Giant Octopus: Diet Choice & Prey Energetics

The Research Problem

We are interested in questions about octopus habitat use, their diet, and their predators. Our work has been supported because of its relevance to questions of management (Do octopus habitats need special protection in light of the Exxon Valdez oil spill in 1989?), and because of scientific interest in the life history of Giant Octopuses (Do shallow habitats play a unique role as nursery areas for juveniles?). Our current interests are in the roles predators play in promoting community diversity.

Predators can be characterized as either specialists or generalists. A specialist has specific behaviors or other adaptations that make it better at foraging on one type of prey, but at some cost of its ability on other prey types. A generalist simply captures and eats whatever prey is encountered, and is not adapted in particular ways to a subset of otherwise suitable prey. For almost all organisms, specialization is the rule, and few (if any) generalists are known. Octopuses have very broad diets, often including many species each of bivalves, crustaceans, gastropods, chitons, fish, and even birds! For this reason, it has been claimed that octopuses are generalist predators. Because a generalist predator typically takes the most common prey, they tend to increase the ability of prey species to coexist together. Whenever one prey species becomes more common and others relatively rare, a generalist predator ends up feeding on the more common species, where a specialist predator might continue to feed disproportionately on the rarer species.

We are therefore interested in how octopuses choose their diet. One very successful way of explaining diet choice in other animals has been to use foraging theory. Foraging theory (Stephens & Krebs 1986) supposes that an animal has been adapted to maximize some aspect of foraging performance, such as energy gained per unit of time foraging. If octopuses are energy rate maximizing foragers, than they are not generalists. To see if octopuses are maximizing energy gain in their diet, we need to know the energy content of their prey as well as the time it takes them to obtain that energy (handling time) and their encounter rates with that prey. We also need to know which prey are eaten in the wild, and which prey species (if any) are avoided.

Diet Choice by Octopuses

To find these answers, we've been working with students (at St. Lawrence University, 1997-1999, and Alaska Pacific Unviversity (2000-present) to determine selectivity in octopus diet and measure the energetic content of octopus food.

Data collected by Megan Reed, pictured at left (St. Lawrence University, class of '98) indicates that octopuses are selective in the prey they include in their diet. In the graph at right, the proportion of prey items found either in the intertidal environment (purple bars) or in octopus middens in the intertidal (blue bars) are plotted against one another. Higher occurence in middens than in the environment indicates preference for a prey; the reverse pattern indicates avoidence.

This is an interesting observation: why should octopuses prefer one species of small crab over another? Why not eat Hapalogaster mertensii, which was so abundant in the intertidal habitats where we find octopuses? One possibility is that octopuses may avoid crabs that are not good food, and prefer crabs that are. Good food may mean that the species is energy rich, or particularly easy to get into (octopuses must pull apart, or bite or drill through their hard-shelled prey to consume them).

Prey Energetics

Several students, including Kate Achilles and Laura Hostteter, pictured at left (St. Lawrence University, class of '99), are analyzing the energy content of important prey species using bomb calorimetry. We have not analyzed our results yet. However, the data collected should indicate whether octopuses are avoiding H. mertensii because these crabs are poor in energy. Because this species is a soft-shelled crab, we suspect that octopuses would not find the crabs difficult to open if they wanted to eat them. However, initial results indicate that the energy content of an item is primarily influence by body size and possibly water content. H. mertensii are soft (possibly high water content), and not terribly large. Still, they are ignored even relative to similar-sized crabs of presumably similar energy content.

After measuring prey energy content, the next step will be to examine handling time - that is, how much time or effort is involved in opening different species of prey. A shell of the clam Saxidomus giganteus found in an octopuses midden indicates the difficulty these animals can have getting at their prey. This work will be done with captive octopuses in the lab.

Finally, octopus foraging behavior and prey behavior may influence encounter rates. We are building Shadow to follow and film octopuses while foraging, so that we can measure actual search efficiency for octopuses on various prey.

Research described on this page was supported at St. Lawrence University by the Merck Research Program/AAAS, the PEW Faculty-Student Summer Research Program, and by St. Lawrence University; by the NOAA West Coast National Undersea Research Program; and at Alaska Pacific University by the At-Sea Processor's Association Pollock Conservation Consortium Fund.