➚ DESCRIPTION: Although zebra mussels
vary in color, they are fairly easy to distinguish by their striped pattern and triangu-lar-shaped bivalve shell. Zebra mussels
can grow to two inches in length, but most
only reach half that size.
➚ ORIGIN: Native to the Black and Caspian
seas, zebra mussels were likely brought to the
U.S. via ballast water from transoceanic ships.
➚ WATERSHED WHEREABOUTS: Zebra
mussels have invaded the Susquehanna
River and its tributaries from New York
to Maryland. The zebra mussel prefers
freshwater, but can survive in slightly
➚ BENEFITS: The filtering power of zebra
mussels can improve water quality.
➚ THREATS: Zebra mussels often grow in
large clumps which can clog intake pipes
at water treatment and power plants. They
colonize very quickly and can encrust
rocks, piers, buoys, and native mollusks.
The sharp shells of dead zebra mussels
that wash up on beaches can be harmful
to people and wildlife. The U.S. Geological
Survey predicts that the cost of managing
zebra mussels in the Great Lakes may
climb to $5 billion per year. To help prevent new invasions, boaters should take
care to clean their hulls when moving from
one water body to another.
ROBERT VIDÈKI/DORONICUM KFT./ BUGWOOD.ORG
➚ DESCRIPTION: This rooted underwater
grass branches freely, especially near the
surface. Toothed leaves, which grow to 3/4
inch, present in whorls of four to eight.
➚ ORIGIN: Hydrilla was likely introduced to
the U.S. after being imported from Korea
and India as an aquarium plant in the
1950s. It was first noted in the wild in
Florida’s Crystal River in 1960.
➚ WATERSHED WHEREABOUTS: Hydrilla is
present in the Chesapeake Bay watershed
from Pennsylvania and Delaware to
Virginia. Dense beds in D.C.’s Potomac
River have been especially newsworthy.
➚ BENEFITS: Where submerged aquatic vegetation was not previously present, hydrilla
can provide habitat for fish and shellfish
and have a positive effect on water quality.
➚ THREATS: The invasive hydrilla can spread
quickly, choking out existing native underwater grasses. Its dense presence on the
water’s surface can make swimming,
boating, and fishing almost impossible.
Some methods of controlling hydrilla—like
mechanical weed harvesters—can actually
encourage the plant’s growth. Hydrilla can
propagate vegetatively through small
pieces of stem or tuber.
➚ DESCRIPTION: The largest birds in the
Chesapeake Bay area, mute swans are
distinguishable by their orange bills with
black basal knobs. An adult bird has a
wing span of up to six feet and an average
lifespan of 12 years.
➚ ORIGIN: Mute swans were imported from
Eurasia in the late 1800s as park and
residential pond decorations.
➚ WATERSHED WHEREABOUTS: Mute swan
populations have been present throughout
the watershed since the 1930s. Not true
migrants, these birds are year-round
inhabitants in rivers, creeks, streams, and
inland ponds and lakes.