Animals - Freshwater Ecology: Concepts and Environmental Applications - page 155

Phylum Cnidaria
The Cnidaria (jellyfish, anenomes, corals, and other polyps, also called
Coelenterates) are mostly marine, but a few species of small jellyfish (Fig.
9.2B), polyps, and colonial hydrozoa occur in freshwaters (Slobodkin and
Bossert, 1991). These animals are radially symmetrical, and all have ne-
matocysts, which are cells that can fire a spine or thread to capture prey
and may contain strong toxins. The cnidarians can reproduce sexually or
asexually. Freshwater species often have a drought-resistant stage that al-
lows for widespread dispersal.
Hydra (Fig. 9.2A) may be the most commonly observed freshwater
cnidarian occurring in streams, wetlands, and lakes. These small polyps are
1-20 mm long with 10-12 tentacles crowning a tubular body. Hydra
species can float in the plankton but are commonly observed attached to
hard substrata or macrophytes in benthic habitats. They can move across
solid surfaces by “cartwheeling”—attaching tentacles, releasing the poste-
rior end, and flipping it over the body to reattach on the other end.
As noted for sponges, Hydra can also appear bright green from the en-
dosymbiotic green alga, Chlorella . This relationship is usually optional for
the Hydra . The alga produces photosynthate in lighted habitats but is
ejected or digested during an extended period of low light. Hydra species
with algal symbionts have been documented to have higher growth rates
in the light than those without algae (Slobodkin and Bossert, 1991).
A unique jellyfish, Mastigias, occurs in a saline, marine-influenced lake
in Palau in the West Caroline Islands (Hamner et al., 1982). The lake is
stratified and has an anoxic, high nutrient epilimnion. This medusa con-
tains an endosymbiotic dinoflagellate. The medusa moves down at night to
follow the copepods it consumes for food and it moves up during the day
to allow the endosymbionts to photosynthesize. During the day, the
medusae migrate up to 1 km horizontally to maximize exposure to light.
Similarly, in temperate, freshwater lakes the freshwater jellyfish Craspeda-
custa can be important in linking the lower and upper strata of water dur-
ing short periods of time. Their role in food webs can be substantial when
they aggregate in swarms of more than 1000 medusae/m 3 (Angradi, 1998;
Spadinger and Maier, 1999).
Phyla Platyhelminthes and Nemertea
The Platyhelminthes includes three classes: the Turbellaria (free-living
flatworms), the Trematoda (flukes), and the Cestoda (tapeworms). The
Turbellaria (Figs. 9.3E-9.3G) are common in freshwaters, with about 400
species found throughout the world (Kolasa, 1991). They do not have an
anus or closed circulatory system, but they do have an intestine and a cil-
iated epidermis over the entire body. The Turbellaria are divided into two
groups—the microturbellarians with about 300 species and the macro-
turbellarians, or Tricladida. Genera of microturbellarians usually have cos-
mopolitan distributions, whereas the triclads are distributed less widely.
Microturbellarians can be found in rivers, ponds, lakes, and subsurface
habitats. Caves and underground waters have many unique and endemic
triclad species; diversity is high in karst regions.