Octopus

An octopus is a soft-bodied, eight-limbed mollusc belonging to the order Octopoda. This order contains approximately 300 different species. They are part of the larger class Cephalopoda, which includes squids, cuttlefish, and nautiloids.

An octopus is bilaterally symmetric, meaning its left and right sides are mirror images. It possesses two eyes and a central beak-like mouth located between its limbs. Because they lack a rigid skeleton, octopuses can radically deform their bodies. This ability allows them to squeeze through incredibly small gaps in the environment.

The anatomy of an octopus is highly specialized for life in the ocean. The body consists of an elongated head and a bulbous, hollow mantle. The mantle is fused to the back of the head and houses most vital organs.

Attached to the head are eight flexible, prehensile limbs known as arms. These arms are connected near their base by a webbed structure. The arms function as muscular hydrostats, meaning they use muscle contractions rather than bones to move. They contain longitudinal, transverse, and circular muscles around a central nerve. The interior of each arm is lined with circular, adhesive suckers. These suckers consist of an outer infundibulum and an inner acetabulum. By sealing the orifice between these two parts, the octopus can secure itself to surfaces or handle objects.

Respiration and movement are closely linked through a process called jet propulsion.

To breathe, the octopus draws water into the mantle cavity through an aperture. The water passes over the gills, which have a lamella structure for high oxygen uptake. The gills can absorb up to 65% of the oxygen in water at 15 degrees Celsius. Once the water is oxygenated, the octopus expels it through a funnel called a siphon. By forcefully discharging water through this siphon, the animal can shoot itself through the water. This process is also used for waste disposal and discharging ink for defense. While swimming, the octopus often trails its appendages behind it as it moves backwards.

The circulatory system of an octopus is complex and closed, meaning blood stays within vessels.

They possess three distinct hearts to manage their blood flow. A single systemic heart circulates blood to the rest of the body. Two smaller branchial hearts pump blood specifically through the gills. This system is necessary because octopus blood is quite viscous. The blood contains a copper-rich protein called haemocyanin, which transports oxygen. Unlike human blood, haemocyanin gives the blood a distinct bluish color. This protein is highly efficient at transporting oxygen in cold environments with low oxygen levels. However, the systemic heart becomes inactive when the octopus swims, which causes the animal to lose energy quickly.

Octopuses exhibit a wide range of sizes and habitats across the globe.

They inhabit diverse environments, from coral reefs and the seabed to the dark abyssal depths. The giant Pacific octopus is a notable example of a large species. An adult can weigh 50 kilograms and possess an arm span of up to 9 meters. In contrast, the smallest species, Octopus wolfi, is only about 3 centimeters long. Most species follow a life pattern of growing quickly and maturing early. Most are short-lived, often dying after a single reproductive cycle. In many species, the male delivers sperm to the female's mantle cavity and then dies. The female then guards her eggs in a den until they hatch, after which she also dies.

These invertebrates are known for their intelligence and diverse behaviors.

They are active predators that hunt fish, crustaceans, gastropods, and bivalves. To survive, they use several sophisticated defense strategies. These include camouflage to blend into surroundings, expelling ink, and using deceit. Some species, such as the blue-ringed octopus, are known to be deadly to humans.

Beyond biology, octopuses have deep roots in human culture. They appear in Norwegian mythology as the Kraken and in Ainu myths as the Akkorokamui. They are also considered a delicacy in many parts of the world, particularly in Asia and the Mediterranean.

The name "octopus" is derived from the Ancient Greek words for "eight" and "foot."

While some people use the term "octopi," linguists note this is etymologically incorrect. Because the word comes from Greek, the standard English plural is octopuses. The term "octopodes" is also used, though it is considered rare or pedantic. This linguistic history highlights how humans categorize and name the many wonders of the sea. From their complex nervous systems to their unique blue blood, octopuses remain one of the most fascinating subjects in marine science.