Octopus (8 page)
Authors: Roland C. Anderson
Settlement of paralarval octopuses to the bottom is a major transition. They drop through the water to the bottom of the ocean, carried by currents, and arrive in new, strange areas. They have new adaptations in their bodies, to help them cope. While coming to new areas may be good for the species, for an individual it can be hazardous. The paralarvae may drop down into the ocean's depths where it is too deep, dark, and cold for near-shore species to survive. They may drop onto animals, such as sea anemones or corals, that eat them. They may drop into a school of fish that might eat them before they can get to a protective den. They may settle onto a coral reef, a boulder field, a bed of gravel, or a field of mud. Still, widespread species like the common octopus and the day octopus survive and reproduce in these new environments. They can camouflage on mud or sand as well as on diverse coral reefs. And they can find or make a den there.
When octopuses settle, they must be able to catch many types of prey
in the new environment. They may have to find crabs, clams, or snails hidden under rocks or buried in the sand. They may need to eat snails washed off rocks from the intertidal zone, or catch crabs that live only on wave-washed intertidal rocks. They must be able to subdue their prey and prepare them for feeding. But it's not quite so all-or-nothing. Jennifer found that the newly hatched Caribbean pygmy octopus swims at the surface and clings to the sides of aquarium tanks for several weeks, and pelagic paralarvae can delay for weeks before settling down for good. The juveniles' body shape, characteristics, innate behaviors, and learning ability are all matched to the place where they will live, up in the surface layers or down on the bottom.
Juvenile octopuses, those that have settled to the bottom or those that hatch out as big enough to live on the bottom right away, probably use several methods to determine whether a habitat is suitable for settling. They may use sight to look for suitable den sites and explore the substrate with their suckers, but settling octopuses are negatively phototactic (avoiding light), seeking dark places. They also must look for prey and watch for predators. The temperature, clarity, or salinity of the water may play a role in their choice, and octopuses can detect these characteristics of seawater. Paralarvae can delay settling to the bottom, but not for long, if they find that they are in a place that is not to their liking.
Aquarium workers such as Snyder raising giant Pacific octopuses found that the paralarvae alternated life styles before settling to the bottom for good, indicating the transition may take some time. The paralarvae still swam as they would in the plankton but gradually spent longer and longer periods attached to the walls of the tank or its bottom over several months. Snyder observed that settlement by one paralarva induced other paralarvae to settle nearby, which appeared to be a social influence on a nonsocial animal.
Martin Wells and Joyce Wells reported in 1970 an interesting settlement by the day octopus in Hawaii. Thirteen juvenile octopuses were found clinging to the underside of a buoy marking the end of a shark-fishing line. They knew these animals were newly landed because the line had been set for a short period. The buoy was set in 210 ft. (63 m) of water, making it highly unlikely that the little octopuses had crawled up from the bottom. Besides, they had a mantle length of just ½ in. (1.3 cm), the same size as the smallest day octopuses found on the bottom. And so we know that these octopuses can live in the plankton until they have reached ½ in. (1.3 cm)
mantle length and that they probably settle at about that size. We also know that they sometimes, perhaps often, make mistakes where they choose to settle. Natural selection will probably eliminate those that don't make good decisions.
The behavior of newly settled juvenile octopuses and those of newly hatched large-egged octopuses that don't drift in the plankton is remarkably consistent among the many species of octopuses. Most are nocturnal. They can change color and even produce skin display patterns. Juveniles of species that as adults have fake eyes on their skin, or ocelli, have them at the juvenile stage as well, and all can use the adult combination of walking and swimming. They hide in tiny shell or crevice dens or make their own. They catch prey, preferably crabs, but now adult prey rather than larvae. They have moved on in the life cycle.
3
Making a Living
F
or all animals, getting and eating food is a critical part of their life. Octopuses are generalists, so it's difficult to study this important behavior category in these animals. They are carnivores, and consume a wide range of animal species. They don't eat kelp or other algae or marine plants since they don't have the enzymes needed to digest them. Pretty much any marine animal on or near the bottom of the ocean is fair game. The prey can be bigger than the octopus, since the octopus's venom is deadly to most animals in the environment.
But octopuses do tend to eat more of specific groups. Researchers from all around the world have noted that octopuses eat a lot of crustaceans. The kind of crustacean varies with the octopus's size and location: pygmy octopuses in the shallow Caribbean Sea eat little hermit crabs, day octopuses of Hawaii catch small coral crabs, and giant Pacific octopuses on the west coast of North America consume Dungeness crabs. Octopuses also eat a lot of mollusks. For a group of common octopuses living near a reef full of mussels in South Africa, Malcolm Smale and Patricia Buchan (1981) found that the mussels were the most common prey. For a giant Pacific octopus on Vancouver Island, digging up clams was its favorite way to find food. For common octopuses in Bermuda, tiny file clams (Lima spp.) were a preferred snack.
The first step in an investigation of what octopuses eat is to find out what specific animals are being caught and eaten. There are three common techniques for attempting to get answers. First, in the lab, you can feed octopuses different prey species, and if they eat them, then you've established their choices, right? Not necessarily. If you were locked up and fed turnips and oatmeal, you'd eat them, but as soon as you got out, you'd probably pass them by in the supermarket. Also, you may love fresh figs but don't get to eat them often because they're not available. Octopuses may choose their foods in the same way. 55
Ambrose verified this contrast between accepting and choosing prey species with his two-spot octopuses in 1984. He compared what they ate by getting a list of their local snail and crab prey in the wild from their middens with what they accepted in the lab. After years of fieldwork, he established that at Bird Rock in the Catalina Islands, they eat mostly top snails (Tegula funebralis). But in the lab, they took crabs first. A comparison of choice and availability showed that these octopuses didn't find many preferred crabs and mollusks out in the ocean, and that probably there just weren't many crabs of the right size at Bird Rock.
A second way to find out what octopuses select to eat is to sample their middens. Most octopuses eat the soft parts of the prey and throw the shells or skeletons out in front of their den. The midden-sampling approach works relatively well for hard-shelled prey like clams and crabs but doesn't apply for soft-bodied animals like polychaete worms that have few remains. Also, shells dumped onto middens don't stay put. We watched wrasse fish eat the shell remains of crabs thrown out after a meal by the Hawaiian day octopus, and charted the removal of light pieces of crab shell by waves and currents for the common octopuses. Ambrose (1986) made artificial middens near his two-spot octopuses' dens, and watched hermit crabs take away the snail shells to use as homes. So visiting an octopus den once a week to assess intake by counting hard-part remains offers an incomplete picture of actual meals. The only way to know consumption from sampling leftovers is to know about the relationship between what the octopuses ate and what you found at their home within a couple of days.
Another way to find out what an animal has eaten is to kill it, open the stomach, and sample the contents of the gut; you can also examine the stomachs of already dead museum specimens. But this approach doesn't yield an accurate description of prey choices, because octopuses only eat the soft parts of prey: they scrape flesh away from the shell or skeleton, and they digest some of it before they eat it. What the researcher has to work with is often semidigested lumps of tissue. Biochemical serological analysis is needed to get a clear idea of what the food species is and where there was a different array of species than the midden samples of other octopuses. And you've killed the octopuses to find the answer to a behavioral question, which is a drastic solution, and you have only one sample.
The underlying problem with the question of prey choice is that octopuses eat a lot of different prey species. Ambrose (1984) found that his two-spot octopuses from Bird Rock ate fifty-five prey species over several years. We found twenty-eight prey species for twelve common octopuses in one small bay in Bermuda over four weeks, and we have collected seventy-five prey species so far from the common octopuses of Bonaire. In addition, what an octopus of one species takes as prey varies across the range of that octopus. Giant Pacific octopuses, for example, range from California to Alas ka in North America and over to Japan. In Japan, they eat fish, shrimp, and crabs; on Vancouver Island, they specialize in crabs, cockles, and Pacific littleneck clams (Protothaca staminea); in Alaska, they eat crabs 75 percent of the time. In each of these cases, there were remains of over twenty-five prey species in middens of a small group of octopuses. Clearly, octopuses like variety, and also sample a different variety in different areas.
Octopus Garbage Heaps
I conducted a study (1991a) of what common octopuses ate in Bermuda and what I found outside the dens a couple of days later. A team of volunteers followed two octopuses constantly from dawn to dusk for about ten days. Each day, we recorded where the animals went and what they ate outside their dens when they stopped to snack. We picked up the shell remains, and recorded what they took back to their dens. We then recorded the daily disappearance of prey remains from the den midden caused by sediment sifting over them or waves washing them away. After ten days, I dug through the midden to see what had stayed and what was buried. I remember coming by one den and seeing the eight bright blue shells from a chiton meal (a chiton has eight shell valves), then watching on subsequent days as the shells slipped down the midden slope and gradually got covered by debris.
From these observations, I made a flowchart of food fate, from capture to visible prey remains at the den. About one-third of prey was eaten in places other than the den, but there was no size selection. Both big and small prey were carried home, depending mostly on how far away the octopus was when it found the prey. Size mattered after pieces hit the midden, since small, light crab shell pieces were more likely to be washed away than heavy zebra mussel (
Arca zebra
) shells. Ultimately, I could backtrack from ten shells of six prey species in the midden visits by a researcher every five days to how many of each species were actually eaten. So I know how much food was probably eaten by an octopus that had a given number of prey remains.
âJennifer A. Mather
Foraging theory developed in 1986 by David Stephens and John Krebs sets forth that any animal must maximize its feeding efficiency: it has to get the most energy from its prey and spend the least energy finding, catching, and handling it. How do octopuses manage this tradeoff, and does this tell us why they eat what they do? The fact that they shelter in dens makes them “refuging” predators, moving out from central shelter to hunt and then returning to feed, leaving us the convenient midden heaps. Their energy is spent on search, capture, and handling of prey and then on digestion of food. Since a refuging predator spends a lot of time searching for food (see plate 10), taking whatever likely species it finds, this behavior partially accounts for the wide diet.
It's partially correct that octopuses simply take what's there. We noticed that different midden heaps in Bermuda were full of crab remains or shells of file clams. The number of remains of different prey species of the giant Pacific octopus varied with den location even at the same place, and an in-depth analysis compared abundance of prey species in den remains and around the home. In general, there were more individuals of one prey species availableâsay, red rock crabsâwhere their remains showed that the octopuses had been eating more crabs.
But availability isn't the only deciding factor. Octopuses learn well. It's possible that the accumulation of shells in front of the den resulted from the octopus having learned a particular hunting strategy and going to a particular area where it worked well. That possibility may have prevailed for the red octopuses we found in beer bottles. With the chance to live in a mud-bottom habitat because of these man-made shelters, they specialized in eating the abundant olive snails available there. Back on the rocks, they might be generalists in their eating habits, but they haven't been studied there. Learning and habit might override a take-it-if-you-find-it approach.
