All ideals being admitted, if terror be an object, the octopus is a masterpiece.

VICTOR HUGO (1802-85)

Octopuses

Invertebrate Vertebrates


Octopuses look about as different from humans as its possible to imagine: no body to speak of: just as head sprouting eight arms (it’s very biologically incorrect to call them legs or tentacles). However, they have larger brains, relative to body weight, than any animals except birds and mammals, and they get bored easily. 
 
If they are kept in environments enriched with natural features, they grow faster, learn faster and remember more of what they learn than if they are kept in bare tanks. They remember places where they might find food, and where they have already hunted. And, although octopuses are mostly solitary, there is evidence that they can communicate and, if kept together, they establish hierarchies and avoid confrontations. For all these reasons, in the UK, octopuses enjoy the same legal status as vertebrates.
 
What they are not so good at is recognising the sex of other octopuses. If you put two octopuses in a tank they will start to copulate regardless of sex. Thirty seconds into a male-on-male encounter there’s usually an unembarrassed disentangling, although some of these gay clinches can last for days.
 
Octopus sex is a polite affair, carried out at arm’s length. One of the male’s eight arms is for mating and it differs from the others by having a groove on its underside and a grasping tip, called a ligula which, in some species, inflates with blood, rather like a mammal’s penis. The arm carefully places a packet of sperm in a corresponding slot in the female’s mantle (the body/head). The ligula then breaks off and remains embedded in the female. The male dies within a few months of mating. Although octopuses can regenerate lost limbs, they can’t grow a new sex arm.

Octopuses are good mimics. Some imitate other dangerous animals like sea snakes and lionfish.

DREW BARRYMORE

Oh, I love hugging. I wish I was an octopus, so I could hug 10 people at a time!

Two-thirds of an octopus’s neurons are in its arms.

Keeping Octopuses Happy


Octopuses are some of the most intelligent marine organisms and can be trained to open screw-top jars, though they forget the trick very quickly and constantly have to be re-trained.
 
Staff at the Sea Star Aquarium in Coburg, Germany believe that an octopus called Otto had been so annoyed by the bright light shining into his aquarium that he climbed onto the rim of his tank and short-circuited it by squirting a jet of water in its direction. Director Elfriede Kummer who witnessed the act said:
 
‘We've put the light a bit higher now so he shouldn't be able to reach it. But Otto is constantly craving for attention and always comes up with new stunts so we have realised we will have to keep more careful eye on him - and also perhaps give him a few more toys to play with.
 
‘Once we saw him juggling the hermit crabs in his tank, another time he threw stones against the glass damaging it. And from time to time he completely re-arranges his tank to make it suit his own taste better - much to the distress of his fellow tank inhabitants.’

OGDEN NASH (1902-71)

Tell me, O Octopus, I begs
Is those things arms, or is they legs
I marvel at thee, Octopus;
If I were thou, I'd call me Us.

All octopus species are venomous to humans, but only one is deadly.

The suction cup on the arm of an octopus has the sense of smell. 

The Walking Octopus


Indonesia’s Octopus marginatus from Indonesia walks using two of its arms (not tentacles, which are longer then arms and only have suckers at the tip) and wraps the other six around its head, perhaps so as to disguise itself as a coconut rolling with the current - no-one has come up with a better explanation, anyway. This behaviour was discovered recently by Christine Huffard from the University of California.

Octupuses or Octopodes


The plural of octopus is 'octopuses', not 'octopi' - if you were being pedantic you'd have to opt for 'octopodes' as the etymology of the word is from the Greek okto (eight) and pous (foot) not from any Latin word which might have a plural ending in -i. If the word were Latin, it would be octopes ('eight-foot') and the plural octopedes, analogous to centipedes and millipedes. 

Three Hearts and Blue Blood


An octopus has three hearts and the blood that it pumps around is blue when oxygenated. It has two small hearts located near the gills which pump oxygenated blood to the main systemic heart. This heart then pumps the refreshed blood around the rest of the body. Mollusc blood is based on the protein haemocyanin (rather than haemoglobin) which is a copper compound. It turns blue when oxygenation changes the Cu(I) to Cu(II).

The arms of an octopus can work and solve problems independently of its brain due to the many neurons it has.

An octopus has horizontal pupils; whatever angle the octopus is at, its pupils always stay aligned with the horizon.