Crassostrea virginica (family Ostreidae) is a prolific member of
estuaries on the eastern coast of the Americas (Meyers and Townsend 2000,
Wilson et al. 2005). The shell of the eastern oyster is thick, flattened,
and highly variable in shape. It grows from round (irregular) to oval and
usually bears concentric ridges. The exterior color of the shell is dirty
white to gray while the interior is bright white with a deep purple or
red-brown muscle scar (Kay 1979).
C. virginica is a key structural component of estuaries (Berquist
et al. 2006) playing a major role in the function of estuary ecosystems
(Dame 1972). Large beds provide habitat for numerous fish and invertebrate
species, reduce water turbidity through filter-feeding, and physically
serve as filter removing large material from the water as it passes over
the reef (Meyers and Townsend 2000, Berquist et al. 2006). The eastern
oyster has been selected as a Ònatural resource of ecological significanceÓ
and methods for management are currently being developed (see the
Introduction to Oyster Reefs for more information) (Wilson et al. 2005).
HABITAT AND DISTRIBUTION
Crassostrea virginica naturally occurs in the Gulf of the St. Lawrence, Canada, along the Atlantic coast of the United States to the Gulf of Mexico to the Yucatan Peninsula, Mexico and to the West Indies and the coast of Brazil (Buroker 1983). It is also found in the estuaries of the main Hawaiian Islands where it was introduced in 1866.
The eastern oyster forms extensive reefs both intertidally and subtidally on the eastern coast of Florida including the estuaries of the Indian River Lagoon (Grizzle et al. 2002, Boudreaux et al. 2006).
LIFE HISTORY AND POPULATION BIOLOGY
Age, Size, Lifespan
Crassostrea virginica has a lifespan of up to 20 years (Buroker 1983) growing to 100 - 115 mm in length in two years. Individuals can reach sexual maturity at 4 months.
Small individuals (spat) of the eastern oyster live in dense aggregations
of up to 1, 416 oysters per m2 in the higher intertidal and 576 oysters per
m2 in the lower intertidal. Larger individuals reaching legal size (75 mm)
for harvesting, were recorded in densities of 336 oyster per m2 in the
higher intertidal and 228 individuals per m2 in the lower intertidal
(Berquist et al. 2006).
Crassostrea virginica spawn in late June to November peaking in June and July when seawater temperatures are warmest (Dame 1972). The eastern oyster has separate sexes (Buroker 1983, Wallace 2001). A female oyster can produce 15 to 114 million eggs in a single reproductive cycle (Buroker 1983).
The fertilized egg of Crassostrea virginica initially develops into a non-feeding swimming trochophore. Within 24 - 48 hrs, the trochophore develops into a veliger larva with a long dispersal period (Buroker 1983, Wallace 2001). The veliger has a thin shell and feeds on phytoplankton (Wallace 2001). After two to three weeks the veliger develops into a pediveliger with a foot and eye spots and begins to test the substratum for a suitable habitat to settle (Buroker 1983, Wallace 2001). The larva will cement itself to whole or broken oyster or clam shells and metamorphose into a spat (tiny oyster) (Wallace 2001). Attachment takes place between March and September in the Indian River Lagoon (Wislon et al. 2005), between July and December in the Gulf of Mexico, and between July and September in the north Atlantic coast of the United States (Buroker 1983). Metamorphosed spat are usually males with some individuals transforming into females after the first or second spawning. In some instances, female C. virginica reverse to males.
The reproduction of Crassostrea virginica is regulated by
temperature change. Spawning occurs in northern oysters when seawater
reaches temperatures between 15.5 and 20°C and in southern oysters spawn
when seawater is above 20°C (Wallace 2001, Wilson et al. 2005).
Crassostrea virginica appears to have a higher tolerance of
salinity fluctuation than other oyster species (Berquist et al. 2006). The
optimal salinity for growth and reproduction is 10 - 28 ppt (Wilson et al.
2005). Larvae will not settle and metamorphose into spat when salinity is
less than 6 ppt (Wilson et al. 2005). Adult C. virginica can live in
salinities up to 35 ppt (Buroker 1983). A study of the population genetics of
the eastern oyster suggests that there is gene selection in some
populations for high salinity tolerance (Buroker 1983).
The eastern oyster is a filter-feeder, removing particulate matter from the
water column (Berquist et al. 2006). It has small laterofrontal cilli that
facilitate the retention of particles between 5-6_m. The filtration rate
of Crassostrea virginica was measured at 6.80 liters of seawater per
hour in laboratory experiments (Riisgï¿½rd 1988). Other sources suggest that
it is as high as 36 liters per hour (Brusca and Brusca 1990).
In laboratory experiments, Crassostrea virginica was shown to
selectively ingest nutritional organic material. The cilli present in the
gills were used to sort out unwanted material from the mucus layer by
pushing it out of the gill (Newell and Jordan 1983).
Oyster reefs built by Crassostrea virginica provide habitats for
numerous invertebrate and fish species (Berquist et al. 2006). The eastern
oyster is also a common host of the symbiotic xanthid crab Tumidotheres
(Pinnotheres) maculates (Kruczynski 1973).
The human pathogen Vibrio vulnificus is also found in the tissues
of some populations of Crassostrea virginica. V. vulnificus
causes primary septicemia in patients with compromised immune systems. The
infection occurs when the oyster are eaten raw (Tamplin and Capers 1992,
Motes et al. 1998).
The total harvest of Crassostrea virginica is approximately 22 million pounds of meat per year, making up 70% of all the oyster harvest. Less than one half of the harvesting is from cultivated populations, making a considerable impact on natural oyster reef. The harvestable size of the eastern oyster is 75 mm. In cultivated oyster beds, individuals of C. virginica will grow to 75 mm in 12 - 36 months depending upon the food supply and environmental conditions. The demand for this fishery has decreased in recent years. This has been attributed to the instance of disease associated with eating live oysters and a general change in the eating habits of the targeted market (Wallace 2001). Despite this, over-fishing has had a major negative impact on oyster reef habitats (Meyer and Townsend 2000, Wilson et al. 2005).
Oyster culture serves two purposes: 1) to provide enough Crassostrea
virginica to meet the demand of the oyster fishery; and 2) to restore
oyster reef habitats in estuaries along the eastern Atlantic coast and the
Caribbean. The eastern oyster is cultured in both natural estuaries and
hatcheries. In some regions, oyster clutches are seeded in estuaries
considered optimal for oyster growth and reproduction. Alternatively,
larvae from controlled spawning events settled on carefully treated shells
in hatcheries are then placed in large mesh bags that are subsequently
taken to a nursery area in natural waters. Nursery areas are chosen for
optimal environmental conditions and the absence of large numbers of
potential predators (Wallace 2001).
Oyster culture has been successfully used in efforts to restore reefs on
the coast of North Carolina. The reefs created by seeding with live oyster
clutches reached the size of natural reefs in adjacent areas within one
year. C. virginica spat readily settled on the created reefs within
3 months and reached harvestable size (>75 mm) within two years. In
addition, invertebrate species known to be associated with C. virginica
oyster reefs were found at densities equivalent to those on natural
reefs within 2 years (Meyer and Townsend 2000).
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