Biota of Rivers and Streams - Hydro-Environmental Analysis: Freshwater Environments - page 172

Scud ( Crangonyx sp . ) Isopod ( Caecidotea sp . )

FIGURE 6.8 Crustaceans. (Photographs from the Department of Environmental Protection, Bureau of
Land and Water Quality, State of Maine, Available at http://www.maine.gov/dep/blwq/docmonitoring/
biomonitoring/sampling/bugs.htm. With permission.)

wide variety of habitats, including pools, rifles, and the hyporheic zone. They are typically scaven-
gers, eating both dead and live plant and animal debris.

6.3.1.1.2 Phylum Mollusca
This phylum includes snails (class Gastropoda), and clams and mussels (class Bivalvia). The gastro-
pods, or snails, are common and most easily recognized by their shells. Some have true gills and
an operculum, a plate that can seal the shell after the soft body has been retracted (prosobranchs;
Figure 6.9), while others take up oxygen through a vascularized mantle cavity that acts like a lung
(pulmonates; Figure 6.9). The pulmonates can trap air, such as from the water surface, in their
mantle cavity and obtain oxygen directly and as a result, they are not as sensitive to polluted con-
ditions. The presence of the pulmonate pouch snail is used by the U.S. Environmental Protection
Agency (U.S. EPA) as an indicator of nutrient-enriched conditions and poor water quality (U.S.
EPA's Biological Indicators of Watershed Health). Most of the aquatic snails are herbivorous, scrap-
ing algae from substrates using a tonguelike structure covered by a ribbon of teeth, the radula
(Figure 6.10; Clifford 1991).
Mussels and clams are bivalves, having a shell separated into two symmetrical valves. They have
a soft body with enlarged gills and a muscular extendable foot that assists them in burrowing or mov-
ing. They typically burrow into bottom sediments, and feed by iltering ine particles from the water.
Clams and mussels are found around the world, but native mussels of the family Unionidae are more
diverse in North America than anywhere else, with nearly 300 species documented (USGS 1999).
Native mussels have a unique life history (Figure 6.11). During spawning, probably prompted by
temperature, males release sperm into the water column, which is taken in by females and fertilizes
their eggs. The fertilized eggs develop into a larval stage, the glochidia, which are stored in the
female's gills for several weeks or months and then released into the water column. The glochidia
irst drift until they encounter and attach to the ins or gills of a suitable ish host. There may be