Most clams remain close enough to the surface that the tips of their siphons are exposed. Siphons, sometimes called the neck, are specialized tubes of the mantle. One siphon brings in water that carries oxygen for respiration and food for growth.
Incoming water passes over the gills where the oxygen is absorbed and where algae and other food particles are filtered out. Food particles are drawn over the gills by the action of cilia that cover the gills and mantle.
Food is trapped by mucus on the gills and carried to the food groove along the ventral edge of the gill, and then to the palos around the mouth. Filtered water and waste are then expelled by the other siphon.
The growth of hard clams is affected by tidal movement and by algal concentrations along the substratewater interface. Ideal conditions are moderate tidal movement with dense algal concentrations and adequate dissolved oxygen levels— above 4 ppm.
The rate of shell deposition is a major factor affecting growth. Clam shells are formed by the deposition of calcium carbonate crystals on an organic material. Shells grow in rings and/or layers deposited on this organic matrix.
Growth is not continuous, but incremental, with periods of shell dissolution. Shell growth occurs only during aerobic respiration when the valve is open. The mantle, which covers the inner surface of the shell, is responsible for shell formation because it secretes organic material that reacts with calcium carbonate to form shell.
New shell is deposited between the inner shell surface and the mantle epithelium. Shells grow laterally (in thickness) when growth is slow. Faster growing individuals usually have thinner shells.
All hard clams, whether natural or cultured, grow at different rates. Hard clams with maximum growth rates attain market size in 12 to 24 months. In 10 to 16 months, fast growers may be twice the size of slow growers.
The relatively long period of time needed to produce market size clams is an important limiting factor in their aquaculture. Growth rate may be influenced by genotype or genetics, because a percentage of any clam population originating from the same spawning matures at a faster rate.
Therefore, scientists have been developing selective breeding programs to establish faster growing genetic lines, which would have economic benefit. It has become apparent that removing slow-growing clams from a production system reduces production costs.
When clams grow at varying rates they must be handled more, which increases stress to the animals and decreases the predictability of cash flow. Environmental factors also influence growth rates. These include water temperature, food availability, salinity, water quality and tidal currents.
Generally, hard clams are 1 to 2 inches long at harvest (at about 2 to 3 years of age), with a meat weight of 18 to 20 grams. Growth slows with increasing age. In 7 to 8 years hard clams may be only 3 inches long. Hard clams are known to live for 30 years or longer.
Predation:
When clams are disturbed, they burrow deeper to avoid predators. Common predators are blue crabs, stone crabs, mud crabs, conch, sting rays, horseshoe crabs and snails, which feed mostly on juvenile or small clams.
Producers use rafts, trays, cages, bags and nets to exclude predators. Biological control of crabs has been attempted by stocking toadfish in culture trays. Toadfish do reduce the number of crabs and increase clam survival significantly, but the economic feasibility of stocking them is questionable.
Marketing:
Hard clams are marketed whole. About 30 percent are shucked and the remainder are marketed for raw or steamed consumption. Unshucked clams command the highest prices on a per clam basis. Higher prices are paid for clams designated in the market as littleneck and topneck clams, which range from 1 to 1 1/4inches thick. Size class designations may vary from state to state.
Hard clam prices fluctuate with the supply, which is dependent on the stocks of clams, access to production areas, weather conditions, harvest seasons, and the bacterial content of the growing environment.
In most southern states, farm-raised clams may be harvested year round, which offers a marketing advantage over wild-harvested clams in some states. Check with your state natural resources agency about regulations for harvesting cultured clams.
Author:
Jack M. Whetstone, Leslie N. Sturmer and Michael J. Oesterling
