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Giant Octopuses: Natural history & Ecology

A general discussion of the life history of the Giant Octopus (giant North Pacific octopus, now Enteroctopus dofleini (Hochberg 1998), previously most often known as Octopus dofleini or Paroctopus dofleini), from hatching to adulthood, based on reviewing literature available in early 1996. Details of life history were drawn largely from Hartwick (1983) and Paust (1988), unless otherwise indicated.

Life history

Spawning may occur throughout the year, although there is some evidence that a spawning peak occurs in winter in the northeastern Pacific and Paust (1988) states that the peak period for egg laying in Alaska is April-May. Evidence from Japan also suggests an additional peak in May through July. Spawning takes place in relatively shallow water (13-30 m). Females lay eggs on the inner side of a rocky den and may lay 20,000 to 100,000 eggs over a period of several days. Eggs are tended, cleaned and aerated by females until they hatch. Incubation may take 150 days to seven or more months, requiring 2300-2600 degree days.

Upon hatching, newly emerged larvae immediately swim toward the surface and begin a planktonic phase, estimated to last from 28-90 days. Planktonic E. dofleini have been reported along the Aleutian Islands (Kubodera 1991). Sampling extended from mid-April to mid-September, but Kubodera reports that E. dofleini were only recorded from June through August. Assuming these juveniles were 4-6 weeks old, they would have hatched mid-April (for June juveniles) to mid-June (for August juveniles). Based on an average temperature for waters off the Aleutian Islands, Kubodera (1991) estimates that spawning juveniles must have occurred in winter one and a half years earlier. Little is known about juvenile mortality in the planktonic stage, although post-hatching survival to 10 mm total length has been estimated at 1%.

Little is known about early benthic life of octopus, but benthic E. dofleini between 3-5 g (50 mm total length) have been reported. E. dofleini grow quickly, and may reach 0.5-1 kg in weight by one year, and 10 kg after 3 years. Predation is believed to be a major source of mortality for small to medium benthic octopus, while predation on very large individuals may be much rarer. Non-fatal predation is also reported from direct observations and the high frequency of mutilations or missing arms. Hartwick, et.al (1988) estimate the population frequency of mutilations or missing limbs as high as 25-60%, and note that octopus in deeper water show a much higher incidence of such injuries.

Gonads mature at a weight of approximately 15kg, with males maturing earlier (at a smaller weight) than females. The third right arm of the male octopus is modified with a long hectocotylus that may be one fifth the length of the arm. Mating apparently peaks in the fall, and divers from the northeast Pacific report seeing pairs and note an increase in aggression toward divers at this time. Mating may occur at depths of 20 to 100m, and take several hours. Males may mate with more than one female and females receive fairly large spermatophores (up to 1m long) during mating. Females appear to prefer larger males as mates. Eggs are laid some time after mating (one report of 42 days in captivity in Hartwick, 1983).

E. dofleini females die before or shortly after eggs have hatched. Males may live a few months after mating. Maximum life span in the absence of reproduction is believed to be five years.

Ecology

Dens are an important resource to octopus during all benthic life stages. They are used both as brooding chambers and as refuges from predators, including other octopuses, various fishes, sea otters, seals, and sea lions. Most dens are naturally occurring spaces under rocks or in crevices. Smaller octopus will often excavate a suitable cavity in sand or gravel (Hartwick, et al., 1978), and are more likely to use artificial lairs than are adults. Octopus will also occupy man-made objects as dens, and this behavior supports a viable fishery for O. vulgaris off Japan. Octopus are large and very mobile predators and return to their dens after foraging excursions. A den may be utilized by the same individual for up to several weeks. Octopus do not appear to be territorial nor to have an established dominance hierarchy. Smaller individuals generally retreat when confronted by larger ones.

Octopus utilize a wide variety of prey species, and estimates of diet breadth seem to increase with the length of the study (Mather & O'Dor, 1991). They consume mostly crustaceans and molluscs, and apparently favor crab and shrimp. Bivalves, snails, fish and other octopus are also eaten. Octopus forage largely at night, and frequently return to a den to feed. Shell and other fragments are discarded in middens at the mouth of the den. For O. vulgaris in Bermuda, prey middens represent a biased sample of octopus diet, because prey species differ in the length of time their discarded remains were retained in a midden (Mather 1991).

Little is known about population variation in E. dofleini, although it seems generally accepted anecdotally that octopus populations are highly variable. Nevertheless, published data supporting this assertion are scarce. The perception that populations fluctuate may also reflect an onshore-offshore seasonal migration (Hartwick, et.al, 1978), an annual cycle in apparent abundance as mature individuals die following reproduction (Hartwick, et al.. 1978; for O. briareus , Aronson, 1986), or the fact that octopus survey methods can produce variable results depending on local conditions (e.g. success of the pot fishery in Alaska, Paust, 1988). Some octopus populations are reported as relatively stable (Hartwick, 1983). This apparently occurs when populations are limited by den abundance, since the number of dens in an area is fixed. Populations limited by predation and recruitment may be more variable.

A seasonal onshore-offshore migration of E. dofleini seems well established, although its ecology and timing remain poorly described, particularly for the eastern Pacific. Migrations may occur as individuals move from shallow to deep water at about one year of age, and onshore movements of mature individuals may be related to mating and spawning needs. Paust (1988) states that octopus may move offshore in the fall and return onshore in spring. Hartwick, et.al (1978) provide data on inshore and offshore surveys that suggest offshore movements in November and December and onshore movements in April through June. An inshore mating season and an offshore migration both in the Fall suggest that spawning in September to October may occur before offshore migration in November and December.

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References


Alaska Pacific University Science Center Home Octopus
2000-2001 David Scheel (APU), 1995-1999 David Scheel (PWSSC)