Info

Figure 21.3. Time-course of the establishment (i.e., first detection in the wild) of alien species in the fresh waters of the Hudson River basin, the Great Lakes basin, and fresh and brackish waters of the San Francisco Bay and Delta. From data of Mills et al. (1993, 1996,1997) and Cohen and Carlton (1998).

of invasion histories, habitats, biological traits, effects, and costs or benefits of alien species.

i. zebra mussel (Dreissenapolymorpha) Zebra mussels are small bivalves with black-and-white striped shells (Fig. 21.4, color plate 5). The life cycle of zebra mussels is unusual for a freshwater invertebrate, and includes a free-swimming larva called a veliger. During warm periods (usually late May to the end of summer in the Hudson), adults shed eggs and sperm into the water, where fertilization occurs. Fertilized eggs develop into veligers, which spend one to several weeks feeding in the plankton. When larval development is complete, these animals settle onto hard objects such as stones, plants, wood, concrete, fiberglass, steel, etc. They are sexually mature after one year, and may live for from four to six years, reproducing each summer. Both adult and larval zebra mussels are suspension feeders, subsisting on phyto-plankton, small zooplankton, large bacteria, and organic detritus. In turn, zebra mussels are eaten by some fish (for example, sturgeons, freshwater drum, some sunfishes and suckers), waterfowl (for example, coots, scaup, goldeneyes), and decapods (for example, blue crabs, crayfish).

Zebra mussels are native to fresh and brackish waters in southeastern Europe and western Asia, and spread throughout Europe since ~1800 as a result of canal-building and other human activities. In about 1985, a ship from a European port released ballast water containing live zebra mussel veligers into Lake St. Clair near Detroit (Hebert,

Figure 21.4. Selected alien species now established in the Hudson estuary: (a) the zebra mussel, Dreissenapolymorpha; (b) the largemouthbass,Micropterussalmoides; (c) abedofwater-chestnut, Trapanatans,inTivoli South Bay; (d) the spinynutofthe water-chestnut; (e) Rangiacuneata;(f) the Japanese shore crab Hemigrapsus sanguineus. Photographs from David Strayer (a, d, e), F. Eugene Hester and the United States Geological Survey (b), Stuart Findlay (c), and Diane Brousseau (f).

Figure 21.4. Selected alien species now established in the Hudson estuary: (a) the zebra mussel, Dreissenapolymorpha; (b) the largemouthbass,Micropterussalmoides; (c) abedofwater-chestnut, Trapanatans,inTivoli South Bay; (d) the spinynutofthe water-chestnut; (e) Rangiacuneata;(f) the Japanese shore crab Hemigrapsus sanguineus. Photographs from David Strayer (a, d, e), F. Eugene Hester and the United States Geological Survey (b), Stuart Findlay (c), and Diane Brousseau (f).

Muncaster, and Mackie, 1989). Zebra mussels have since spread rapidly into lakes and rivers in eastern North America, especially in waters that are used heavily for navigation or recreation, activities that readily spread zebra mussels. Zebra mussels were first seen in the Hudson near Catskill in May, 1991. By the end of 1992, they were found everywhere in freshwater and oligohaline parts of the Hudson estuary, and had a biomass greater than the combined biomass of all other consumers in the river (Strayer et al., 1996). Zebra mussels have remained abundant on hard substrata throughout the freshwater and oligohaline Hudson estuary since 1992.

The theoretical daily filtration activity of the zebra mussel population during the summer has been 25-100 percent of the volume of the freshwater estuary.

Zebra mussels caused vast changes to the freshwater tidal Hudson (Strayer and Smith, 1996, 2001; Caraco et al., 1997, 2000; Strayer et al., 1999, 2004; Smith et al., 1998; Pace et al., 1998; Findlay et al., 1998). Populations of phytoplankton and small zooplankton fell sharply because of direct consumption by zebra mussels (Table 21.4). In contrast, populations of copepod zooplankton did not change, and populations of planktonic

Table 21.4. Changes in the Hudson River ecosystem caused by the zebra mussel invasion. Data are summertime (June-August) means; all biomasses given as g dry mass/m2

Variable

Pre-invasion

Post-invasion

Change

Light extinction0 (m-1)

0 0

Post a comment