The octopus is one of the most recognizable animals from the planet. There are around 300 different species found in nearly every ocean habitat. In tropical regions, octopus make their home on coral reefs and on shallow flats. These environments suit the octopus, which often use the features on reefs and flats as a form of natural defence. When an octopus lives in these natural structures, it can be called dens. All octopus are part of a class of animals called cephalopods. They are symmetrical, with one of its two eyes either side of its mouth and beak, which lies at the centre point of the octopus's eight arms. The head of the octopus contains the brain and the mantle, which is the soft portion of the octopus that falls behind the eyes. This contains most of the internal organs. The stomach, the kidneys, the hearts and the sexual organs are all protected within this soft mantle at the top of the octopus. It's not only the mantle of the octopus that is soft. Octopus are invertebrates, which means they do not have skeletons. Instead, the body of an octopus is made almost entirely of soft tissue and muscle. This gives them the ability to extend and shorten their own bodies and squeeze their incredibly small gaps. Even the largest of octopus have been observed squeezing into gaps as small as 2.5cm. Octopus have three hearts. Two that pump blood through each of its two gills and one which circulates oxygen-rich blood around the entire body. This heart cannot be used while the octopus swims, so often the octopus will crawl along the sea floor to save energy and use its arms to help him maneuver. The eight arms of an octopus increase its versatility further. They contain muscles which can extend and contract, twist left and right, bend in any direction or be held rigid at any point, giving them exceptional maneuverability. They are also covered with suckers, as many as 1800, that allow the octopus to stick itself to objects or manipulate them with ease. Octopus use these suckers to feel their way across the reef, searching for prey. Taste receptors on these suckers mean they can also taste as they search to help identify what is edible and what is not. Octopus eat many small animals, including crabs, small fish and sea cucumbers, and use chemicals to dissolve prey into a mush, making them much easier to eat and digest. One of the most impressive things an octopus can do is change their appearance by altering the colour and texture on their skin to match its surroundings. Octopus do this by expanding and shrinking millions of pigment cells and flexing the many tiny muscles in their skin to change texture. This allows the octopus to camouflage itself to protect itself from predators and sneak up on prey. Octopus also produce ink, which is stored in an ink sack near the guts. When startled, ink is ejected from the sack, passes through additional glands where it is mixed with mucous and creates a thick cloud of ink in the water. Octopus frequently ink to escape predators or when they feel threatened. Octopus are short-lived species with most only living for one or two years. However, larger species like the Pacific Octopus can reach five years of age. Scientists can get an idea about how old an octopus is by looking at its size and weight. But getting an accurate age can be difficult without using special techniques that count the growth rings that grow on the hard parts of an octopus. One such hard structure are called stylists. Stylists are a pair of fine hard rods found inside the muscle of the mantle. Not much is known about them, but they appear to be a unique feature to the octopus. Data collectors at Blue Ventures can be trained to carefully remove stylists from the mantle and preserve them in alcohol. They are then shipped to a lab, sliced until they are very thin and examined under the mantle. This is a new form of a microscope. At higher magnification you can see the daily growth rings of an octopus, which are similar to the annual growth rings on a tree stump. Counting the rings can provide a reasonable estimate of the octopus's age. Octopus grow quickly, doubling in size every month. By six months of age, octopus are near full sexual maturity. This is a new form of a new form of an octopus. Next, we will explore this further by looking at the life cycle of the octopus. The octopus is a new form of a new form of an octopus.