Oyster reefs, often referred to as oyster bars, are common
submerged habitats in the southern United States. Oyster reefs in Florida are
found in nearshore areas and estuaries of both coasts, but grow especially
vigorously near estuarine river mouths where waters are brackish and less than
10 meters deep. For example, the Apalachicola River in northern Florida is a
particularly productive area for oysters, and supplies over 90% of the state’s
annual oyster catch. Within the Indian River Lagoon, oyster reefs may be found
in the vicinity of spoil islands and impounded areas. In addition to being
commercially valuable, oyster reefs serve a number of important ecological roles
in coastal systems: providing important habitat for a large number of species;
improving water quality; stabilizing bottom areas, and influencing water
circulation patterns within estuaries.
Oyster reefs are built primarily by the eastern oyster,
Crassostrea virginica, through successive reproduction and
settlement of larvae onto existing reef structure. Oysters in Florida spawn from
late spring through the fall. The planktonic larvae that develop require a hard
substratum to settle upon in order to complete development to the juvenile
stage, and prefer to settle on the shells of other oysters. Thus, over time,
continued settlement and subsequent growth of generations of oysters may form
massive reef structures consisting of staggering numbers of individuals. Luntz
(1960), estimated that 5,895 oysters, the equivalent of 45 bushels, occurred
within a single square yard of oyster reef.
As successive generations of oysters settle and grow,
reefs become highly complex, with many structural irregularities and infoldings
that provide a wealth of microhabitats for many different species of animals.
Wells (1961) listed 303 different species utilizing oyster reef as habitat in
North Carolina. Common Indian River Lagoon species associated with oyster reefs
include bivalves such as the hard clam (Mercenaria mercenaria) and bay
scallop (Argopecten irradians concentricus); space competitors such as
the scorched mussel (Brachidontes exustus), ribbed mussel (Geukensia
demissa), the jingle shell (Anomia simplex), and barnacles of the Balanus
genus; gastropod mollusks such as the conchs (Melongena spp. and Strombas
spp.) and rocksnails (Thais spp.); numerous sponge species; flatworms;
polychaete worms; amphipods; isopods; shrimp; and fishes such as blennies,
gobies, spadefish, snappers, drum, and seatrout, among others.
Beyond providing smaller organisms with habitat, oyster
reefs also provide food to a wide variety of secondary consumers. Many species
of fish prey upon oyster reef associates; while others such as the black drum (Pogonias
cromis) and cow-nosed ray (Rhinoptera bonasus) prey upon oysters
themselves. Other species that utilize oyster reefs for foraging and feeding
include the xanthid crabs, also known as mud crabs; swimming crabs of the genus Callinectes;
mollusks such as the thick lipped oyster drill (Eupleura caudata), the
sharp-rib drill (E. sulcidentata), the Atlantic oyster drill (Urosalpinx
cinerea), the Tampa drill (U. tampaensis), the knobbed whelk (Busycon
carica), the lighthire whelk (B. contrarium), and the pear whelk (B.
spiratum pyruloides); flatworms such as oyster leeches (Stylochus spp.);
boring sponges (Cliona spp.); and annelid worms (Polydora spp.).
Oyster reefs also contribute to improved water quality
in areas where they occur. Oysters are filter feeders which strain microalgae,
suspended particulate organic matter, and possibly dissolved organic matter from
the water column over their gills in order to feed. Under optimal temperature
and salinity conditions, a single oyster may filter as much as 15 liters of
water per hour, up to 1500 times its body volume. Spread over an entire reef,
for an entire day, the potential for oysters to improve water clarity is
immense. Additionally, since oysters are sessile, and bioaccumulate some
potential toxins and pollutants found in the water column, they have been used
to assess the environmental health of some areas.
Over-harvesting, as well as persistent diseases such as
MSX and Dermo have taken a devastating toll on many oyster populations along the
east and Gulf coasts. In recent years, oyster reef restoration has been a
concern for resource managers all along the East Coast of the United States, but
especially in areas where oyster harvesting has historically been commercially
important. In the late 1800s, for example, annual oyster harvests in the
southeastern United States routinely topped 10 million pounds per year, and
peaked in 1908 when the harvest was nearly 20 million pounds. However, annual
harvests since that time have declined steadily. Today, annual harvests for
oysters in the southeast averages approximately 3 million pounds per year. In
many areas, efforts are underway to revitalize depleted oyster reefs and
encourage growth of new reefs. For example, the Florida Department of
Agriculture has stockpiled calico scallop shells from processors and placed
these on depleted oyster reefs from the spring through the fall spawning
periods, when larvae are most abundant in the water column. Oyster larvae,
having a preference for settling on shell material, then attach themselves onto
the newly placed shells and metamorphose to the juvenile stage. These young
oysters, under optimal conditions, will grow to marketable size in as little as
18 – 24 months.
[A more detailed look at some emerging human-induced threats facing the oyster reefs of the IRL is available here.]
Click a highlighted link to read more about individual
Bahr, L.M. and W.P. Lanier. 1981. The Ecology
of Intertidal Oyster Reefs of the
South Atlantic Coast: a Community
Profile. U.S. Fish and Wildlife Service, Blot.
Program, Washington D.C. FWS/OBS 81/15. 105 pp.
Burrell, V.G. 1986. Species Profiles: Life
Histories and Environmental Requirements
of Coastal Fishes and Invertebrates (South Atlantic):
American Oyster. U.S. Fish
and Wildlife Service. Biological Report 82(11.57). U.S. Army Corps of
Engineers. TR EL-82-4. 17 pp.
Kumari, Siva, and C. Solis. 1995. The State of
the Bay: a Characterization of the
Galveston Bay Ecosystem. Rice university,
Houston, TX. Accessed on-line at:
Livingston, Robert J. 1990. Inshore Marine
Habitats. In: Ecosystems of Florida,
Ronald L. Myers and John J. Ewel, Eds. University of
Central Florida Press,
Orlando, FL. Pp. 549-573.
Lunz, G.R., Jr. 1960. Intertidal Oysters. Wards
Natl. Sci. Bull. 34(1): 3-7
Wells, H.W. 1961. The Fauna of Oyster Beds with
Special Reference to the Salinity
Factor. Ecological Monographs 31(3): 239-266.