1.6.1 Net-Building Filter-Feeding Caddis Flies
In ecosystems that are made up of flowing water, organisms can minimize energy investment in capturing food by constructing nets. Filter feeding can yield low energy costs for food capture, but there are several physical and biological constraints. These constraints include hydraulic forces, variability in the physical conditions, abundance of drifting food, quality of drifting food, filter net design, general lack of net portability, and the ability and energy costs for an organism to maintain its position in a flow regime. Within these constraints are trade-offs when caddis flies exploit different habitats and types of food.
Often the most abundant and consistently available source of drifting food is in high flow-rate environments. The process of natural selection has produced several species of caddis flies that can utilize these environments. Adaptations include terminal hooks for securing the invertebrate to the substrate, silken safety cords to prevent drift when dislodged, and portable cases that act as ballasts in high water flow. Additional adaptations are found in net design because the fly must build a net that has a size, shape, and mesh size that functions most effectively at capturing the particle sizes available. As a result, filter feeding by caddis flies does not always yield high – energy returns. Net design is important in that hydraulics regulates the size of food that is transported in water column. As flow rate increases, the size of particles transported by water increases through increased kinetic energy. Larger food items are typically of higher quality (animal in origin) and are usually preferred by filterers, but the combination of large particles and high flow can destroy the net. Once energy has been invested in a net, the environmental conditions may leave a fly with a net that does not capture food effectively. This fly would then have to take an energetic loss and either abandon the habitat for better conditions, create a new net, or continue filtering with a net of marginal quality. Filter feeders have a large potential to minimize the cost of food capture, but the fly may not reach this potential in an environment that is temporally and spatially dynamic.
1.6.2 Filter-Feeding Mussels
Filter feeding using energy to pump water into and out of clams and mussels can also be a very profitable feeding strategy. To examine this, the basal metabolic rate of mussels can be estimated in the laboratory during a time when they are not feeding. Small plastic beads are then added to an aquarium container containing mussels; the mussels start to filter out the particles, increasing their oxygen consumption. Real food particles added to the chamber in place of plastic beads allow measurement of growth rate and food consumption. The quantity of food energy consumed per unit energy invested in feeding is then calculated by difference. The highest energy return in investment into feeding is estimated at about 500% (Table II).