Animals - Freshwater Ecology: Concepts and Environmental Applications - page 174

FIGURE 9.12 Cyclomorphosis of adults of the cladoceran Daphnia retrocurva over a sea-
son in Bantam Lake, Connecticut, during 1945. Only body shape was traced (from Brooks,
1946).
because of their roles in processing organic matter and as an indicator of
water quality in both surface and subsurface waters.
The Amphipoda (scuds and sideswimmers) are similar in size to the
isopods, but they are flattened laterally (Fig. 9.11A). There are about 800
freshwater species worldwide (Pennak, 1978). Eyes usually are well devel-
oped (except in the subterranean species). The amphipods are omnivorous
and often considered scavengers. They are mainly nocturnal benthic species
and can be present in numbers up to 10,000/m 2 . Amphipods are very im-
portant as fish food.
The Mysidaceae (opossum shrimps; Fig. 9.11D) and Bathynellaceae are
usually minor components of the freshwater fauna. The mysid shrimps are
planktonic zooplanktivorous species that are several millimeters long. Twenty-
five species occur in freshwaters, whereas 780 occur in marine habitats.
Mysids can be an important food source for fishes and strong competitors of
other planktivores in some temperate lakes. The bathynellids are microscopic
to macroscopic invertebrates that can be found in the hyporheic zones of
some streams and other groundwaters. Because of their cryptic habitat, they
often have been ignored, and new species are actively being described.
PHYLUM CHORDATA, SUBPHYLUM VERTEBRATA
The fishes are considered the most important freshwater animals by
many people. Other types of vertebrates also live in or rely on freshwater
habitats. These will be discussed only briefly.