Behavior and Interactions among Microorganisms and Invertebrates - Freshwater Ecology: Concepts and Environmental Applications - page 379

assemblage, may waste their competitive edge by putting energy into chem-
ical defense. If predators do not eat selectively (as is likely with a more di-
verse prey assemblage and if predators are very large and individual prey
very small), the defended cells will be eaten anyway and will grow slower
than those that put energy into growth and reproduction rather than
defense.
A unicellular algal bloom provides a situation in which production of
toxins to deter grazers may be advantageous for planktonic organisms
eaten by filter feeders. This is probably why planktonic cyanobacteria pro-
duce broad-spectrum toxins that can harm or even kill grazing zooplank-
ton (Hietala et al., 1995; Ward and Codd, 1999). Given the complete dom-
inance of cyanobacteria in many eutrophic habitats, the energy used on
chemical defense from predation is not wasted.
Chemical defenses may be useful deterrents to planktonic cells that are
eaten by protozoa. Investments in chemical defenses confer an evolution-
ary advantage in this case because predators select individual cells. For ex-
ample, one unicellular marine alga has a concentrated compound (dimethyl
sulphoniopropionate) that is converted to dimethyl sulfide and toxic acry-
late when the cell is grazed (Wolfe et al., 1997). Presumably, such defenses
also occur in freshwater algae.
Spines and large size are defenses because predators cannot ingest the
cells efficiently. If the epilimnetic waters of a mesotrophic lake are exam-
ined in midsummer, the majority of the phytoplankton is composed of
large cells or aggregates that are difficult for zooplankton grazers to ingest.
Such mechanical defenses are less useful in benthic habitats, where grazers
such as snails, insect larvae, crayfish, and fish are large enough to be un-
affected by microscopic spines or large colony size.
Chemical cues can lead to development of mechanical defense in the
green alga Scenedesmus (Lüring, 1998; von Elert and Franck, 1999). When
Scenedesmus actus was grown in water filtered from a Daphnia culture,
colonies of four to eight cells formed rapidly. Solitary algal cells were pro-
duced when the alga was grown without exposure to Daphnia water. This
defense against Daphnia grazing by formation of large colonies occured
only when the predator was present. Larger colonies settle more quickly,
so there is an evolutionary cost associated with larger colonies that is off-
set by resistance to predation (Lürling and Van Donk, 2000).
Parasitism
Parasites or diseases afflict all organisms. Naturalists often treat these
opportunistic diseases and infections as oddities. It may be difficult to
study parasitism because infections can be sporadic and unpredictable. Dis-
eases of fishes have attracted attention (Table 18.2), particularly those as-
sociated with aquaria and fish culture (Untergasser, 1989). Human para-
sites with intermediate aquatic hosts have also been studied extensively.
Schistosomiasis is one of the worst human diseases with an aquatic or-
ganism serving as an intermediate host. About 200 million people world-
wide have this infection, with about 10% having severe clinical disease
(World Health Organization, 1993). Five species of Schistosoma trema-
todes can cause human disease, and each requires a different species of