Octopus by Roland C. Anderson

almost all giant Pacific octopuses are coveredwith small hermit crabs, likely scavenging on leftovers from the octopuses’ food. And these crabs are free to forage in close proximity to the large octopuses, because they are too small for them to bother eating.
    A midden advertises the presence of an octopus den. Scuba divers have long used den middens to spot giant Pacific octopuses. Likewise a hungry fish or marine mammal may use a midden to detect its potential octopus prey. Dens are also evident by a pile of natural material, usually sand and gravel, excavated from inside the dens, causing the “volcano effect,” since it looks like the talus slope of a volcano emerging from the crater of a den. Den middens are a good source of information for scientists about the food that octopuses have eaten. Giant Pacific octopuses make particularly large middens in front of their dens, a treasure trove for studies of prey choice and food handling techniques.
    An octopus usually has a strong grasp on its shell home. Some years ago, while studying Kennerley’s venus clam (Humilaria kennerleyi), we collected some live specimens and took them back to the Seattle Aquarium. While they are normally found buried in shallow gravel and sand, many live clams were lying on the ocean’s bottom, presumably having been dug up by sea stars that were then unable to open this strong-muscled, thick-shelled clam. While diving, we judged whether a clam sitting on the gravel was alive by attempting to pull the valves apart. Confident that we had live clams, we transferred them to holding facilities of the aquarium. On examining the clamshells the next day, we were surprised to find that five of twelve were inhabited by octopuses rather than clams—four by red octopuses and one by a small giant Pacific octopus. They had been holding the shells tightly shut with their suckered arms and were almost as strong as the clams themselves.
    Den locations may be the result of many different activities, such as reproduction. One or several males may make dens close to a female, as if waiting for her to be ready to mate or looking for the opportunity to mate with her. In the case of a giant Pacific octopus, dens may be as close as 3 ft. (1 m) apart. When we watched Hawaiian day octopuses to assess their feeding in Coconut Island, we saw a male and female establish dens half a yard (half a meter) away from each other.
    Some species of octopuses bear planktonic paralarvae that do not have much of a choice where they make a den if water currents sweep them into poor areas—one reason that octopuses are so adaptable. They have to make the best of a bad situation when they settle to the bottom and there isno readymade den. If water currents carry an octopus paralarva out onto a sand flat and it has to settle out of the water column, it may be dangerously exposed. Although it can camouflage itself by matching the color and texture of the sand, it needs to find or make a den in which to rest and sleep, a place of protection where it can relax its camouflage, which takes muscle contraction. We have seen giant Pacific octopus dens that were created by carrying and blowing sand out from under a sunken log resting on the sand bottom or inside hollowed-out sunken pilings on the sea floor under the Seattle Aquarium. Jennifer has seen common octopuses in Bermuda living in pipes and tin cans.
    Lurking in Shipwrecks
    Octopuses even live in wrecked ships. If we believe what we see in Hollywood films, every sunken shipwreck has a giant octopus lurking in its murky cabins, perhaps even guarding a sunken treasure chest. Although few sunken ships in the North Pacific have treasure chests, many shipwrecks have one or more giant Pacific octopuses living in them. I write a shipwreck column for a Puget Sound–based dive magazine and have dived on many shipwrecks and seen many octopuses living on or under them. These giant Pacific octopuses can get as large as 400