Octopus

Octopus (can be common octopus, Octopus vulgaris)

Scientific classification
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Cephalopoda
Superorder:	Octopodiformes
Order:	Octopoda Leach, 1818[1]

What are octopuses?

Octopuses are molluscs (Phylum Mollusca) like snails, slugs and clams; and cephalopods (Class Cephalopoda) which include squids and cuttlefish. The correct plural term for octopus is octopuses and not octopi.

Where seen?

Most people are surprised to hear that octopuses are quite commonly encountered on many of our shores. Even the most 'beat up' looking shore eventually turns up a resident. They are generally more common in areas with coral rubble, but may also be seen in seagrass areas. You need patience and some experience, however, to spot an octopus. These marvellous masters of camouflage are shy and generally only active at night. During the day, they are often well hidden in some cosy den. Some octopuses seen are as large as 1m across with their arms outstretched. Others are tiny, less than 10cm across.

Awesome Octopus:

The octopus is a hunter with many tricks. Among its formidable weapons is its brain! The octopus is in fact considered the smartest known invertebrate. It has a well-developed brain and excellent eyesight. Studies show that the octopus can learn, not only by itself but also from one another!

Armed and Dangerous:

An octopus searches for prey mostly at night, spreading out its eight long arms to feel into crevices for crabs, prawns, snails, clams and other such morsels. The highly flexible arms have strong suckers to grip objects so that the octopus can slowly 'creep' over the surface as it stealthily investigates all hiding places (octopuses use jet propulsion when they are in a bigger hurry, see below). The arms also have numerous receptors sensitive to taste and touch.
The arms are joined together near the head with webbing. An octopus uses this webbing like a net. For example, to envelope a little mound of rubble where some small titbit might be hiding. When the prey attempts to escape, it is literally surrounded by octopus! Prey is killed with a bite of its sharp, hard beak. It is often then hauled back to the octopus' den for a leisurely meal.

Octopuses bite!

Although octopuses have a hard beak and a radula (ribbon of teeth), they don't chew their food. Digestive juices are injected into the prey which soften the tissues. Some octopuses can drill a hole through a snail’s shell to get at it. Others crush shells and crack crabs with their hard beaks.

Three Hears!
The octopus has three hearts. Besides the usual heart, it has two additional hearts, each pumping extra blood through the gills. Its blood is blue due to concentrations of copper-based pigments that transport oxygen.

Shell-less but not helpless:

Unlike most other molluscs, the octopus does not have a shell at all. This is actually an advantage as the octopus can then squeeze into all kinds of impossibly tight hiding places. The octopus, however, has many other ways to deal with danger.
In the first place, an octopus is generally very difficult to spot. It can change its colours and even the texture of its skin to blend with its surroundings. And change these rapidly as it moves to a new location.
When spotted, some octopuses make sudden drastic colour changes to confuse the predator. They then zoom off using jet-propulsion; squirting a jet of water out of a funnel to zoom off in the opposite direction. When particularly alarmed, an octopus may release a cloud of ink to disorient predators. The ink may contain substances that affect the senses of other sea creatures. In the clouded water, the octopus makes its getaway.

Octopus babies:Octopuses have separate genders. To mate, the male uses a special arm called a hectocotylus to insert a sperm packet into the female's body. While doing so, he usually keeps as far away from the female as possible, and he is usually pale, a sign of stress. The female uses the sperm to fertilise her eggs as she lays them. In most octopus species, the eggs are laid in capsules attached to hard surfaces. Here are some photos of cephalopod egg capsules.
In bottom-dwelling octopuses, the female looks after her eggs; keeping them oxygenated, free of algae and bacteria, and defending them from predators. Some even carry their eggs with them. The female does not feed during this time and usually dies after the eggs hatch. Most octopuses breed only once in their life, and many die after doing so.
The eggs do not hatch into free-swimming larvae. Instead, miniature octopuses emerge. Some are rather well-developed and settle down soon after hatching. Others may drift with the plankton before settling down.

Credit: http://www.wildsingapore.com/wildfacts/mollusca/cephalopoda/octopus.htm