Animals - Freshwater Ecology: Concepts and Environmental Applications - page 158

Rotifers can serve as a food source for zooplanktivores, but they are
generally too small for planktivorous fish to capture. Some species produce
long spines in response to predation risk; consequently, body form can vary
considerably (Fig. 9.4). Other species have adaptive behavioral responses
to avoid predators such as making rapid jumps when contacted.
Phylum Nematoda
Nematodes or roundworms are found in freshwaters, soils, and marine
habitats. Probably because of their small size and complex taxonomy, they
have not received much attention in freshwater habitats. Two thousand
freshwater species have been reported (Pennak, 1978). Nematodes are usu-
ally benthic but can be found in most aquatic habitats, including those as
extreme as hot springs and snowmelt pools (Poinar, 1991). The nematodes
often reach densities of 1 million/m 2 .
The nematodes are nonsegmented, worm-like, cylindrical in cross sec-
tion, and possess a complete alimentary tract and a body cavity (Fig.
9.3H). They also have a well-developed nervous system, excretory system,
and musculature. Most freshwater nematodes are less than 1 cm in length.
Reproduction is parthenogenic in some species, and sexual reproduction
can also occur.
A wide variety of feeding strategies occur among the nematodes. Some
feed on detritus, some feed on algae, many feed on aquatic plants, and
some species are carnivorous. Predatory nematodes may be the biggest con-
sumers of nematodes in their natural habitats. Nematodes can be impor-
tant parasites of humans, other animals, and plants. The species parasitic
on humans use mosquitoes, black flies, or midge larvae as hosts. The ne-
matode that causes river blindness (onchocerciasis) is transmitted by black
flies (Simuliidae) and infects 120 million people, most living in Africa.
Phylum Nematomorpha
Members of the Nematomorpha are known as horsehair worms (Fig.
9.3I) and are parasites. Some species are parasitic on humans, but many
invertebrates or other vertebrates serve as hosts. The free-living adults are
FIGURE 9.4 Change in body form (cyclomorphosis) of the planktonic rotifer Karatella qua-
dratica in successive generations in laboratory culture (reproduced with permission from
Hutchinson, 1967).