The clever enigmatic octopus and what we know about the species so far

The octopus is an eight-armed cephalopod mollusk of the order Octopoda. Octopuses are widely distributed in oceans worldwide. Generally solitary creatures, they spend much time in dens – small holes and crevices in rocks.

Octopuses are very old in evolutionary terms. The oldest known octopus fossils are of an animal that lived 296 million years ago, before the age of the dinosaur. The octopus is very clever and the common octopus (Octopus vulgaris) is rated the most intelligent of all invertebrates.

Octopuses range in size across their different varieties from about 5cm to 5.4m long, the longest with an arm extension of 9m. They generally feed on crabs and other crustaceans, although some are plankton-feeders. Each octopus arm has two rows of fleshy suckers of great holding power.

The octopus body-plan is quite unusual – see article by Rachel Nuwer, Smithsonian Magazine. For example, in addition to eight arms the octopus has three hearts. Two smaller hearts pump blood to the gills while a larger heart pumps blood to the rest of the body.

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It can change the colour and texture of its skin pretty much instantly, camouflaging itself against its surroundings

It can also be considered to have nine brains. In addition to the central brain, each of the eight arms is a mini-brain that can act semi-independently. For example, the central brain, which forms a ring around the oesophagus, can signal to an arm – “search that hole for crabs” – and it will search the hole tasting and feeling with its suckers, while the octopus occupies itself with other activities.

If the octopus loses an arm it can generate a replacement in 100 days. One hundred and eighty million neurons in the central brain are connected to 40 million neurons in each of the eight arms. The octopus has blue blood, adapted to cold low-oxygen water by using haemocyanin, an oxygen-carrying copper-containing protein – vertebrate blood uses the iron containing haemoglobin.

Impressive defence strategies

The octopus arms are joined at the base by a web known as the skirt, at the centre of which lies the animal’s mouth. The mouth has a pair of hard horny beaks and a file-like organ (radula) for drilling shells and rasping off flesh. The octopus has an amazing capacity to squeeze its body through narrow cracks. Basically, it can pass through any opening that is narrower than it’s only hard part, the beak.

The octopus confronts its aggressors with very impressive defence strategies. It can change the colour and texture of its skin pretty much instantly, camouflaging itself against its surroundings. And the mimic octopus (Thaumoctopus mimicus), in addition to disguising itself can accurately impersonate other animals feared by its predators. Also, when it feels in danger the octopus squirts out an inky substance as a screen. This screening ink can also paralyse the sensory organs of the attacker.

Octopuses come in two sexes, male and female. The male octopus has one modified arm, the hectocotylus, that inserts sperm directly into the female. The fertilised female lays more than 100,000 eggs, each about 0.3cm long, under rocks and in holes. The eggs hatch after four to eight weeks, during which time the female guards the eggs. Procreation is also a tragic event for the octopus. Males fade away and die some months after fertilising a female and females die soon after their eggs hatch.

Octopuses live only two to three years. This is mysterious in view of their large brains

The brain to body ratio of the octopus is the largest among invertebrates. It is also greater than many vertebrates, although not mammals. Tests have demonstrated that octopuses can negotiate mazes and can, for example, unscrew caps from containers to get at the food inside. In 2009, veined octopuses (Amphioctopus marginatus) were observed excavating half-coconut shells from the ocean floor and carrying them away for use as portable shelters. This is the first documented use of tools by an invertebrate.

Octopuses live only two to three years. This is mysterious in view of their large brains. Why invest so much in developing the capacity to learn about the world when they have so little time to use this information?

Octopuses are eaten in Korea, Japan and Mediterranean countries. In order to provide continuity and volume of supply, it is proposed to establish industrial farms to rear octopuses for the food industry. But surely industrial farms are very inappropriate for these curious, active and intelligent animals. Indeed, is it not highly inappropriate to eat these animals in the first place?

William Reville is an emeritus professor of Biochemistry at UCC