounds) in weight was found on the beaches of the British Columbia (western Canada). The average size is achieved in three years.

3. The males of argonaut octopuses are 2 cm (0.8 inch) long, while the female is huge in comparison, with her maximum 10 cm (4 inch) in length.

4. Octopuses are solitary, territorial agile hunters, feeding mainly on crustaceans (crabs and lobsters) and mollusks (clams, squids, cuttlefish), sometimes fish (including sharks). They can fast up to 6 months. Octopuses turn active at the twilight. During the day they hide in hollows, whose mouth they tap with a stone. They digest their food in 12 hours at a water temperature of 18-19 degrees Celsius. The hard parts of the prey are not swallowed.

Octopuses are preyed on by sperm whales, seals, fish (conger eels, moray eels) and sea birds.

5. Octopuses have mimic ability overcoming that of a chameleon. Hatchlings have 65 chromatophores; at the age of one year, an individual has 1-2 million chromatophores. This are starred pigment cells in the skin, colored in yellow, orange, red, brown or black, and activated through nerves. When contracted, their color disappears, when spreading, their color dominates. Leucophores form white spots on the skin.

Octopuses can mimic not just the color, but also the shape of the environment, imitating, for example, a stone covered by algae, making the approach to their prey and escaping predators easier.

Iridophores are found in the skin; they refract light and give the octopus an iridescent brightness. Adults can show off colors as signs of aggression.

6. The octopus is the most intelligent invertebrate. Its brain is formed by 500 million large neurons (the human brain is made of roughly 100 billion smaller neurons), but their intelligence is comparable with that of the apes. If a cat has to get the reflex of opening a door after having watched the action many times, an octopus can detect the solution by itself, displaying "conceptual thinking". An octopus opens the plug of a bottle to get to the lobster inside. Besides mammals and birds, octopuses are the only animals that play.

Because octopuses are solitary, the offspring do not have a learning process based on direct transmission. The speed of learning is astonishing: an octopus found by itself in about 12 minutes how to get out of a cage. The next time, it escaped from the cage in 90 seconds, proving an unmatched memory. Octopuses have two memories, cerebral (brain) and one connected to their cups. It is known how curious octopuses are to scuba divers, a behavior typical to intelligent animals.

Octopuses live only 3-5 years, so they have limited time for accumulating information. Some researchers say that if octopuses had lived more, they, and not humans, would have been the dominant intelligence on Earth.

7. Octopuses can change body shape to escape traps. An octopus's only solid part is its cartilaginous "skull". So, if a hole allows the passing of the skull, the octopus will escape, quite a problem in aquariums.

8. The octopus's eye is the most sophisticated amongst invertebrates, approaching as structure and function to that of the vertebrates. Still, they cannot see the colors.

9. These animals have three hearts, but the vascular system is poorly developed, explaining the short lifespan. Another thing: their blood is blue, as it contains hemocyanin, a copper containing respiratory pigment.

10. The octopus' penis, made by the male's third arm on the right, is called hectocotylus and is detachable. Mating can last two hours for several times, and the male introduces the hectocotylus into the gills' cavity where it deposes the spermatophores (pouches filled with millions of sperm cells), but the partners keep the distance. Partners recognize each other through smell and touch. During the mating, the male loses its hectocotylus, but till the next season, he will regenerate a new one.

They usually mate once in a lifetime: the female lays clusters of eggs, usually 100-500,000 eggs. A cluster of eggs can be 10 cm (4 in) long and contain 2,000-3,000 eggs. The clusters are defended and oxygenated by her with sprays of water for 24 -125 days (incubation time depends on the water temperature), a time of fasting for her, when she survives from her fat reserves. When the offspring hatch, she has just died of starvation. The same happens with the squids and cuttlefish. The hatchlings feed on shrimp larvae. . 90 % of them die soon after hatching.

Because females die after breeding, they live just 1-2 years, and the males up to 5.

In argonaut octopuses, the male does not even approach the female. When the spermatophores are formed, the hectocotylus breaks off from the male while away from the female, heading towards her and entering alone into its mantle cavity to fecundate the eggs. The argonaut octopuses are also called paper nautiluses, referring to the paper-thin eggcase that females secrete. This structure lacks the gas-filled chambers present in chambered nautilus shells and is not a true shell, but an evolutionary innovation unique to these octopuses.

10.The 1,600 cups found in the arms are not just tactile organs, but also olfactory. In other words, octopuses smell with their arms. A lost arm is regenerated.

11.When attacked by predators, octopuses can detach their arms. The amputated arm grows back in a few weeks. They also expel ink, which can persist for 10 minutes.

12.All octopuses produce venom, which is harmless for men in most species. Still, one of the smallest species, the blue-ringed octopus (Hapalochlaena) (20 cm (8 inch) long, 26 g heavy, the size of a golf ball), living in tidal areas in northern Australia has a powerful neurotoxic venom that kills humans and for which no antidote is known. The venom blocks the breathing muscles.

13.An abyssal octopus, the Vampire Squid (Vampyroteuthis infernalis) is covered completely with light-producing organs (photophores), controlled by nerves. It can produce disorienting flashes of light for fractions of a second to a few minutes.

14. There are no octopuses in polar waters.