Clam

Clam is a general term for almost any burrowing bivalve. They belong to Phylum Mollusca, Class Bivalvia (meaning two shelled). Clams are strictly aquatic creatures and they can come in a whole variety of sizes. Most of these creatures spends almost their entire lives buried in the sand except during their developing stages when they swim freely about their environment. They are a popular seafood dish worldwide and can be prepared in hundreds of different ways. Clams typically live about four to eight years although one clam was found which is believed to have lived for four hundred years, making it the world’s oldest living creature. 

Anatomy
The clam is a flat, round invertebrate that possesses a shell which consists of two separate valves (or halves. The two valves are held together by a hinge, and the shell is kept closed by a tense abductor muscle. Each valve is created by secretions from the mantle and they both consist of three layers (Porch, p468). The first, outermost layer is meant for protection and has a rough, bumpy texture. The middle layer is a hard, protective layer made of calcium carbonate. Lastly, the third, innermost layer is made of a smooth, pearly substance. Because it is a mollusk, the clam has a foot which it uses to burrow in the sand. The clam has two tube-like siphons that it uses to filter water. Water is brought into its body through the incurrent siphon and the water is later removed from its body through the excurrent siphon. The clam breathes using its gills. Water containing oxygen is brought into the gills and the many blood vessels within the thin-walled gills exchange carbon dioxide. The gills also assist in obtaining food. The incurrent siphon brings water into the gills and mucus secreted by the gills and the palps, which are organs surrounding the clams mouth, capture tiny bits of food. The food is then moved by cilia on the palps through the esophagus and into the stomach. The food particles are later digested by the digestive gland attached to the stomach. The intestines take care of indigestible materials and expels them through the anus. A clam has an open circulatory system with a pumping heart that sends its clear blood through the blood vessels to distribute food and oxygen. A clam also has one kidney to filter the waste from the blood. Within the clam’s nervous system are three ganglia which are connected by nerves and sensory organs, including light and touch receptors on the mantle's edge.

Reproduction
Clams reproduce sexually, and like most mollusks they are hermaphroditic; although some soft shelled clams are dioecious (meaning they have distinct male and female genders). Some clams begin as males and as they grow older turn into females. Clams mate in the spring when the water temperature begins to rise. The clams release both their sperm and their eggs into the water to be fertilized. Once a free-swimming sperm fertilizes an egg, the cells within the egg begin to divide until they form a free-swimming larva called a trochophore (Porch, p469). The trochophore shows no resemblance to its parents because it lacks a shell and is more round than flat. The trochophore matures into a veliger, which is the second stage of its development (see image at left). The veliger begins to resemble its parents with the beginning stages of a shell. The veliger has a unique way of feeding. A formation of cilia surrounding the mouth(called the velum) helps the veliger capture food particles. The velum is also used to push the veliger through the water (Russell, p5). The third and final larval stage is called the pediveliger. The pediveligar is no longer a swimming larva, instead it uses its ciliated foot to burrow. The pediveligar burrows into the sediment where it develops into a mature clam.

Ecology
From oceans to rivers to ponds, clams can be found in almost any aquatic environment. There are over 15,000 species of clams worldwide. Both freshwater and marine clams spend their lives burrowing freely through the sediment near the shore. Almost all clams are motile and can move about freely through the sand; although there are a few exceptions such as the Giant Clam, which is sessile and spends the majority of its life fastened to one spot. Clams are filter feeders, meaning they feed on plankton strained from the water using their siphon. Clams can vary in sizes but are typically small in comparison to other animals. The smallest known clams, the Condylocardia have shells that only measures 0.1 mm, while Giant clams (Tridacna gigas) have shells that can grow to be 1.2 m across. There are many natural predators of the clam including eels, starfish, sea snails, otters, and humans.

Clam Chowder
The word "chowder" is suspected to have originated from the old English word "jowter", which was at first a term for fish peddlers. Chowder was originally considered to be a poor man's meal and it consisted simply of fish and vegetables stewed in a pot. People have been consuming chowder since the 16th century and probably even before that. There are many varieties of clam chowder. In a way it is similar to curry in the fact that there are many different ways to prepare it. New England chowder is typically made with cream or milk combined with a mixture of onions, flour, bacon, and clams. Adding tomatoes is an option, but this idea was once shunned so strongly in New England that many proposed to the Maine legislature that it be made illegal. Unlike the thick, milky New England chowder, the Manhattan clam chowder has a clear broth and tomatoes are added to give it flavor and color. The less popular Rhode Island version of chowder is clear and consists of quahogs (hard clams), broth, potatoes, and onions. There are many other varieties of chowder and the way they are prepared depends on the chef and the person eating the chowder.