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II. HABITAT AND
DISTRIBUTION
Regional Occurrence:
C. costata occurs in shallow waters along the eastern U.S. coast from
Massachusetts to Florida and into the Gulf of Mexico to Texas. It also occurs in
Cuba, the West Indies, Central America, and South America to Brazil (Gustafson
et al. 1991). It is most common in the low intertidal zone, at or just below the
low tide line, where it can burrow to a depth of approximately 3 feet in soft
sand or muddy substrata.
IRL Distribution:
Although C. costata is common along Florida's west coast, it is
patchily distributed along much of Florida's east coast, including the Indian
River Lagoon and Banana River, Florida (Gustafson et al. 1991).
III. LIFE HISTORY AND POPULATION BIOLOGY
Age, Size, Lifespan:
C. costata grows to 10 - 20 cm (4 - 8 inches)
and is noted for its rapid growth rate (Amos and Amos 1997). In aquaculture
trials, post-settlement juveniles reached 2 mm shell length (SL) within 32 days,
and 11.6 mm SL within 60 days. The growth rate of post-settlement juveniles in a
land-based aquaculture system averaged approximately 0.05 mm per day, while the
growth rate of field planted juveniles was between 0.5 - 0.7 mm per day (HBOI
report to Florida Sea Grant 1992). Gustafson et al. (1991) reported juvenile
shell length increased under optimal temperature, food, and salinity conditions
at a mean rate of 0.19 mm per day. Field planted juveniles reached a market
size of 5 - 7 cm (2 -3 inches) within 6 months, approximately 1/3 the time
required to achieve market size in the widely cultured hard clam, Mercenaria
mercenaria.
Abundance:
C. costata can be locally
abundant, but
is relatively patchy in its distribution throughout most of its range.
Locomotion:
Upon settlement into soft sand or muddy substrata,
juveniles burrow as much as 3 feet below the surface. C. costata remain
buried throughout their lives, and are unable to re-burrow themselves after they
attain 10- 15 mm SL.
Reproduction:
C. costata in Virginia have been observed to spawn from May to September. In
Florida and throughout the warmer regions of its range, the gonads of C.
costata tend to remain ripe year-round, except during June through August,
the months of prime spawning . Recruitment in Florida peaks from September to
December (Creswell and Shilling 1985).
In the laboratory, C. costata has been
induced to spawn by either inducing thermal shock (i.e., temperature of culture water is
increased rapidly to approximately 29 ° C), or by the addition of sperm to the
culture water.
Embryology:
In C. costata, a brief trochophore stage follows fertilization, with D-stage
veliger larvae developing within the first 24 hours. Eggs have a limited amount of yolk, which
requires that larvae begin to feed after the first day. Larvae remain free
swimming for 16 - 21 days before metamorphosis to the pediveliger stage which is
competent to settle (Creswell and Shilling 1985; Tiu et al. 1989; Gustafson et
al 1991; HBOI report to Florida Sea Grant 1992). Pediveliger shell length averages approximately 317 um (0.01 inches) (Gustafson et al. 1991). Competent
larvae apparently require sand as a settlement substratum, as
attempts to induce settlement of C. costata pediveligers on artificial or bare
substrata resulted in the complete mortality of cultures across all
temperature and salinity combinations within 7 days (Creswell and Shilling 1985;
Gustafson et al. 1991).
In the laboratory, a 1-hour exposure to a 10-3
M solution of epinephrine induced over 95% of all competent larvae to settle on
a sand substratum (Gustafson et al. 1991).
IV. PHYSICAL TOLERANCES
Temperature:
Temperature is the primary factor determining survival and
growth rates in C. costata larvae (Gustafson et al. 1991). In larval
growth trials, optimum shell length increase occurred at 30 °C over the entire
16-day larval period. However, maximal larval survival through metamorphosis
occurred at a
temperature of 25°C (Gustafson et al. 1991). Larvae do not survive at water
temperatures greater than 35°C (Creswell and Shilling 1985; Gustafson et al.
1991).
Salinity:
In growth trials, optimum shell length increase for C.
costata larvae occurred at a salinity of 20 ppt. in the first 8 days of larval life, and
at 25 ppt. in the next 8 days. More than 70% of larvae survived in salinities of
between 15 - 35 ppt (Gustafson et al. 1991).
V. COMMUNITY ECOLOGY
Trophic Mode:
C. costata, like other bivalves, is a filter feeder, and uses its
incurrent siphon to transport microalgae and tiny zooplankton into its body.
Food is "strained" from the water on the gills and is moved toward the
mouth by cilia. Filtered water, waste products, and gametes are passed out the
excurrent siphon.
Competitors:
C. costata competes with other
sessile filter feeders such as
the hard clam, Mercenaria mercenaria. It is also likely to compete
with suspension
feeders such as bryozoans and some species of polychaete worms.
Habitat:
Burrows up to 3 ft. deep in soft, sandy mud (Creswell
and Shilling 1985; Amos and Amos 1997).
VI. SPECIAL STATUS
Special Status:
None
Benefit in the IRL:
C. costata is indigenous to the
Indian River Lagoon and has been targeted as a recreational fishery throughout
its range, from Massachusetts to Brazil. However, its fragile shell, deep
burrowing habitat and its limited shelf life have thus far prevented effective
commercial harvesting of this species. Its commercial importance in the Indian
River Lagoon could increase should it ever become widely accepted as a
marketable aquaculture product.
Economic Importance:
C. costata and other pholadid clams are harvested commercially
throughout the world. C. costata supports a commercially important fishery in Mexico, Cuba, Puerto Rico and Chile (HBOI report to Florida Sea Grant
1992). Although it is not widely cultured in the United States, preliminary
aquaculture trials (Creswell and Shilling 1985; Gustafson et al. 1991; HBOI
report to Florida Sea Grant 1992) found C. costata an attractive species
for aquaculture due to its fast growth rate and generally unspecialized physical
requirements.
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