Octopus (10 page)

We found that the giant Pacific octopus uses another trick to get at its food. Pulling apart mussels and Manila clams (Venerupis philippinarium) is easy. But Pacific littleneck clams (Protothaca staminea) have a thicker shell and stronger muscles to keep their valves together. Roland tested clam strength on a “clam rack.” He glued one valve to a plate at one end and a second one to a hook attached to a strain gauge and a screw mechanism. He put more and more pressure, measured by the gauge, into pulling the valves apart, and stopped when the clam just gaped, giving a good strength reading. Pacific littleneck clams were indeed stronger than mussels or Manila clams, and presumably because of this fact the octopuses likely tried pulling and failed, then drilled, though they seldom drilled either of the other species. But they also chipped with their beak at the edge of the valve, again not breaking much shell off but enough to make a hole that presumably would let in venom. And these chips, like by the drilling of the red-spot night octopus, were at the anterior and posterior shell edges as near to the adductor muscles as the octopus could get.

We also tested whether the clam's resistance to the pull would affect what the octopus chose for food. Steer and Semmens had looked at octopus size selection of clams within a single species and had found, as we did, that large octopuses took a wide variety of clam sizes. Our octopuses also ate more mussels and Manila clams than they did the strong Pacific littleneck clams. We wondered what would happen if we took away the problem posed by the shell and opened the shells for them. When we delivered these clams on the half shell to the octopuses, they changed their choices, eating many more of the Pacific littleneck clams and nearly none of the mussels, leading us to suspect that mussels don't taste that good to octopuses but they are easier to open. And Pacific littleneck clams aren't necessarily worth all the trouble, but they certainly taste good if someone else does the work.

Once the octopus eats its food, it continues the process of digestion. Some digestion happens as saliva eats away at the muscles before the meat is taken in. Food that's already begun to be broken down is passed into the gut, where digestive enzymes break it down even more. Over a lifetime, the octopus eats and absorbs more and more food, first to build muscles and later to build eggs or sperm. Since the octopus can't digest fats, they are passed out of the intestine in a string of feces and blown forcefully out the funnel. These fatty substances are snapped up by scavengers like
wrasse, because fish can digest and use them. Octopuses have a high conversion efficiency of food to energy, meaning that they are good at laying down body tissue.

We suspect that when an octopus experiences a food shortage, it may just move on, catch all the easily available prey in one range, and then try another range. If true, this would help explain why octopuses don't hold and defend territories. Behavior ecology theory says that an animal only defends an area that has resources it will keep on using. If an octopus samples an area for easily caught prey and then moves on, that area would not be worth defending.

If food gets really scarce, the common octopus may have a very different strategy for coping. It will stop foraging, and if it's a female, it may even develop and lay eggs earlier than normal. This may be because of this species' cold-blooded metabolism and short lifespan. An octopus uses energy mainly for eating and digestion. If the available food isn't enough to keep it going, it might as well get on with its one-time reproduction; it's better to have some offspring than none at all. This set of food-gathering strategies works for the octopus.

4

In the Den

W
e can think of an octopus home much as we think of our houses. The octopus uses its den for rest, safety, feeding, and a place to shelter young—eggs in the case of octopuses. We have known for millennia that octopuses take refuge in dens, and fishermen have often taken advantage of that habit. Egyptians fished for octopuses 4000 years ago using clay pots for artificial dens. They lowered the jars on strings, then drew them up after a period of time to harvest the octopuses resting in them. Hawaiians dangled a cowry shell just outside an octopus in a den and speared the animal when it came out to the lure.

Most shallow-water octopuses take refuge in a den for long periods of each day—and about 70 percent of their time—over most of their lives. The den can be as simple as a burrow in the mud, a hole in or under a rock, an empty shell, or even a beer bottle. Some octopuses inhabit a previously existing den, but they will meticulously clean it, if necessary. Often the octopus will partly reconstruct the den or will elaborately clean out and modify an existing crevice.

Octopuses may have started taking up residence in dens about 65 million years ago, when they lost their shells. Before that, ancient cephalopods were protected by either an exterior coiled shell or an interior structural shell. Their shells protected them from predators, so they didn't need dens.

An important function of an octopus den is to provide a place for egg guarding. Many female octopuses lay their eggs in a den and frequently block up the entrance to the den, remaining there while they are guarding the eggs. They select rocks or shells small enough to keep other octopuses and possible predators out but large enough to allow good water circulation between them. Female octopuses are very flexible in their choice of den location and construction, possibly because of their adaptability. For instance, in the lab, the Caribbean pygmy octopus prefers to lay eggs in hard shells with small entrances, such as snail shells. But on the sand flats
of St. Joe Bay in the Florida panhandle, it settles for cockleshells since there's competition with hermit crabs and blenny fish for snail shells.

We used to think that shallow-water octopuses made their dens in rocky areas or on coral reefs—places with hard substrate of some sort. We now know that a number of octopus species live on sand and mud and make their burrows there, such as the mimic octopus (Thaumoctopus mimicus) and the wunderpus of Indonesia (see plate 12). They may even consolidate the sand of the burrows' sides with mucus that they produce.

Many shallow-water octopuses choose dens with hard walls: a crevice in a rock wall, under a boulder, a hole in a coral head or in a rock, a clamshell, or snail shell. So they generally live in areas where rocks, coral, or shells are available (see plate 13). In the Puget Sound area of Washington, giant Pacific octopuses live where there are rock outcrops or ledges or large underwater boulders they can hide beneath known as “monument rocks” that were left behind by glaciers. These octopuses' dens are in areas with fast tidal currents, which sweep away sediments that expose the rocks, carry away food and animal wastes, and bring in a good supply of oxygenated water. The presence of oxygenated water also means that there is a good supply of prey to find on foraging expeditions.

It is difficult to say what makes a good octopus den: there are so many octopus species in so many habitats, and they are so flexible in their behaviors. But most octopus dens have a few basic characteristics: dens are relatively small, and they have a maximum volume two to three times that of the octopus. Giant Pacific octopuses, for example, live in dens of different sizes as they mature: small ones live in small dens and large ones live in large dens. Keep in mind that it's difficult to measure an octopus's den: usually we can only make a rough assessment of the volume of a structure because the dens are hidden away. And the octopus must be out of the den in order for us to be able to measure it.

Shallow-water octopuses choose dens small enough so that they can keep in contact with the walls with their suckered arms. The Caribbean pygmy octopus sits sideways in its chosen cockleshell and holds the valves of the shell closed by pulling on each side. Small giant Pacific octopuses sit upright in their adopted shells, holding onto the shell above and below them. Either way, they keep the shell closed with their strong arms. This contact with several hard surfaces is also seen in laboratories and public aquariums. In the lab, octopuses placed in a bare tank will settle into a corner, either an upper corner where they have contact with two walls and the surface of the water, which acts as a third surface, or a lower corner where they are in contact with two walls and the bottom.

In the wild, octopuses make their dens in a variety of places. Dens have been found in rock crevices or holes in rocks, shells, kelp holdfasts, and even concrete blocks used as mooring anchors for small boats. But small octopuses and juveniles of larger species of shallow-water octopuses often live in empty mollusk shells. Many shell collectors have had to evict an octopus before taking home a prize shell found when they were diving, and shell collectors use the octopus's den middens as sources for shells. Jennifer has found that the Caribbean pygmy octopus in northern Florida much prefers to live in snail shells than bivalve shells, but she found that in the sandy bays where there were few snail shells available in the turtle grass beds, they made do with clamshells.

Octopuses seem to prefer a den with a small opening. Because they have no fixed skeleton, they can squeeze through a narrow opening to access
a wider den within. We have often seen giant Pacific octopuses inside a narrow vertical crack in a rock wall. Caribbean pygmy octopuses will choose brown beer bottles with the tapered neck as dens, and small juvenile octopuses sit in the neck, keeping in contact with a hard surface with all arms.

In Hawaii, the day octopus may make a den in coral rubble with an upward-facing opening from which it will peer out, or it may close the opening with coral rocks when it wants protection while it takes a nap. The common octopus also will make a home in coral rocks. These dens and those of most other octopus species frequently face outward to deeper water, maybe to keep an eye out for predators or potential prey. We sometimes saw a Hawaiian day octopus in a den do vertical head bob movements, using the change in viewpoint to estimate the distance to a faraway object or animal, and then jet directly to and capture a crab 10 ft. (3 m) or more away. The octopus seemed to have been on watch for such an opportunity.

Sometimes octopuses make their own dens where natural dens are limited. Like the Hawaiian day octopus, they may dig a den into rubble or under a coral rock or gather rocks together. A giant Pacific octopus making a den under a large boulder will construct it by blowing and carrying out sand and gravel from underneath the boulder. We observed this same den-making behavior in a Hawaiian day octopus that we were keeping in an outdoor pond on Coconut Island in Kaneohe Bay, the research station of the University of Hawaii; it built a den under a cement block. Octopuses can blow sand and gravel out of their den with jets of water from their funnel, or they tuck material in the webbing between their arms and carry it to the entrance. We found that the day octopuses in Hawaii and the common octopuses in Bermuda were good housekeepers; they spent time cleaning out their dens each day. When a den began to look messy, we suspected that its occupant had moved on. When an octopus excavates its own den, it can make it the exact size, shape, and volume it prefers. Some octopuses, like the giant Pacific octopus, build several entrances to their dens, while pygmy octopuses' dens have only one entrance.

Compared with the giant Pacific octopus, smaller octopus species spend shorter amounts of time in any one den. Jennifer found that the common octopus in Bermuda stayed in a den an average of ten days, with a range from one day to one month. One octopus switched between two dens daily. The octopus that stayed longest in its den lived under a fishing dock where crabs came from afar to eat fish remains, so the octopus probably stayed because of the good food supply. The Caribbean reef octopus (Octopus briareus) also only spends days in a den. Brief occupancy in a den makes ecological sense, since the octopuses are likely to eat all the crabs, clams, or snails in their small home range if they spend too much time in a single den. They eat up all the easy-to-catch prey, and then they move on.