The other common aquatic group of arthropods are the crustaceans. Some crustacea ns are quite small, almost microscopic. Common types include cladocerans like da phnia which have clear shells or coverings that are not segmented. You can see t he internal organs through the exoskeleton. Another small crustacean is the cope pod. These organisms generally have two antennae and have segmented bodies that end in a forked tail. Ostracods are small crustaceans that are enclosed in a hin ged shell (they look like tiny clams).

Larger crustaceans (easily seen without magnification) include amphipods such as Gammarus and isopods such as Asellus which you should see in Frick park. Crayfi sh, shrimp and lobsters are other large crustaceans. Crustaceans usually have mo re than six legs. Insects are not common in marine environments but you will see lots of crustaceans.

7. Rotifers (Phylum rotifera).

This group of microscopic animals are about the size of the protozoans, but are multicellular. They vary in shape from wor m-like to ovals that have a transparent cuticle or outer covering. Rotifers have a crown of cilia at the anterior end. When they feed, you can see one or more c ircles of cilia beating to create a current which collects small cells and parti cles on which the rotifer feeds. Rotifers also usually have a small forked tail. Some are found at the bottoms of ponds and lakes, others swim or float in the w ater.


PERCEIVING STREAM QUALITY

We normally decide if a stream is clean or dirty based on visual cl ues like turbidity (muddiness) or sense of smell (good or bad). Water is rarely 100% clean or 100% dirty. We will provide you with some information and skills t hat will help you understand the quality of Nine Mile Run.

Benthic Organisms (bottom dwelling species) give us a sense of the long t erm water quality. Finding a diversity of species indicates our stream is in ve ry good shape. Finding less diversity may indicate either some very hardy benth ic organisms or a stream that has potential but is in need of some attention.

Chemistry can indicate immediate water quality. When we sample the water we have an immediate understanding of the chemical "signature" of a particula r section of the stream on the day at the minute the sample was taken.