Free Octopus by Roland C. Anderson Page A

eaten by an octopus that had a given number of prey remains.
    â€”Jennifer 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 available—say, red rock crabs—where 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.
    Learning may account for the fact that the octopuses we studied in Bonaire were what we labeled specializing generalists. We were studying squid, but we couldn’t ignore the octopuses moving under us as we squid-watched. So we sampled the remains of what they ate, and over three years there were seventy-five different crab, clam, and snail species. But some of the octopuses specialized, often narrowly. One ate immature queen conch snails (Strombus gigas), and another dug the fragile pen shells (Pinna carnea)out of the sandy mud. Habitat couldn’t account for these differences, since they were all living in a mixed rock-rubble-sand-mud habitat. Maybe each octopus learned a special foraging technique that fit particular species. It’s ironic, though, that the species is generalist but some individuals are specialists.
    Besides learning where to find prey, octopus choices of areas to hunt may be restricted by mammals in the area, like sea otters, both as predators and as competitors for the same food species. We wondered whether a predator would learn where to find common octopuses. The octopuses we watched in Bermuda had very small home ranges but shifted them on average every ten days, maybe because they had used up all the easily accessible food, or perhaps because they were working out a balance of energy spent in hunting and energy gained by eating. We wondered whether they shifted home ranges because ten days is about right for a predator fish to catch on to where an octopus lived. And in fact, this speculation has foundation. As we got to know the area as the local fish might, we learned to gather clues to an octopus’s possible den site. A fish might find an octopus in this way, then grab it when it’s out hunting. The octopuses didn’t spend much of their time carefully balancing energy tradeoff and