Biota of Rivers and Streams - Hydro-Environmental Analysis: Freshwater Environments - page 176

Zebra
mussel

Quagga
mussel

Zebra mussel occurrences
Quagga mussel occurrences
Both species occurrences
Zebra mussels eradicated

FIGURE 6.14 Distribution of zebra and quagga mussels ( Dreissena polymorpha and D. rostrifor-
mis bugensis ) sightings as of July, 2011. (From Benson, A.J., Zebra mussel sightings distribution, 2011.
Retrieved from http://nas.er.usgs.gov/taxgroup/mollusks/zebramussel/zebramusseldistribution.aspx. With
permission.)

and crustaceans (Figure 6.15), impacting feeding, reproduction, and growth, and some native mus-
sels have been found with more than 10,000 zebra mussels attached to them. They also encrust on
water pipes and other surfaces, and densities of 100,000-500,000 mussels m -2 have been reported
in Lake Erie (Alexander et al. 1997). In addition to the impact of the encrustations, zebra mussels
are ilter feeders, each iltering up to a quart of water per day. Considering the enormous numbers
often present, zebra mussels can then essentially ilter all of what is the food for other organisms,
such as native mussels.

6.3.1.1.3 Phylum Annelida (Worms and Leeches)
This group includes the Hirudinea (leeches), Oligochaeta (aquatic worms), and Nematomorpha
(horsehair worms), of which the Oligochaetes are perhaps the most important in stream habi-
tats (Figure 6.16). Of this group, perhaps the best known are the aquatic sludge worms (family
Tubiicidae). The sludge worms are very tolerant of pollution, particularly pollution resulting from
sewage efluents with high concentrations of organic materials, and they are commonly used as
environmental indicators. Like the midge larvae, also commonly found in highly polluted waters,
the sludge worms have hemoglobin in their blood and are also called bloodworms. In polluted areas,
the sludge worms may dominate the aquatic population (low numbers of species and large numbers
of sludge worms).

6.3.1.2 Functional Groups
In addition to their taxonomic classiication, benthic macroinvertebrates are also often classiied by
their method of feeding, as illustrated in the RCC (Figure 6.2). Basing identiication on function
rather than taxonomy is often done since identiication may be more rapidly accomplished. Often,