Habitat ecology

Studies on habitat ecology of octopuses and their relatives are arranged by taxon. Enteroctopus is presented here, and studies on other species of OCTOPUS and SQUIDS can be found in other sections.

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Research study 1

graph of den volume vs. live mass for octopuses Octopus dolfleini in Clayoquot Sound, British Columbiaphotograph of octopus E. dolfeini courtesy Shawn Robinson, Simon Fraser University, British ColumbiaDens of octopuses Enteroctopus dofleini in Clayoquot Sound, British Columbia are mostly located at 4-8m below chart datum. Removal of 46 octopuses from 36 dens results in the re-occupation of 6 dens within 42d.  Den sizes are measured and compared with sizes of occupants (see graph). The authors suggest that the relationship "is exponential with larger octopuses requiring proportionately more space", as opposed to what seems more obvious from a brief consideration of the data, and that is, a simple linear relationship.

Mean size of the 46 octopuses is 5.4kg.  Interestingly, the sex ratio of the octopuses is widely disparate: 30 females and 9 males (7 escaped before being sexed).  Hartwick et al. 1978 J Fish Res Bd Can 35: 1492. Photo of E. dolfeini courtesy Shawn Robinson, Simon Fraser University, British Columbia.

NOTE  a tide level set at the Lowest Astronomical Tide, or the level below which the tide seldom falls.  Theoretically, it should never fall below this level, but does so occasionally as a result of local meterological events.  Chart Datum is commonly used in Canada, Britain, and other countries.  It is lower (by up to 1.5m) than the comparable designation commonly used in the United States, namely, mean lower low water (MLLW).  MLLW is the average of the 2 low tides each day measured over a 19-yr period

NOTE  the authors may have meant to say "disproportionately more space", which would correspond to the notion of an exponential relationship, as eye-fitted to the data in the above graph. An exponential relationship implies that as an octopus grows larger, its need for space would, for some reason, increase out of proportion to its size. The greater liklihood is that, as an octopus increases in size, its need for space increases geometrically. Thus, a 2-unit size octopus would require twice the space of a 1-unit size one, and half that of a 4-unit size one. This is shown by the linear curve, also eye-fitted to the data in the above graph. Thus, a linear fit to the data suggests that space needs are proportional to body size, in this case, by an estimated factor of about 6. A test of isometry versus allometry can easily be done on the data, but both may be non-significant owing to the not unexpected high variability in the data

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Research study 2

histogram of number of octopuses occupying dens for various lengths of timeA year-long study of tagged octopuses Enteroctopus dofleini on the west coast of Vancouver Island, British Columbia provides information on duration of den occupation and some information on extents of immigration to, and emigration from the population.  In total, the researchers tag 151 individuals and recapture 74 at least once.  The sex ratio of these recaptured individuals is 3:1 in favour of females (55 females:19 males), about what the researchers would expect based on other data.  Thirty-six individuals are recorded as changing dens 1-2 times, and one octopus is noted as occupying 4 different dens.  Average distance moved in a den change is 13m.  Most octopuses (59%) spend a month or more in the same shelter (see histogram).  One advantage to being such home-bodies, as suggested by the authors, is that it allows locations of prey to be learned, thus enhancing foraging success.  Although tag losses could not be identified as either mortality or emigration, the authors present a picture of an octopus population moving around on a fairly large scale, interspersed with periods of residence in a relatively small area.  However, there is no evidence of either onshore or offshore migrations in these shallow-water populations.  Hartwick et al. 1984 Mar Behav Physiol 11: 95.

NOTE research visits are made every 2wk

Research study 3

graph of activity of octopuses Enteroctopus dolfleini at different times of day monitorred by sonic tagsUse of sonic “tags” for monitoring Enteroctopus dofleini in Saanich Inlet, British Columbia provides information on size of home ranges and daily activity of 4 individuals estimated to be in their second year of a 3-4yr life cycle.  The habitat is rocky with a maximum depth of 10m.  The octopuses inhabit individual dens and have home ranges of 215-250 m2.  According to the author's estimates, the ranges overlap considerably (6-60%) suggesting that Enteroctopus is not defensive of home-range area, but yet is not particularly social either. In fact, other studies cited in this section indicate that pairs of individuals may share common dens.  The home ranges are stable over 13-15d of observation (for 3 of the individuals).  Mather et al. 1985 Mar Behav Physiol 11: 301.

NOTE  the sonic units send a continuous signal for 1mo, weigh 18g underwater, and are attached to an upper arm of the octopus with hooks.  Monitoring is done every 30min over 281 hours (approx. 12d) using directional hydrophones and triangulation methodology.  Information on daily activity of these octopuses can be found elsewhere in this section of the ODYSSEY: PREDATORS & DEFENSES: HIDING AWAY and NOCTURNAL BEHAVIOUR

NOTE  the 4, comprising 2 males and 2 females, weigh between 4-9kg, and are monitored for periods of 15, 13, 12, and 4d, respectively.  The 4-d individual’s unit fell off and lodged in a crevice, but still provided useful information on the precision of the tracking, which was done from surface boats

Research study 4

As part of a study into the viability of Enteroctopus dofleini for commercial harvest, 2 populations are compared at Clayoquot Sound, British Columbia, one inshore at depths of 7-19m and the other offshore at depths of 37-110m.  Samples from each population are taken using both baited and unbaited graph showing size distributions of octopuses Enteroctopus dolfleini in inshore and offshore locations at Clayoquot Sound, British Columbiatraps.  Results show that individuals in the offshore population are more numerous and larger in size than individuals in the inshore population (see graph on Left). 

Another feature of interest is that individuals in the offshore population exhibit a much higher incidence of arm mutilation (57%) than ones in the inshore population (26%; see pie diagrams on Right). The authors credit this to greater abundance of predators in the offshore regions, including seals, sea lions, sea otters, dogfish sharks, and lingcods.  Recruitment at the 2 sites appears to extend throughout the year.  Both sites show a constant influx of new octopuses, with the greatest immigration occurring in early summer.  The authors record a considerable and unexplained bias towards females in the population, averaging about 77% at the 2 sites. Hartwick et al. 1988 Malacologia 29: 57.

Research study 4.1

histogram showing live mass of octopuses Enteroctopus dolfleini at different depths in AlaskaAn investigation of depth distributions and habitat preferences of giant octopus Enteroctopus dofleini in Prince William Sound, Alaska show that most individuals inhabit dens shallower than 10m, but some are as deep as 180m, and that commonest habitats are among boulders on non-rocky substrata.  In such areas dense kelp cover is common.  Scheel 2002 Mar Ecol 23: 185.

Research study 5

photograph of part of an octopus mantle with 2 types of tags insertedWant to tag an octopus for mark-recapture ecological study and don't know how?  Researchers at the University of Alaska review the efficacy of 2 types of tags, Peterson numbered disc-tags and visible-implant elastomer (VIE) tags.  The researchers tag 97 Enteroctopus dolfleini with both types of tag (69 individuals) or just VIE tags (28), and release them into Kachemak Bay, Alaska.  The photograph shows both types of tag on a single animal.  The elastomer method uses 4 injections of differently coloured liquid elastomers that harden within a few moments to pliable biocompatible solids (colours of red, orange, yellow, and green enable simple codes to be devised).  The Peterson tag involves fastening a numbered tag near the mantle opening with 2 plastic nuts and bolts.  Only 3 tagged individuals are recovered after about 8wk at sea.  However, as one had lost its Peterson tag (with considerable tissue damage at the tag site) and another with intact Peterson tag also showed tissue irritation and necrosis at the tag site, all had retained the VIE tag, so the authors conclude that visible-implant elastomer tags are the way to go.   Barry et al. 2011 Fisheries Res 109: 370.

NOTE  the authors list several tagging methods employed in the past, including external/internal implants (electronic, discs on plastic cable ties, spaghetti tags), and tattoo/branding (latex injection, visible-plastic elastomer).  Problems with the various methods include falling out, being ripped out by the octopus, and inducing significant wound-infection and mortality

Research study 6

histogram comparing average use area by octopuses Enteroctopus dofleini by sex and size in AlaskaFurther investigation by researchers from Pacific University, Anchorage, Alaska of habitat use by octopuses Eteroctopus dolfleini employs sonic transmitters to track movements of individuals for up to 20d.   Forty individuals are tagged and monitored at variable intervals over a 7yr period yielding 566 octopus-days of data.  Major findings are: 1) individuals are stationary (hiding) for 94% of the time), 2) individuals are active during the day, but somewhat more from 2400-0500h, 3) maximum movement following release is 4.8km (approximate 10wk monitoring period), and 4) maximimum and minimum sizes of areas traversed over excursion periods of 2-20d range from 4,300m2 for the smallest schematic showing tracks of 3 individuals recorded by sonic transmitterssized individuals (<2kg) to 50,000m2 for the largest sized ones (>10kg; measured as areas of polygons, see histogram).  These data suggest that for large individuals, at least, these Alaskan populations may not be so stay-at-home as population in southern British Columbias (see previous Research Studies).  A common pattern in daily behaviour is for den-switching to occur at night, feeding/territorial excursions to take place in early morning (0100-0400h), and return to the den at dawn.  Some of the tracking data suggests that an individual may strike off in a general direction to a certain “use” or activity area, but return to its den more directly following contours in the habitat (see map on Right, individual B).  In contrast, other individuals move more haphazardly (A and C).  In general, the preferred habitat type of these Alaskan octopuses is one with greater kelp cover.  The study is valuable in providing detailed  data on movements of octopuses in unconstrained environments over relatively long time-periods.  Scheel & Bisson 2012 J Exp Mar Biol Ecol 416-417: 21.

NOTE  these are attached to the mantle and have an average of 83% retention over periods ranging from 1-88d

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