Hard clam aquaculture is the largest and most valuable of the shellfish aquaculture industries on the East Coast. It accounts for more than $50 million in economic value annually.
Hard clams are bivalve mollusks that live in saline (>25 parts per thousand) waters and cannot tolerate low salinities or freshwater for an extended period. Hard clams occur naturally all along the Atlantic coast from Nova Scotia to Florida.
They have been introduced along the shore of the Gulf of Mexico from Florida to Yucatan, as well as along the West Coast of the United States, in the British Isles, and in parts of France. The distribution of hard clams is determined by hydrodynamic factors and, possibly, sediment types and depth.
Sediment characteristics affect the number and types of both invertebrates and their fish predators. Hard clams support a major commercial fishery along the entire East Coast.
New York, Rhode Island, New Jersey and Virginia are the leading states for hard clam commercial landings. The southeastern states are the leading aquaculture producers of hard clams.
Reproduction cycle:
Hard clams usually spawn in the spring, summer or fall. The optimal range of water temperatures (79 ºF or 26 ºC) occurs at different times of the year at different latitudes. Clam reproduction occurs earlier in the year at lower latitudes. Dimodal (two peaks) or polymodal (multiple peaks) spawning takes place in southern populations, and spawning may occur more than once per spawning season.
There are several stages in the reproductive cycle, which for discussion purposes may begin with the resting and/or spent stage. In this stage the clam completely or almost completely lacks gametes (eggs and sperm).
The resting stage is followed by the early development stage, during which the follicle walls thicken and immature gametes develop. In the late development stage, the follicles rapidly expand to accommodate the larger and more numerous gametes. In the ripe stage, the follicles are fully expanded and thin-walled.
The lumen of female follicles contains mature ova (eggs), while mature sperm dominate the lumen of male follicles. The germinal ducts have begun to expand and may contain a few mature gametes. Mature gametes are released and fertilization takes place externally. The ripe stage is followed by the resting and/or spent stage to complete the reproductive cycle.
The development of the veliger larvae is complete 24 hours after fertilization. These larvae swim, but are moved primarily by tidal currents. The larvae grow to a maximum size of 200 to 275 micrometers.
By the sixth to tenth day, the skin-like outside tissue, called the mantle, starts to form two shells and the umbo. The umbo is the rounded area of the shell just above the hinge. With the extra weight of the shell, larvae no longer swim freely and settle to the bottom.
Only 10 percent of fertilized eggs survive to this stage. During metamorphosis, the clam “seed” burrows where it remains mostly immobile. Clams prefer a combination of mud and sand as substrate but other suitable substrates are pure sand, gravel and mud.
Larval setting:
Many bivalve species attach to sand grains or other debris by one or several strong byssus threads. Byssus threads are thin strands secreted by a gland in the middle line of the surface of the foot.
Hard clams lose their ability to make byssus as they grow older. Clams may release from this attachment and may crawl or be moved by currents to another location where they attach again. As clams mature, they use the muscular foot to burrow into the bottom sediments.
By alternately extending, swelling and contracting the foot, the body is pulled down into the substrate. The whole body usually is below the substrate, except for the incoming and outgoing siphons. The body of the clam is completely enclosed in a mantle, which is subdivided into lateral lobes that secrete a calcium carbonate shell.
The mantle covers the foot and visceral mass and is connected to the shell by pallial muscles close to the edge of the shell. This line of mantle attachment to the shell is seen on the inner surface of the shell as the pallial scar line.
The two valves of the shell are joined dorsally by an elastic hinge ligament, which acts as a spring that forces the valves apart when the adductor muscles relax. The dorsal margin of each valve bears a prominent point near the hinge ligament called the umbo. This is the oldest part of the shell, and around it are the concentric lines of shell growth.
Each umbo may point slightly to the anterior so that it is usually possible to determine right and left valves.
Authors:
Jack M. Whetstone, Leslie N. Sturmer and Michael J. Oesterling
