An octopus’s life is comparatively short, with the animals rarely living longer than two years. However, since octopods grow very quickly (some species gain 1 to 2 per cent body weight in a day), they can still reach considerable sizes during this period. The size spectrum of octopods ranges from a few centimetres to seven metres arm span in the case of Enteroctopus dofleini (giant Pacific octopus) which is probably the largest octopus species in the world. The current size record is said to be 71 kg. The legendary Architheutis is even bigger, but in fact it is not an octopod at all and belongs to the subgroup of squids. Probably the most obvious difference between octopods and the two other cephalopod groups is the number of their arms, which are also called tentacles. While squid and cuttlefish always have ten arms, octopods have eight (from the Greek ‘októ’ for eight), which can be moved extremely flexibly in all directions. In the course of evolution, the muscular foot typical of molluscs has developed into highly flexible arms with suckers, which serve as sensitive tactile organs for gripping things and are more versatile than an elephant’s trunk. There are hardly any tasks at which an octopus will baulk. Their arms can fulfil a wide range of purposes. Octopuses use their arms for locomotion, for tracking, and for examining and holding prey. Some octopods sneak up to their prey, stalking carefully on the tips of their arms in order to grab it in a sudden attack. Others crawl over their prey with outstretched tentacles between which a thin membrane is stretched like an umbrella. An octopus uses its arms to rummage in rock crevices for food, which it holds in position with its suckers and then carries to its mouth. Some species can even use their arms to move nimbly and skilfully on land. The tentacles are also an effective tool for warding off competitors and predators. For a sudden escape the octopus can by means of a jet of water shoot swiftly backward with outstretched arms. When danger threatens it uses its muscular skin mantle to squirt a powerful jet of water out of a movable funnel within its body. Since octopuses have no solid body structures (apart from their bird beak-like horny mouth parts) they are very malleable and can even force themselves through narrow gaps and openings. There are one or two rows of sessile suckers on their muscular arms, which are usually much longer than the body. The arms and suckers are well equipped with nerves and ganglia, and this makes their movements “autonomous” and independent of central control by the brain. It is still not known how this control works in detail. Almost all octopuses have a “favourite arm” which they use more often than their other tentacles. In the males of a large number of octopus species the third right arm (less often the left) develops into the so-called hectocotylus with which the spermatophore is transferred to the female during mating.
Ability to change body colour enables perfect camouflage
Octopuses have three hearts which are as it were “connected in series” and together drive the animal’s blood circulation. The two hearts at the base of the gills (branchial hearts) pump oxygen-enriched blood to the main heart which is positioned in the visceral sac in the middle of the body and has two outlets for arterial blood. One of these main arteries leads to the head, the other supplies the internal organs. Pulsating veins additionally support the blood circulation. This effort is necessary to supply the internal organs and the long tentacles of the octopus with oxygen right down to the tips. Unlike fish, the blood of the octopus contains blue haemocyanin, which cannot transport as much oxygen as the haemoglobin of red blood. In contrast to many other molluscs, octopuses have very good eyes with lenses in a liquid-filled casing (“camera eyes”). Furthermore, octopuses can perceive certain differences in brightness with their body surface. Their skin contains lightsensitive proteins (opsins) which are also found in the visual pigment of the eyes. Although octopods are not able to recognise sharp images or contours with their skin, their sensitivity is sufficient to adapt the skin surface to the respective environment. This process is controlled autonomously without any detour via the brain, which enables particularly rapid colour changes. This ability makes octopuses real masters of camouflage: their skin not only emulates colouring and pattern but also the surface structures of their surroundings in a deceptively realistic manner. Because the colour change is much faster than in a chameleon the animals can also use it for intra-species communication. By changing colour they can signal their current mood to their fellow octopuses, or indicate their willingness to mate. They colour themselves as dark as possible, for example, when competing for territory. Inferior males sometimes disguise themselves as females. This remarkable imitation talent results from the unique structure of the skin which consists of several layers. The uppermost layer contains innumerable differently coloured pigment cells (chromatophores) which can be reshaped and connected with fine muscle and nerve fibres so that each individual cell can be specifically targeted. Depending on the state of tension in the muscles the pigment cells are stretched (their colour is then visible from outside) or compressed so that only the second skin layer underneath is visible. This layer contains cells that function as special filters and amplify or absorb the proportion of light reflected by the chromatophores depending on the angle of incidence. The third and lowest layer consists of white skin cells which, like a canvas, provide a high-contrast background. Complex colour patterns are created by the interaction of several pigment cells, comparable to a flat-screen TV, whose colour spectrum is also produced by combining a few basic colours. The Southeast Asian octopus species Thaumoctopus mimicus is rated the most outstanding master of camouflage and deception, and can even imitate dangerous marine animals to protect itself from predators.
Octopuses spawn only once in a lifetime and die shortly afterwards
Octopods feed mainly on crustaceans, crabs, snails and occasionally fish. Large species devour almost whatever crosses their path as they hunt. They grab their prey with their tentacles, bite out mouth-sized pieces with their sharp parrot beak-like jaws or chop them up with a rasping tongue. Some octopuses lurk hidden for unsuspecting prey, others hunt actively and sometimes even leave the water to search for crabs, snails and other animals in tidal pools. Stories about octopuses strangling people with their tentacles or even pulling ships down to the sea bed are an expression of our subliminal fears and belong in the realm of legend. Such stories culminate in the fantasy figure of Davy Jones, that hybrid creature of man and octopus, whose face has tentacles instead of whiskers – as Hollywood’s “Pirates of the Caribbean 2” cleverly visualized. Most octopods are neither dangerous nor aggressive. A bite by the parrot- like beak can be painful but is usually harmless, although recent research shows that probably all species produce a toxin in their salivary glands. However, only the bite of the blue ringed octopus Hapalochlaena maculosa and H. lunulata, whose deadly toxin maculotoxin is chemically very similar to the tetrodotoxin of puffer fish, is life-threatening for humans. Within two hours it causes severe nausea with vomiting, paralysis and respiratory arrest. An antidote is not known. However, accidents caused by poisonous octopuses are extremely rare. Between 1950 and 1995 only eleven cases were documented, two of which were fatal. Octopuses reproduce only once in a lifetime. They usually reach sexual maturity at the age of two or three years and after mating lay up to 400,000 eggs depending on their species. From then on they devote their efforts to taking care of their young. The breeding season lasts on average 30-65 days and the octopuses die soon after. For mating, the male inserts his third tentacle arm (which has developed into the hectocotylus) into the mantle cavity of the female and transfers a capsule filled with sperm (spermatophores). As soon as it bursts, the sperm are released and fertilise the eggs. In some species of octopus, the spermatophores can be quite large; in Octopus dofleini, for example, about one metre long. After hatching, the larvae drift freely in the open sea for one or two months before settling on the bottom and trying to occupy their own territories.
Octopod meat tastes good and is a high-quality food
People’s interest in octopuses is due not only to their fascinating biology but at least as much to their culinary qualities. In the Mediterranean region and in Asia, but increasingly also in other regions of the world, some squid or octopus species are among the most sought-after marine delicacies. Octopus meat is firmly elastic when raw, has a pleasant smell of the sea and – if prepared correctly – is extremely tender. It contains a lot of protein, omega-3 fatty acids and iodine, and its flavour is reminiscent of veal. As a rule, only the eight tentacles are eaten because although the head is edible it usually remains quite hard. The tough, rubbery consistency that discourages some people from eating molluscs – whether octopus, squid or cuttlefish – is the result of incorrect preparation. Octopods have very fine muscle fibres which lie on top of each other in several layers and are embedded in connective tissue. The collagen contained in them is relatively hard and only softens under long-lasting mild heat. Strong heat causes the tissue to contract irreversibly, which makes the meat unpleasantly tough. That is why patience is necessary when preparing octopus: the cooking water must not boil! Octopus cooks perfectly when simmered over slight heat (as a rule of thumb, one kilogram of octopus should cook for about one hour). Alternatively, it can also be braised slowly in the oven at medium heat. “Alla Luciana” or “en vinagreta”, octopus is a special, and in some regions also rare, pleasure. Due to strong demand, octopod stocks are overfished in some areas. The global catch has almost halved in the last three decades. Great effort is needed to catch the solitary creatures, either in shallow water by hand or in deeper water by divers with spears and harpoons. Probably the principle method used for catching octopuses is, however, with traps, mostly earthen pots, which serve as hiding places for the octopuses. Baits are unnecessary because the octopuses are actually searching for protection more than food. If they like the new home they are offered they will inhabit the traps quite quickly after they have been set.
Fry production is a bottleneck in commercial aquaculture
Fishing alone can hardly meet the growing demand and so interest in octopod aquaculture is growing. In recent times the suitability of some octopus species, including Octopus maya, O. bimaculoides, O. ocellatus and O. mimus, but especially the common octopus Octopus vulgaris, has been investigated. Technically speaking, nearly all of the species proved to be quite suitable. They can cope with cultivation conditions without any major problems, they grow rapidly (growth rate of 5% body weight per day is possible), and convert 30 to 60% of the food they eat into their own body weight. Nevertheless, the breakthrough in octopod aquaculture has been delayed because the animal’s life cycle has not yet been completely closed. It seems to be very difficult to reproduce the animals successfully and to achieve high survival rates of the paralarvae. Although it was possible to establish some octopus aquaculture projects in the past they were mostly based on wild stocking which does not relieve the natural stocks but increases the risk of overfishing even more. However, at the end of 2018, scientists from the Spanish Marine Research Institute (IEO) reported that after 20 years of research they had succeeded in breeding common octopus (Octopus vulgaris) in captivity for the first time and had managed to bring about half of the hatched paralarvae through the problematic first phase of life. It is possible that the long hopedfor breakthrough has finally been achieved. But is what now seems biologically feasible also ethically and morally justifiable? Already voices are emerging that warn against “mass breeding of highly intelligent beings”, of which the octopods are indeed one. In fact, octopuses – like all cephalopods – are unusual exceptions among invertebrates in terms of their mental abilities. Octopods plan their actions and use tools, they deceive their hunting victims, memorize their territories and imitate the behaviour of other sea creatures in order to protect themselves from enemies. Their brain is capable of amazing feats. And we can’t even say exactly where their capabilities begin or end because the 500 million or so neurons of their nervous system are not only located in the head, but two thirds of them are distributed in the outer extremities and function like the body’s own internet. Each arm has, so to speak, its own sensors and controls, and around 10,000 nerve cells are attached to each sucker. Octopus vulgaris has an amazing spatial memory and a good sense of orientation. This is why it solves many maze problems more efficiently than most mammals. It’s no wonder that octopuses are considered the “smartest” molluscs, whose intelligence is said to be comparable to that of rats. One of their special and often described abilities is the opening of screw caps, which becomes faster and faster the more often the animals perform this exercise. Their social behaviour is highly complex and largely based on individual learning. Since most octopus species are solitary creatures and die after reproduction they cannot establish parentchild relationships like birds or mammals to pass on the painstakingly learned skills to the next generation. Their offspring must always acquire everything anew themselves. Allegedly, the clever creatures can even distinguish human beings from each other and occasionally develop special sympathies for some. Most popular are those who regularly feed them which does not, however, stop an octopus from spraying a jet of water at a keeper out of disappointment if they come empty-handed.