Learn about the ecology, anatomy and reproduction of hard clams in Florida.
The hard clam, commonly called a quahog, is a bivalve (two shells) mollusc that lives in bays and estuaries along the Atlantic Ocean and Gulf of Mexico. There are two species of hard clams found in Florida, the northern hard clam (Mercenaria mercenaria) and the southern hard clam (Mercenaria campechiensis) and in areas where both species occur, they can form hybrids (or hybridize). Both clams have dirty-gray or whitish colored exterior shells, but the southern clam generally has larger, thicker shells. The interior of the shells are usually white, although the northern clam may have purple shading along the outer edges.
Of the two species, the northern clam is preferred by wholesale dealers because of its longer shelf life. Because of declines in the number of wild clams (northern) in Florida waters, hard clam aquaculture – large-scale raising of clams in a hatchery – supplements a large portion of Florida’s clam production.
Clam shells are hinged and held together by a ligament attached to its shells, which have interlocking grooves called cardinal teeth that help them fit together snugly. The ligament acts like a spring to push the shells open and each clam has two adductor muscles that it uses to pull the shells tightly closed. A soft membrane called the mantle lines the shell and encases the animal. The mantle has three folds, and the outermost secretes the calcium carbonate and protein that form the hard shell. Clams have no eyes, but they respond to touch, vibrations and chemical cues in their environment. They have two siphons, or tubes, that can be extended or retracted in response to threats from predators or fluctuations in water quality. Water flows in though one siphon carrying oxygen and food particles over the gills and mantle. Millions of cilia (tiny hairs) on the gills work together like a pump to push water out the other siphon, which expels the filtered water and waste. Because clams are filter feeders, they absorb and concentrate whatever pollutants are present, making them good indicators of the overall health of a body of water. Clams have a single, hatchet-shaped foot made of muscle tissue that is used to burrow into sand or mud.
Clams are protandrous hermaphrodites, meaning they begin life as males but often change to females. Approximately half of the population will undergo this sex change, which usually occurs by the end of the first year. In Florida, spawning occurs in spring and fall when water temperatures reach about 73 degrees Fahrenheit. The release of sperm into the water by male clams stimulates females to expel eggs. A female may spawn several times each year, producing millions of eggs.
Within 12 to 14 hours, the fertilized egg hatches into a microscopic creature called a trochophore, which is shaped like a pear and fringed with cilia that help it move. In less than a day, it transforms into a veliger, a free-swimming animal that has tiny wing-like lobes that propel it through the water. The foot, shell and body organs begin to form during the veliger stage, which lasts approximately 6 to 10 days. As the tiny shell develops, the veliger drops to the sea floor and sheds its lobes. When it touches bottom, it sends out thin filaments to hold it in place. As the clam matures, a muscular foot will replace these filaments, allowing the clam to bury itself in the sediments with only its siphons protruding.