The heavy shell of the southern quahog, Mercenaria campechiensis, is ovate trigonal and inflated with valves of equal size (Andrews 1994). Several concentric growth lines are present on the gray to whitish exterior. Three cardinal teeth (the middle one split) on each valve help to hold the shell together at the hinge. In empty shells, the interior color is often porcelaneous white with purple marks occurring rarely. Two muscle scars are present on the interior surface, both attached to the pallial line.
Potentially Misidentified Species:
The southern quahog may be confused with its commercially important relative, the northern quahog, M. mercenaria. However, M. campechiensis is heavier and more inflated, and lacks the smooth central area on the outside of the shell that is characteristic of M. mercenaria (Dillon, Jr. 1992; Abbott & Morris 1995).
II. HABITAT & DISTRIBUTION
The range of the southern quahog extends from Chesapeake Bay to Florida, Texas and the Yucatán Peninsula to Cuba (Andrews 1994). Populations are normally found from intertidal sand flats to offshore depths of up to 120 feet (Abbott & Morris 1995). The clam uses its muscular foot to burrow into the sediment, where it is camouflaged from potential predators.
Indian River Lagoon (IRL) Distribution:
The range of the southern quahog extends south to the St. Lucie Inlet (Stewart 1981), although most populations are probably located in the northern IRL.
III. LIFE HISTORY & POPULATION
Age, Size, Lifespan:
The southern quahog is approximately 7 to 15 cm (Andrews 1994), although some individuals have been documented at over 17 cm in length (Stewart 1981). Lifespan varies with environmental conditions and other factors, but the maximum documented age for the southern quahog is around 22 years old (Stewart 1981). Most growth occurs in the first 2-3 years of life.
Reproduction & Embryology:
The southern quahog is a protandrous hermaphrodite, with about 98% of individuals beginning life as males and changing to females as they grow older (Stewart 1981). Like most mollusks, M. camphechiensis reproduces via external fertilization, releasing gametes into the water column. Spawning events generally occur in warmer months during neap tides (Stewart 1981). After fertilization, larvae pass through three main planktonic stages before developing into settled, juvenile clams. The first of these is the trochophore stage, which is formed 12 to 14 hours following fertilization. At this stage, larvae are cylindrical and ringed with a row of tiny, beating hairs called cilia. After approximately one day, larvae enter the veliger stage, growing lobes or paddles that resemble butterfly wings. During this stage, the shell and internal organs form. After 6-10 days, larvae develop a foot, are considered competent to settle, and are termed pediveligers. Where their ranges and spawning times overlap, southern and northern quahogs may hybridize with each other to produce the subspecies, M. mercenaria texana (Dillon, Jr. 1992; Abbott & Morris 1995).
IV. PHYSICAL TOLERANCES
Based on its range, the southern quahog likely prefers and/or requires warmer waters in order to thrive.
Compared to M. mercenaria, the southern quahog seems to prefer the oceanic waters found near inlets and in offshore habitats. Studies have also shown that M. campechiensis grows rapidly under highly saline conditions (Arnold et al. 1996), suggesting that salinity plays a role in the distribution patterns of the species. Some reports suggest that age may be a factor in salinity tolerance, with older clams thriving under a wider range of salinities (e.g. Stewart 1981).
V. COMMUNITY ECOLOGY
The southern quahog is a filter feeder, sieving microscopic plankton (mainly diatoms and other microalgae) from the water column. When buried in the sand, the clam extends two siphons above the surface of the sediment. The incurrent siphon imports food and dissolved oxygen from the surrounding water column, while the excurrent siphon expels waste (e.g. Stewart 1981).
Little information is available detailing the predators of M. campechiensis, but the clam is likely preyed upon by a variety of fishes and crustaceans.
Although there are no obligate associations documented between the southern quahog and other species, M. campechiensis is commonly found alongside organisms from the various coastal marine habitats in which it resides. For more extensive information on these ecosystems and their associated species found in and around the IRL, please visit Habitats of the IRL.
VI. SPECIAL STATUS
In addition to their value as a fishery species, the occurrence of northern and southern hard clam shells in the archeological remains of the region make these species a useful resource for reconstructing climate, environmental conditions and ecological relationships via isotopic analysis (Surge & Walker 2006).
Although the related hard clam, Mercenaria mercenaria, is more sought after, the southern quahog also forms an important commercial fishery in Florida. Depending on the location, harvest of clams can be unrestricted or fall under permit and bag limit regulations (e.g. Stewart 1981). During red tides and/or when pollutant levels are high, harvest of clams and other filter feeders may be prohibited due to public health concerns. To meet fishery demands, aquaculture of hard clams (especially M. mercenaria) has become prevalent. More information on the Florida hard clam fishery can be found on the northern quahog page.
VII. LITERATURE CITED & OTHER USEFUL REFERENCES
Abbott, RT & PA Morris. 1995. Shells of the Atlantic and Gulf Coasts and the West Indies, 4th Edition. Houghton Mifflin. New York, NY. USA. 350 pp.
Andrews, J. 1994. A Field Guide to Shells of the Florida Coast. Gulf Publishing. Houston, TX. USA. 182 pp.
Arnold, WS, Bert, TM, Marelli, DC, Cruz-Lopez, H & PA Gill. 1996. Genotype-specific growth of hard clams (genus Mercenaria) in a hybrid zone: variation among habitats. Mar. Biol. 125: 129-139.
Busby, D. 1986. An overview of the Indian River Clamming Industry and the Indian River Lagoon. Florida Sea Grant Extension Program. Technical Paper No. 44. Project IR-85-7. 45 pp.
Dillon, Jr., RT. 1992. Minimal hybridization between populations of the hard clams, Mercenaria mercenaria and Mercenaria campechiensis, co-occurring in South Carolina. Bull. Mar. Sci. 50: 411-416.
Stewart, VN. 1981. Sea-stats: a summary of information and statistics on Florida’s marine organisms and the marine environment. No. 7: Clams. Florida Department of Natural Resources. 11 pp.
Surge, D & KJ Walker. 2006. Geochemical variation in microstructural shell layers of the southern quahog (Mercenaria campechiensis): implications for reconstructing seasonality. Palaeoecol. 237: 182-190.
Report by: LH Sweat,
Smithsonian Marine Station at Fort Pierce
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Page last updated: 28 September 2010
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