Clams and oysters are members of the phylum Mollusca, a group of over 100,000 species that also includes snails, octopods and squid. Clams and oysters are bivalved animals, those animals having two shells. The hard clam or quahog, Mercenaria mercenaria , and the eastern oyster, Crassostrea virginica., are native to the very productive intertidal and subtidal estuarine ecosystems along the East Coast of the United States.The habitat of the eastern oyster extends to the Gulf of Mexico.
Anatomy of the Clam
Hard clams live beneath the surface in the sand. They use a muscular foot to burrow and have a siphon that reaches out of the sand and acts like a straw for sucking in and discharging water from their body cavity. As they feed, water is drawn in the incurrent siphon passed through their gills and discharged out of the excurrent siphon. The gills, aside from having a respiratory function, (taking up oxygen and expelling carbon dioxide) are also used in feeding. Microscopic plantlife called phytoplankton that are present in large quantities in the estuarine waters are trapped in the tiny gill filaments as water is passed through the clam’s body. The tiny particles of food are then moved by little cilia or hairs from the gills into the clam’s digestive system. Waste that is expelled by the digestive system are washed out via the excurrent siphon. Oysters also feed and respire using their gills but they do not live beneath the sand. Oysters are epibenthic which means that they live on the surface either singly on the sand or reefed together in clumps. Oysters do not have discrete siphons, water is simply drawn in and expelled from the body by currents that are created by the beating of the gill cilia.
In the spring, the sun warms the water of the shallow tidal flats triggering oysters and clams to spawn. Both species are broadcast spawners meaning that males and females just release sperm and eggs into the water. When they meet by chance sperm and egg join to become a zygote or fertilized egg. The fertilized egg metamorphoses into a small (about 0.5 mm) swimming larvae called a veliger. These larvae, despite the fact that they can swim are planktonic and are moved along with the currents and tides being distributed throughout the local embayment and in some cases many hundreds of miles from the parents beds. During the next one to four weeks the veliger larvae matures and finally drops out of the water column onto the sand and begins its life as a tiny little bivalve. Oysters cement themselves onto hard substrates like shells or rocks or other oysters. Hard clam seed attach themselves to sand grains with tiny little byssal threads until they are large enough to burrow beneath the surface.
Hard clams take about two years to reach the littleneck size (1″across the hinge), four years to reach cherrystone size (roughly the size of your fist) and 6 years to reach quahog size (fill your open hand).
Sex change is quite usual in many species of clams and oysters. Quahogs are serial hermaphrodites. Young quahogs are males. As they grow, they switch to being female and remain so for the rest of their lives. The eastern oyster, C. virginica , is not hermaphroditic sexes are fixed and separate through the life of each individual. In contrast,European oysters,Ostrea edulis , switch sex back and forth throughout their lives.