Species Description: 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.
Regional Occurrence: 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.
IRL Distribution: 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.
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.
Abundance: 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.
Locomotion: Mercenaria has limited locomotion in that it is able to burrow via use of its muscular foot; however, they are generally sedentary if left undisturbed.
Reproduction: 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).
Embryology: 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).
Temperature: 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.
Salinity: 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. That is, 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.
Trophic Mode: 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).
Habitat: Prefers sand or mud substrata in depths from intertidal flats to 15 m (50 ft.) in depth.
Special Status: Fisheries; aquaculture
Fisheries Importance: 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 pounds harvested.
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 $10.7 million in 1995.
Brevard County annually accounts for the largest percentage of the catch with 84% in total (Figure 2), followed distantly by Indian River County, which accounts for 13% of the total and Volusia County, which accounts for 4.5% of the total. St. Lucie and Martin Counties combined collected less than 0.5% of the harvest. Of note is the fluctuating nature of the hard clam fishery in the IRL as a whole. While the fishery in Brevard County from 1995 - 1997 was apparently declining, in Indian River County, the harvest increased.
Tables 1 and 2 show the commercial value of the hard clam harvest to Indian River Lagoon counties in both dollars (Table 1) and percentage by county (Table 2).
|VOLUSIA||BREVARD||INDIAN RIVER||ST. LUCIE||MARTIN||TOTAL|
|YEAR||Value ($)||Value ($)||Value ($)||Value ($)||Value ($)||Value to IRL|
|VOLUSIA||BREVARD||INDIAN RIVER||ST. LUCIE||MARTIN|
|YEAR||% Total||% Total||% Total||% Total||% Total|
|VOLUSIA||BREVARD||INDIAN RIVER||ST. LUCIE||MARTIN|
Broad-scale Cost/Benefit: 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 River Lagoon.
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