A. Some generalities 1. Passive mechanisms a. Filter feeding b. Wait and lurch predation 2. Active mechanisms a. Filter feeding b. Predation c. Herbivory 3. Facultative versus obligative a. facultative implies useful if it happens, but not necessary for survival b. obligative implies that the organism must practice or use a particular function/food source B. An example of how a passive mode can be both facultative and obligative 1. Corals typically use the symbionts in tissues for much of their metabolic input of nutrients, but at the same time passsively feed on microcrustaceans that come in contact with the oral disk. This latter is in a sense a mode of filter feeding, somewhat implied by the circular nature of the oral disk. If they loose their symbionts (called bleaching, something which is happening in the Carribean at this moment in time), they rather than facultative lie in wait predators, they become obligate predators. C. Some ideas concerning active modes of foodgetting 1. Whole aspect can be thought of in terms of economics, i.e., the benefits gained from eating an article of food must be greater than the costs of finding and ingesting the item. If cost = benefit then there would be no growth and thus no reproduction. If cost > benefit, the organism essentially starves. 2. Costs include identification of prey material, searching for prey material, and handling prey material once found. 3. Benefit is the useful caloric value associated with the food item. 4. Identification of food item involves the concept of a search image, that is, a mental image in the predator of what the food item looks like. The predator then needs to know where in the environment that organism might be found. Then it needs to travel to that location to successfully obtain the object. 5. Handling involves immobilizing the prey item, discarding unuseful components and ingesting the rest. 6. Also aspects above involve a "learning" process on the part of the predator. At the same time, we might expect the prey to learn "avoidance" behaviors. D. Optimal Foraging Strategy 1. All the above are associated with an "optimal foraging strategy" on the part of the predator. The organism should take the food item that maximizes caloric intake (benefit) and minimizes search and handling time (costs). 2. This implies that an organism should stick with one species of prey, until the costs of taking that prey exceed the benefits. 3. The big question then becomes, "When should a predator switch?" This question is more complicated that perhaps one might think, as organisms may be available through a short search, but may not be nutritionally very good. 4. When a predator switches, it implies a new learning curve associated with the new prey species E. A specific example--the Green Shore Crab (Carcinus maenas) 1. shore crab typically eats mussels and worms in the intertidal zone. Small mussels have thin shells, but little meat, large mussels have thick shells, but lots of meat. Best returns come from middle-size mussels which have thicker shells than small mussels, but more meat than small mussels. 2. An examination of mussel size distribution compared to size eaten supports this contention, as when mid-size mussels are most common, they are most commonly eaten, with few individuals large than the median size taken. As the median size mussels decrease in frequency, the crab than takes either larger or smaller mussels, but of course has to take large numbers of smaller mussels to compensate for their lower caloric value and the increased handling time (even though shell is thinner, it has to handle more mussels to get the equivalent caloric value).