||Mercenaria mercenaria Linnaeus, 1758
The hard clam, Mercenaria
mercenaria, burrows shallowly in sediments of either mud or sand. It
is among the most commercially important species of invertebrate. Like other
clams, it is a filter feeder. Mercenaria mercenaria has a large, heavy
shell that ranges from being a pale brownish color to shades of gray and
white. The exterior of the shell, except nearest the umbo is covered with
a series of growth rings. The interior of the shell is colored a deep
purple around the posterior edge and hinge.
Potentially Misidentified Species
Can be confused with the Southern hard clam, M. campechiensis which
closely resembles M. mercenaria. However, M. campechiensis
is slightly larger, and its shell lacks purple coloration on the interior
surface. It also lacks any smooth areas near its umbo.
HABITAT AND DISTRIBUTION
The natural range of M. mercenaria
is from the gulf of St. Lawrence, along the east coast of the United States,
around the Florida peninsula and into the Gulf coast of Texas.
Hard clams are common in sandy bottoms throughout the Indian River Lagoon.
They are also farmed commercially from Indian River County northward. Other
important growing areas for clam aquaculture in Florida are Cedar Key in south
Florida, and Lee and Charlotte Counties on the west coast of Florida.
LIFE HISTORY AND POPULATION BIOLOGY
Age, Size, Lifespan
Most hard clams become reproductively active at approximately 1 year of age
(Loosanoff 1936, 1942) and will continue to produce broods throughout their
lives, with no reproductive senescence observed (Walker and Hefferman, 1996).
Owing to their commercial importance, the typical lifespan of Mercenaria
is approximately between 4 and 8 years of age, the peak of their commercial
marketability. At this age, most hard clams measure between 2 - 4 inches, which
places them in the littleneck (2.0 - 2.9 inch) and cherrystone (3.0 - 4.0 inch)
categories. Those hard clams which grow above 4.0 inches are termed chowders or
quahogs (Busby, 1986). The natural lifespan of Mercenaria is generally
unknown; however, counts of growth rings indicate that the hard clam, in the
absence of predation or commercial exploitation, may live as long as 40 years.
Growth has been observed to cease after the age of 15 years, with annual growth
at this age slowed to approximately 1mm per year (Loesch, and Haven, 1973).
Size classes for Mercenaria mercenaria
have been designated as follows to standardize commercial conventions: Seed
clams: < 1"; Beans: 1.0 - 1.5 "; Buttons: 1.5 - 2.0";
Littlenecks: 2.0 - 2.5"; Topnecks: 2.5 - 3.0"; Cherrystones: 3.0 -
4.0"; Chowders: >4.0".
Local variations in growth rates are extreme throughout the entire geographic
range, and seasonal differences in growth also occur (Ansell, 1968). In
northern areas of the geographic range, growth in M. mercenaria occurs
only during the summer when water temperatures approach 20 °C, the optimum growth
temperature for this species. During winter, growth ceases altogether in water
temperatures below 5 - 6 °C. In southern areas of the range, growth is more
continuous. For example, in South Carolina and Georgia, hard clam growth is
rapid in fall and spring, tends to slow throughout the winter months, and is
slowest during the summer. Mercenaria in Florida may have growth rates 3
times those for Mercenaria in more northern waters (Barile et al., 1986)
Water temperature sets the limits for growth;
however other factors, such as food availability and degree of crowding also
influence growth rates. Crenshaw et al., 1996 showed that hard clams reared at
high densities (>360 per square foot) tended to take longer to reach the same
size as those reared under more moderate (~30 per square foot) conditions.
In the IRL as in other areas within its range, Mercenaria mercenaria
is most abundant in shell-containing soft bottoms. They are also found (in
decreasing order of abundance) on sand flats, sand/mud flats and on muddy
bottoms (Wells 1957; Pratt 1953). A study by Peterson et al., (1984) also showed
that densities of 0 - 2 year old hard clams in eelgrass (Zostera marina)
beds of North Carolina was more than 5 times the average density of clams in
nearby sand flats (9 per square meter in eelgrass, vs. 1.6 per square meter in
nearby sand flats. Further, hard clams from Zostera beds appeared to be
somewhat larger, on average, than those from sand flats. Hydrodynamic baffling
by seagrasses may be at least partially responsible for the observed
result (Peterson et al., 1984). Reduction in currents near the benthos enhances
the deposition of fine sediments and suspended materials between blades of
seagrass, especially near patch edges. Hydrodynamic baffling therefore provides a
rich food source for juvenile clams.
Mercenaria has limited locomotion in that it is able to burrow via
use of its muscular foot; however, they are generally sedentary if left
Mercenaria mercenaria is a protandric hermaphrodite, with the male
line developing first. Approximately 98% of all juvenile clams begin life
as males; however, with increased age and size, sex ratios in the population
even out, and approximately half of the males later change to females (Loosanoff
1936, 1942; Barile et al. 1986; Walker 1995).
The hard clam spawns during summer throughout
its geographic range. Carriker (1961) found that water temperatures between 22
and 30 °C allowed hard clams to spawn with maximum frequency. In the Indian
River Lagoon, however, spawning occurs in the autumn after water temperatures
drop below 23 °C (Busby 1986).
Eggs of M. mercenaria measure approximately 70 - 90 um in diameter
and are surrounded by a gelatinous envelope (Walker and Hefferman 1996).
Fertilized eggs become trochophore larvae within the first 12 hours; shells
develop within 26-30 hours. The veliger stage is reached in another 8-12 hours.
Veligers are planktonic for approximately 12-14 days before settling. Larvae
that are competent to settle measure approximately 200-210 um. With settlement, the
velum disappears and use of the foot shifts from aiding in swimming to burrowing
and crawling (Loosanoff and Davis 1949).
Ansell (1968) found growth is optimized at a temperature of 20 °C, with no
growth occurring above 31 °C or below 9 °C. At 4 °C, Mercenaria enter
"hibernation", but are able to survive somewhat lower temperatures.
Maximum growth of larvae is achieved at
water temperatures between 22.5 - 36.5 °C,
with a minimum temperature requirement of at least 12.5 °C.
The distribution of bivalves is
particularly influenced by variations in salinity due to the fact that most
bivalves are unable to emigrate from the adverse environmental conditions which
occur when salinity drops. The tolerance of M. mercenaria to
decreases in salinity INCREASES with the age of the clam, but is inversely
proportional to temperature. Thus, development and survival rates decrease
sharply when salinity is low and temperature is high (Barile et al. 1986). Eggs
develop normally within the range of 20 - 32.5 ppt. Over 35 ppt, only 1% of eggs
develop to the larval stage; at salinity below 17.5 ppt, none do.
Maximum growth of larvae is achieved at
salinities between 21 - 30 ppt. Below 15 ppt, growth ceases and larval mortality
is high. However, late stage larvae tend to have decreased tolerance to
temperature changes compared to embryos, but are increasingly tolerant of lower
salinity. One important exception is the pediveliger stage, which has a salinity requirement
of at least 20 ppt in order to successfully complete metamorphosis
to the juvenile stage (Barile et al. 1996).
Juvenile hard clams (1.8 - 3.6 mm) are
increasingly less vulnerable to lower salinity than are larvae, but will die
when subjected to extended periods of salinities below 15 ppt. Older hard clams
fare better at low salinity, but growth slows at salinities below 20 ppt.
Adult Mercenaria are able to withstand
long periods of low salinity due primarily to their ability to close their
valves. Adults have been shown to survive salinity as low as 10 ppt for up to
4-5 weeks, and are able to balance their internal osmotic conditions with that
of the external medium. However, extended valve closure leads to decreased
growth rates and reproductive capacity (Barile et al. 1986).
Other Physical Tolerances
Oxygen levels of at least 0.5 mg/L are required for normal development of
embryos. Below this level, at approximately 0.2 mg/L, 100% mortality occurred
(Morrison 1971). At 0.34 mg/L, development proceeded only to the trochophore
stage, with no subsequent shell formation. Larval growth was poor below 2.4
mg/L, but was optimized above 4.2 mg/L.
Davis (1969) found that in larvae, the pH optimum was between 7 and 8.75. In
adult hard clams, this range is broader.
Hard clams feed primarily on single celled algae and diatoms which are taken
in by the inhalant siphon, filtered over the gills, and eventually passed to the
mouth via cilliary tracts. Hard clam consumption of particulate organic carbon (POC) is on the order of approximately 1292 Kcal per square meter per year. Of
this total energy intake, 59% is deposited as feces/pseudofeces and passed
directly to nitrifying bacteria (Hibbert 1977).
Prefers sand or mud substrata in depths from
intertidal flats to 15 m (50 ft.) in depth.
The hard clam has the highest value of any fishery species in the Indian River
Lagoon. The statewide commercial catch of wild harvested hard
clams, Mercenaria mercenaria, between the years 1987 - 2001 was 13.5 million pounds, with a
dollar value of over $98.9 million. The 5 county area
encompassing the IRL (Volusia, Brevard, Indian River, St. Lucie and Martin
Counties) accounts for a large portion of this figure, with 10.5 million pounds
harvested, and a dollar value in excess of $70.3 million. This
ranks the hard clam first in commercial value within the IRL, and ninth in
Figure 1 below shows the dollar value of the hard clam
fishery to IRL counties by year. All size classes of clams were combined
in this dataset. As shown, commercial catch ranged from a
low of $960,303 in 2001 to a high of over $10.7 million in 1995.
Brevard County annually accounts for the largest percentage of the catch with
84% in total (Figure 2), followed distantly by Indian River County, which
accounts for 13% of the total and Volusia County, which accounts for 4.5% of the
total. St. Lucie and Martin Counties combined collected less than 0.5% of
the harvest. Of note is the fluctuating nature of the hard clam fishery in the IRL as a
whole. While the fishery in Brevard County from 1995 - 1997 was apparently
declining, in Indian River County, the harvest increased.
Tables 1 and 2 show the commercial value of the hard
clam harvest to Indian River Lagoon counties in both dollars (Table 1) and
percentage by county (Table 2).
Figure 1. Annual dollar value of the commercial catch of hard clams to the 5-county area of the Indian River Lagoon.
Figure 2. Total hard clam dollar value and percentage by county for the years 1987 - 2001.
Table 1. Total dollar value of IRL wild-harvested hard clams, Mercenaria mercenaria, between 1987 -2001.
||Value to IRL
By-county annual and cumulative percentages of the hard clam harvest for the years 1987-2001.
Table 3. By county cumulative dollar value and percentage of total for the IRL clam harvest from 1987 - 2001.
In terms of aquaculture, hard clams account for a large
percentage of total aquaculture production in Florida, ranking third in dollar
value behind tropical fishes and aquatic plants. In 2003, 259 growers
(including seed clam producers) produced 134 million aquacultured clams with a
commercial value of $12.9 million.
A 1993 study by Mojica and Nelson
found no significant detrimental effects from clam aquaculture in the Indian
River Lagoon. The presence of clam farms was associated only with a decrease in
mean sediment sizes within 1 meter of growout bags. Beyond this limit, no
effects could be observed. Furthermore, no changes were observed in the
surrounding benthic, soft-bottom community underlying growout bags, suggesting
few adverse environmental conditions resulted from clam farming in the Indian
Ansell AD. 1968. The rate of growth of the hard clam Mercenaria mercenaria throughout the geographical range. J Conseil 31: 364-409.
Barile DD, Rathjen W. 1986. Report on rainfall event of September and October 1985 and the impact of storm discharge on salinity and the clam population (Mercenaria mercenaria) of the Indian River Lagoon. Marine Resources Council, Florida Institute of Technology.
Busby D, ed. 1986. An overview of the Indian River clamming industry and the Indian River Lagoon: A collection of papers prepared in connection with the Brevard County Clam Industry Workshop held September 7, 1985 at the Florida Institute of Technology, Melbourne, FL. Florida Sea Grant Extension Program, Univ Florida.
Carrikcr MR. 1961. Interrelation of functional morphology, behavior, and autecology in early stages of the bivalve Mercenaria mercenaria. J Elisha Mitchell Soc 77: 168-241.
Crenshaw Jr JW, Heffernan PB, Walker RL. 1996. Effect of growout density on heritability of growth rate in the northern quahog, Mercenaria mercenaria (Linnaeus, 1758). J Shellfish Res 15: 341-344.
Davis HC. 1969. Shellfish hatcheries - present and future. Trans Amer Fish Soc 98: 743-750.
Hibbert CJ. 1977. Growth and survivorship in a tidal-flat population of the bivalve Mercenaria mercenaria from Southampton Water. Mar Biol 44: 71-76.
Loesch JG, Haven DS. 1973. Estimated growth functions and size-age relationships of the hard clam, Mercenaria mercenaria, in the York River, Virginia. Veliger 16: 76-81.
Loosanoff V. 1936. Sexual phases in the quahog. Science, 83: 287-288.
Loosanoff VL. 1942. Seasonal gonadal changes in the adult oysters, Ostrea virginica, of Long Island Sound. Biol Bull 82: 195-206.
Loosanoff VL, Davis HC. 1950. Conditioning V. mercenaria for spawning in winter and breeding its larvae in the laboratory. Biol Bull 98: 60-65.
Mojica R, Nelson WG. 1993. Environmental effects of a hard clam (Mercenaria mercenaria) aquaculture site in the Indian River Lagoon, Florida. Aquaculture 113: 313-329.
Morrison G. 1971. Dissolved oxygen requirements for embryonic and larval development of the hardshell clam, Mercenaria mercenaria. J Fisheries Board Canada 28: 379-381.
Peterson CH, Summerson HC, Duncan PB. 1984. The influence of seagrass cover on population structure and individual growth rate of a suspension-feeding bivalve, Mercenaria mercenaria. J Mar Res 42: 123-138.
Pratt DM. 1953. Abundance and growth of Venus mercenaria and Callocardia morhuana in relation to the character of bottom sediments. J Mar Res 12: 60–74.
Walker RL, Heffernan PB. 1995. Sex ratio of the northern quahog according to age, size, and habitat in coastal waters of Georgia. Trans Amer Fish Soc 124: 929-934.
Wells HW. 1957. Abundance of the hard clam Mercenaria mercenaria in relation to environmental factors. Ecology 38: 123-128.
Report by: K. Hill,
Smithsonian Marine Station
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Page last updated: November 8, 2004