Animals - Freshwater Ecology: Concepts and Environmental Applications - page 164

Phylum Annelida
Annelids are segmented worms with a tubular body and a specialized
digestive system with a terminal mouth and an anus. Their body cavity has
thin transverse septa that delineate the segments. They generally reproduce
sexually by cross-fertilization and are often hermaphroditic, but many re-
produce asexually by budding. The freshwater annelids include the
oligocheates, the leeches, and several other less diverse classes.
Aquatic oligochaetes (Fig. 9.5G) are very similar to their terrestrial
analogs (earthworms). They usually have four bundles of chaetae (hairs)
on each segment (Brinkhurst and Gelder, 1991). Most of these worms bur-
row through sediment and ingest organic particles in lotic and lentic habi-
tats, but some are important algal feeders or predators. Some oligochaetes,
particularly the tubifid worms such as Tubifex, are highly resistant to low
O 2 and high levels of organic pollution. Thus, they are used as indicator
species of polluted waters and can be components of biotic indices to as-
sess ecosystem health. Oligocheates can also be vectors for important par-
asites such as whirling disease, which infects trout.
Leeches (Hirudinae) are mostly predators that feed on midge larvae,
amphipods, oligochaetes, and mollusks, but some are parasites that feed on
the blood of vertebrates, including humans. Species of leeches that utilize
blood are being investigated for the pharmacological value of the anticoag-
ulants used during feeding (Davies, 1991). Leeches are distinguished by sev-
eral characteristics, including dorsal-ventral flattening (Fig. 9.5I), an oral
sucker and usually a posterior sucker, usually 34 true body segments, and
a muscular body. Leeches are commonly found in shallow warm waters, in-
cluding slow-moving streams and rivers, lakes, and wetlands (but generally
not acid peat bogs). Some species live in moist terrestrial habitats.
When parasitic leeches attach to prey for a blood meal, they attach
with the posterior sucker and explore for a suitable feeding spot with the
anterior end. The oral sucker is then attached, three painless cuts are made
with the jaws, and anticoagulants are injected. The leech eats its fill and
then drops off the host. Frequent meals are not necessary, and specimens
have been kept alive without feeding for more than 2 years (Pennak, 1978).
Phylum Bryozoa
Bryozoans (also known as Phylum Ectoprocta) generally are sessile (at-
tached to bottom) colonial invertebrates that use ciliated tentacles to cap-
ture suspended food particles. This group is primarily marine with more
than 4000 species worldwide, about 50 of which are freshwater species
(Pennak, 1978). These animals are composed of many individual zooids,
each of which is approximately tubular and has a crown of tentacles (Figs.
9.5K-9.5M). The Bryozoa are generally restricted to warm water and can
be found in still and running waters (Wood, 1991). They require solid sub-
strata for attachment. The colonies can reproduce asexually by formation
of encapsulated dormant buds and can also reproduce sexually once a year.
The bryozoa are fairly resistant to predation.