Octopus (11 page)

Octopus dens attract other animals as well. Giant Pacific octopuses share their dens with several species of fish and invertebrates, such as sea stars, crabs, and scavenging snails. Jennifer found that the wrasse dubbed “Slippery Dicks” were den associates for the common octopus of Bermuda. The day octopuses of Hawaii also attracted wrasse to their food remains, and hermit crabs, perhaps to take the discarded shells of snail prey for new homes. The fish may have used octopus dens for protection against predators through the presence of the octopus, even though the octopus could also prey on them. At Cape Flattery, on Washington's northwestern coastal edge, the midden piles of almost all giant Pacific octopuses are covered
with 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 is
no 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.

Octopuses make their dens in many other human-donated items. Common octopuses of the Mediterranean Sea live inside ancient amphorae, sometimes all that is left of sunken Roman galleons on the sea floor. In the northeastern Pacific, the little red octopus may move into garbage on the sea floor. A female red octopus trawled up from the Friday Harbor area, in Washington's San Juan Islands, was living in an old shoe. This was a female with eggs, so she was quickly named Mother Hubbard by the college class that found her.

Roland once found a giant Pacific octopus living in and blocking the active outfall of a sewer pipe in Tacoma, and a retired professional diver reported seeing several giant Pacific octopuses inside the boat locks between Lake Washington and Puget Sound during routine cleanings of the locks. Octopuses are frequently caught in crab traps in the North Pacific and in lobster pots in other parts of the world. They don't intend to use the traps as dens, but on their foraging expeditions they frequently pause in a temporary minimal shelter to quickly eat whatever prey they've caught, rather than take it home to eat it. And so a crab or lobster trap can become a snacking stop, replete with a handy supply of food. Unfortunately for the octopus, the trap is sometimes lifted to the surface before the animal has finished its snack and moved on. The octopus may then be used as bait for future catches.

Our trash can be useful for supporting octopus populations. On a recent scuba dive in Puget Sound, Roland saw eight beer bottles littering the bottom, and each had a small red octopus in it. Since the beer bottles at that dive site had 100 percent occupancy, the animals were utilizing a new resource. There was very little else for them to hide in other than a few large snail shells that were already occupied by hermit crabs, so lack of suitable dens could have stopped them from living there. Our trash may be increasing the range of this octopus to include areas that have no natural den sites. Secure in its beer bottle home, the animal is temporarily safe from predators. Ironically, the species may lately be facing a housing shortage. Environmental societies in coastal areas are cleaning up trash from public beaches, and as part of these cleanups scuba divers are picking up underwater trash, including beer bottles and cans.

The phenomenon of octopuses living in our beer bottle trash has been useful to researchers. Janet Voight did population studies in 1988 on Diguet's pygmy octopus (Octopus digueti), which normally live in shells on intertidal sand flats in the northern Gulf of California. Strings of beer bottles
were set out at low tide and monitored for a year. The octopuses captured in this manner were not harmed as they are by other methods such as trawling, dredging, or noxious chemicals squirted into their dens. Dark beer bottles with narrow necks, tapering to an opening less than 1 in. (25 mm) wide, were used. About 14 percent of the bottles examined were occupied by the octopuses.

We have found that red octopuses in Puget Sound prefer stubby beer bottles to the long-necked variety and brown bottles rather than clear or green ones. Furthermore, they prefer “aged” bottles that are covered with a growth of barnacles or sea anemones. These characteristics make the containers darker inside, so perhaps the octopus feels safer than in transparent glass.

Dens aren't just places for octopuses to hide. Other animals are attracted to the site of the octopus's feeding activity, and some animals, such as juvenile parrotfish, aren't found in those sites because of habitat disruption by the digging octopus. Jennifer found that octopus dens in Hawaii attracted scavenging wrasse fish waiting for the scraps after the octopus had finished eating. And the dens also attracted hermit crabs, which need an empty snail shell in which to hide their soft abdomen, and what better place to find one than in the midden of a snail-eating octopus? The dug-up area with a diameter of 3 ft. (1 m) around the den will have less growth of algae, so herbivorous fish won't bother to hang around in that area. These observations remind us that octopuses are part of a complex web of underwater life. They are attracted to specific den sites because they need to avoid being eaten by some animals. And then when there, they change the area, and therefore have influence on other species.

5

Getting Around

O
ctopuses are a remarkable sight when they are out hunting. They seem to flow across the rocks, holding on with some of the eight arms and extending other arms into crevices and under rocks, occasionally lifting off the bottom and gliding about 3 ft. (1 m) or so. Anyone who has taken a long look at an octopus moving this way across the ocean floor or an aquarium tank has likely admired the animal's fluid movement and flexibility.

There are two kinds of octopus movement: one is about control and coordination—how an octopus gets all the arms to do what it wants them to do. The other is locomotion—how the octopus moves itself around in the environment. Regarding locomotion, the octopus looks as if it has a disadvantage because of its molluscan heritage. Snails and clams just don't move fast, and the molluscan foot, unlike the limbs of vertebrates and arthropods, isn't designed to produce speed of movement. Biologist Richard Mc-Neil Alexander, in his 2003 overview of movement styles among animals, points out that each species makes compromises in balancing the demands for speed and maneuverability with the need to limit energy output. These compromises that octopuses make, based in their phylogenetic history, are intriguing aspects of their means of getting around their environment.

One obvious aspect of locomotion is maximum speed, which varies depending on the medium you are moving through. Cephalopods don't have much speed, comparatively. The cheetah, for example, can manage a top speed of 90 ft. (27 m) per second, which is very fast. Still, the cheetah tires very quickly, and so a prey animal that dodges and runs from it may avoid capture. The pronghorn antelope of the North American prairies specializes in sustained speed and can keep a speed of over 50 ft. (15 m) per second for over ten minutes. Air speed can be faster than land speed since air is not dense, and birds manage sustained speeds near 60 ft. (18 m) per second. A diving peregrine falcon can achieve a gravity-assisted top speed of 170 ft. (50 m) per second, the fastest known. Sea animals are at a disadvantage
because of the high density of water: dolphins manage 30 ft. (9 m) per second, and pike can accelerate to 12 ft. (4 m) in less than a second. Squid, the fastest cephalopod, can only move 8 ft. (2.4 m) per second, and octopuses 1 ft. (
m) per second. Clearly, cephalopods can't always out-swim fish.