Giant Pacific octopus

Name and taxonomy

Gerhard Wülker of Leipzig University described the species in 1910, in a work on Japanese cephalopods. He chose the name dofleini to honour the German scientist Franz Theodor Doflein. Wülker noted a great deal of variation within the species, a point that later DNA work would return to. Eric Hochberg moved the species into the genus Enteroctopus in 1998. GBIF now gives the family as Enteroctopodidae, while Wikidata still places the genus in Octopodidae.

The species is a cephalopod, a class of molluscs that also holds the squids, cuttlefish and nautiluses. Within the octopuses it belongs to the suborder Incirrata, the octopuses without fins or cirri on the arms. Other English names are the North Pacific giant octopus and the giant octopus, and divers often say GPO. In Japanese it is mizudako, and in Korean it is muneo.

Evolution and relatives

The genus Enteroctopus holds the giant octopuses, and E. dofleini is the biggest of them. Like other coleoid cephalopods, it descends from shelled ancestors, and it has lost the shell entirely. DNA analysis suggests that the species may in fact be three subspecies: one in Japan, one in Alaska and one in Puget Sound. Genetic techniques are now used to trace the evolutionary past of the species.

One study built a DNA test to tell this species from two other octopuses that are sold as food. Its large spermatophores, more than 1 metre long, are typical of the genus and probably mark a shared ancestry. Like other octopuses, it has a large, folded brain, which is thought to have evolved separately from the vertebrate brain.

Appearance

An adult usually weighs about 15 kilograms, and the arm span reaches 4.3 metres. The record specimen weighed 270 kg, with a radial span of 9.8 metres. ADW gives a typical mantle length of 50 to 60 centimetres. The skin is usually reddish, but the animal changes colour and texture in an instant to match the rock or the kelp around it. The body is soft and has no bones, so an octopus can push through any gap larger than its beak, down to 50 millimetres.

The sack-shaped mantle holds the brain, the gut, the reproductive organs and two eyes with very sharp vision. Eight arms extend from the mantle, and each carries up to 280 suckers. The largest suckers measure about 6.4 centimetres across, and each can hold 16 kg. A hard beak of chitin sits at the centre of the arms. The male's third arm ends in a hectocotylus, a section without suckers that transfers sperm.

Habitat and range

The species lives in the coastal North Pacific. On the American side it ranges from Baja California in Mexico north through California, Oregon, Washington and British Columbia to Alaska and the Aleutian Islands. On the Asian side it lives off Kamchatka, in the Sea of Okhotsk, around Japan and Korea, and south to the East China Sea. It occurs from tide pools down to 2,000 metres deep, though ADW puts most animals above 110 metres.

The octopus does best in cold water rich in oxygen, at about 8 to 12 degrees Celsius. Its blue blood carries oxygen with copper rather than iron, and it carries oxygen poorly, so warm water is a problem. The ideal sea floor is mud, sand or gravel with large boulders for dens, close to dense kelp. Dens can be caves, holes dug under rocks, or rubbish such as bottles, tyres, pipes and barrels.

The home range is small, under 5 square kilometres. GBIF holds records from 23 countries, but that count includes aquariums and misidentified animals, and it is not the native range. iNaturalist holds 2,839 observations, most of them from divers in the United States and Canada.

Diet and hunting

The giant Pacific octopus eats shrimp, crabs, clams, cockles, scallops, abalone, snails, lobsters, fish, squid and other octopuses. It is a generalist, and the shells piled outside a den show that clams, crabs, fish and squid make up most meals. In captivity it has caught spiny dogfish up to 1.2 metres long, and dogfish remains turn up in wild middens too. In 2012 a photographer recorded a wild octopus seizing and drowning a seagull.

It hunts by sight, and it stalks, chases or waits in camouflage to ambush prey. It grips prey with its suckers and then bites with the beak. To open a shell it pulls the halves apart, crushes them, or drills a hole and injects toxic saliva. It carries food back to the den to eat in safety, and it dumps the remains at the entrance. Scientists and divers use this den litter to find an octopus.

Behaviour and social life

The species is solitary, and each adult keeps its own den. An octopus stays hidden or still 94% of the time, in a den, in kelp or under camouflage. It is active through the day but most active from midnight to early morning. It holds one den for at least a month and often longer, and it returns after short trips away. When it does move house, the new den is usually close, on average about 13.2 metres away.

The octopus crawls on its arms over the sea floor. In open water it swims by jet propulsion, drawing water into the mantle and forcing it out through the siphon. Large animals travel farther than small ones, and they choose rocky ground with dense kelp. Off Japan the population moves inshore in early summer and winter and offshore in late summer and winter. The Alaskan and north-east Pacific populations show no such migration.

In aquariums it opens childproof bottles, solves simple puzzles and uses tools. It also opens tank valves and takes equipment apart, and keepers report distinct personalities from playful to destructive. It is timid with people, and it rarely acts aggressively unless challenged.

Communication and senses

The eyes give very sharp vision, and the octopus hunts and navigates by sight. It finds its way by landmarks such as cliff edges and the shape of the sea floor. Each sucker holds thousands of chemical receptors, so the arms taste and touch at the same time. The brain has folded lobes with separate centres for visual and tactile memory, and each arm carries its own cluster of nerves.

In aquariums it recognises the people it meets often, and it jets water or changes texture at particular individuals. ADW reports that it is the only invertebrate known to learn by watching. Like other octopuses, it signals mood through colour and skin texture, and a pale animal is probably calm while a dark one is alarmed. Ink is a last resort: the octopus fights with its arms first, then jets away and leaves an ink cloud as a screen.

Reproduction and life cycle

The octopus matures at one to two years, and it breeds only once. Breeding happens all year, with a peak in winter. The male uses his hectocotylus to place two spermatophores, each about 1 metre long, inside the female's mantle. Mating takes 2 to 3 hours. The female can store the sperm for months, and one female in Seattle kept it for seven months before laying. Egg clutches show that both sexes mate with several partners.

Over several days the female lays 20,000 to 100,000 eggs, though Wikipedia cites figures up to 400,000. Each egg is shaped like a grain of rice, and she hangs them from the den roof in clusters of 200 to 300. She blows water over the eggs, cleans off algae and never leaves to feed. Hatching takes about 6 months in most water, and from 150 days to almost a year depending on temperature. The female starves during the brooding and dies soon after the young hatch. The male dies within three months of breeding.

Hatchlings are about 9.5 to 10.1 millimetres long, and they swim up to live as plankton for 1 to 3 months. Then they settle on the sea floor and grow fast, from 0.03 grams to 20 to 40 kg as adults. The whole life lasts 4.5 to 5 years, in the wild and in aquariums alike. After breeding, both sexes enter senescence: they stop eating, wander, and develop white lesions before they die.

Predators and threats

Harbour seals, sea otters and sperm whales all eat the giant Pacific octopus, and Pacific sleeper sharks take it too. Scavengers try to eat the eggs even while the female guards them. The paralarvae are eaten by birds, fish and other plankton feeders. Adults have few predators apart from people.

Fishing bycatch is the biggest threat named in our fact file. Warmer water speeds up egg development and can shorten the lifespan by up to 20%. Each autumn in Hood Canal, Washington, dying plankton use up the oxygen and create a dead zone. Oxygen there falls to 2 parts per million, against a normal 7 to 9 ppm. Ocean acidification cuts the clams, crabs and scallops that the octopus eats. A small fall in pH also reduces how much oxygen its copper-based blood can carry. Researchers have found heavy metals and PCBs in its tissues, probably from red rock crabs that live in polluted sediment.

Population and conservation

The IUCN lists the species as Least Concern, and it is not on any CITES appendix. NatureServe rates it N4, apparently secure, in Canada. Nobody has counted the population, because the animals are solitary and hard to find. Scientists use catch numbers as a proxy, but mislabelling in the seafood trade makes even that unreliable.

In Puget Sound a legal catch caused a public outcry, and Washington then protected the octopus at seven sites. The Puget Sound population is not considered threatened. Seafood Watch rates Alaskan giant Pacific octopus as a Best Choice. Some research suggests that fishing of other species has helped the octopus, by removing its predators and competitors.

Relationship with humans

People have caught octopuses for thousands of years with lures, spears, pots, nets and bare hands. Octopus, squid and cuttlefish together supply about 3.3 million tonnes of food a year, worth about 6 billion dollars. This species is fished commercially in parts of the Pacific and eaten in several countries. In the 1950s and 1960s fishermen used it as bait for Pacific halibut, but that practice has faded.

The species is a favourite in public aquariums because of its size and its behaviour. It is also a subject of research into intelligence and learning. The species is wild, and it is harmless to people: ADW records no adverse effect on humans. Divers describe it as shy, and it will usually retreat into its den rather than approach.

In culture and myth

The giant Pacific octopus is one source of the Norse kraken, the sea monster that dragged ships under. In Ainu tradition the Akkorokamui, a huge red octopus, lives in Funka Bay in Hokkaido. Hawaiian tradition links the sea god Kanaloa with the octopus. Hokusai's print of a fisherman's wife with two octopuses is the most famous image of the animal in Japanese art.

Today the octopus stands for intelligence, adaptability and mystery. Its escapes from aquarium tanks are widely reported, and they probably do more for its reputation than any myth. Nature films and books on animal minds often use this species, probably because it is large and easy to keep on display.