Behavior and Interactions among Microorganisms and Invertebrates - Freshwater Ecology: Concepts and Environmental Applications - page 377

animals do not have the ability to digest cellulose directly and thus rely on
the bacteria and fungi growing on detritus for their nutrition.
Organisms that scrape biofilms can consume a wide variety of micro-
bial species. Scrapers include snails and some species of aquatic insect lar-
vae. Scrapers may not discriminate among individual cells because the
biofilms are composed of many microbial species. However, they may dis-
criminate on the basis of the dominant particles in the biofilm (i.e., some
biofilms may be more attractive than others) or the efficiency of digestion
of various types of particles.

Filter Feeders
Organisms such as Daphnia are able to filter particles from the medium
and maintain a continuous input of food (Fig. 18.10). As with protozoa, the
ingestion rate is a function of particle size and concentration. Rotifers (Fig.
18.11) and Daphnia have many feeding appendages that circulate and filter
water. These appendages capture particles and move them toward the mouth-
parts. Some of the particles are actually filtered, but many others may stick
to individual feeding appendages rather than become lodged between them
(i.e., impaction rather than filtration is used to capture some particles).
Larger Daphnia are more efficient feeders on larger particles than are
smaller Daphnia (Burns, 1969; Hall and Threlkeld, 1976). Thus, in the ab-
sence of fish, they can dominate planktonic communities. This feature is
essential to the trophic cascade in lakes described in Chapter 19. Other or-
ganisms, such as blackfly larvae, clams, mussels, and net spinning caddis
fly larvae, can feed on microbial particles.

Selectivity of Particle Feeders
There are many small particles in aquatic ecosystems, ranging from in-
organic such as clays to living organisms such as phytoplankton and bacte-

5 4

Filter rate
Ingestion rate

4

3

3

2

2

1

1

0 0

10 3 10 4 10 5 10 6

Algae (cells cm -3 )

FIGURE 18.10 Filtering and ingestion rates of algal cells (Chlamydamonas reinhardtii) by
Daphnia magna as a function of algal concentration. Although the filtering rate falls off
sharply with increased concentration, the ingestion rate increases up to a point and then be-
comes constant (reproduced with permission from Porter et al., 1982).