Animals - Freshwater Ecology: Concepts and Environmental Applications - page 157

Phylum Gastrotricha
The gastrotrichs can be tremendously abundant in freshwaters
(10,000-100,000 m 2 ), but are poorly studied (Strayer and Hummon,
1991). About 250 species in 11 genera have been described from the fresh-
waters of the world (Pennak, 1978). Morphologically, they are about
50-800 m long and bowling pin shaped. Gastrotrichs usually have a dis-

tinct head with sensory appendages and a cuticle that is covered with scales
or spines (Figs. 9.3C and 9.3D).
Gastrotrichs are found mainly in benthic habitats and are common in
the shallow benthos of lakes. They are among the few animals that can
withstand extended anoxia and concomitant exposure to sulfide (Strayer
and Hummon, 1991). Gastrotrichs feed on bacteria, protozoa, algae, and
detritus. Most genera have cosmopolitan distributions.

Phylum Rotifera
About 2000 species of rotifers occur in freshwaters, and members of
the phylum generally have cosmopolitan distributions. Rotifers can be
found in all freshwater habitats. They are more diverse in fresh than ma-
rine waters, and some species inhabit saline lakes (Wallace and Snell, 1991).
Rotifers are small (60-250

m long) and distinguished by a ciliated
head region (corona) that moves water in a circular fashion. Their body
can range from worm-like to vase-shaped (Figs. 9.3A and 9.3B). They have
a well-developed digestive system that includes a mastax to grind food, a
stomach, an intestine and an anus. Rotifers have a small brain and eye-
spots that allow them to respond to environmental stimuli with moderately
complex behavior. Rotifers can also have a “foot” that extends ventrally
and several appendages called “toes” that can be used for movement or
attachment.
Some rotifers reproduce only sexually, others asexually, and yet others
mostly by asexual parthenogenesis with occasional sexual reproduction.
Amictic generations reproduce without any recombination of DNA. Sex-
ual reproduction often results in formation of a resting cyst. Thus, sexual
reproduction may be a strategy to avoid undesirable environmental condi-
tions. It is noteworthy that the bdelloid rotifers are the largest metazoan
taxon completely without sexual reproduction (Welch and Meselson, 2000).
This verifies that sexual reproduction is not a requirement for multicellu-
lar animals to maintain stable species over the tens of millions of years the
bdelloid rotifers have been a distinct group.
Several species of rotifers are adapted to drying. These animals can sur-
vive decades of desiccation and revive within minutes to hours after rewet-
ting. Such species are important animal components of temporary waters
and well adapted for dispersal.
Many species of rotifers are omnivorous filter feeders. These species
use their cilia to actively filter large volumes of water. Some benthic species
act as “sit and wait” predators that engulf prey when it swims near (Wal-
lace and Snell, 1991). Not all captured particles are consumed; rotifers feed
selectively and reject unsuitable food particles. The rotifers are important
consumers of bacteria in aquatic habitats.