by Goutham Krishna
Bivalves are marine or freshwater molluscs characterized by a shell that is divided into two valves that are inter-connected to one another at a hinge. Bivalves are species that lack head and typical molluscan organs like radula and odontophore. Some of the primitive bivalves ingest sediments. The absence of the head and other molluscan organs of the bivalves can be explained by this largely sedentary and deposit-feeding or suspension-feeding lifestyle. Bivalves are also referred to as Lamellibranchiata and Pelecypoda in the scientific literature of the earlier centuries.
The majority of bivalves are marine and can be found in or on practically any substrate at any depth. Bivalves are common on rocky and sandy coastlines in shallow seas. Bivalves range in size from one millimeter (0.04 inch) to the enormous clam Tridacna gigas, which can grow to be more than 137 centimeters (54 inches) long. Bivalve species can be mainly categorized into clams, scallops, oysters, and mussels. Oysters are widely used for edible purposes by humans and other species. Also, bivalves are highly important in maintaining the marine food chain. Apart from marine habitats, bivalves are also seen in fresh water and brackish water.
Various species of Bivalvia are commonly seen on the Chennai coast. Some of them like Peacock mussel and White hammer oyster are used by the natives for edible purposes. Names and details of some of the bivalves common to the coromandel coast are listed below.
They are marine bivalvians belonging to the family, Arcidae. Arc shells are characterized by boat-shaped shells with long, straight hinge lines bearing many small, interlocking teeth. They are mainly found in tropical seas and hence are a common sight on the Chennai coast. They are harmless to human populations in any manner.
Two-toned Cardita, known as “Vaṇṇa vari maṭṭi” in Tamil is another bivalve that is commonly seen in Chennai coast. As its name suggests, Shells of two-toned Cardita are in dual-tone. They are used for fisheries purposes and are harmless to human beings.
This species is mainly seen in shallow water on beach sand. It travels between high and low tide marks with the tides, burying itself in the sand every time the waves expose it. it is usually about 2.5 cm long and is commonly found on the shores of Chennai.
Pinna bicolor is a species of bivalves belonging to the family Pinnidae. It is a tropical Bivalvia, majorly found in the Indian ocean. Henceforth it is a common sight on the coromandel coast.
Siliqua radiata is a Bivalvia belonging to the family Tellinidae) is commonly called a sunset shell. They inhabit the sandy bottom of beaches in small burrows.
The Peacock mussel, is an economically important mussel, a bivalve belonging to the family Mytilidae. It is harvested for food but is also known to harbor toxins and cause damage to submerged structures such as drainage pipes. It is native in the Asia-Pacific region and is also common on the Chennai coast.
This species can be distinguished by its elongate anterior dorsal margin and slightly posterior umbo. The overall shape is elongate, ovate. The posterior margin is nearly straight and forms a distinct angle at its confluence with the ventral margin, and belongs to the family of Veneridae.
The windowpane oyster (Placuna placenta) is a bivalve marine mollusk in the family of Placunidae. They are edible but valued more for their shells (and the rather small pearls). The shells have been used for thousands of years as a glass substitute because of their durability and translucence.
Mactra is a large genus of medium-sized marine bivalve mollusks or clams, commonly known as trough shells or duck clams. The word “trough” in the common name refers to the fact that all Mactra shells have a large ligamental pit at the hinge line, which in life contains a large internal ligament. Most bivalves in other families have an external ligament instead.
The White Hammer Oyster is one of the most unusual types of marine bivalve molluscs and is easily recognized by its greatly elongated hinge extensions and corrugated valves. Internally the shell valves exhibit a nacreous (pearly) appearance. The closely related species, the Black Hammer Oyster (Malleus malleus) has a much darker shell than the White Hammer Oyster, but sometimes occurs in the same localities
A beautiful scallop is usually found in the Indian ocean at shallow depths. They come in an array of gorgeous colors and patterns. The first occurrence of the Tranquebar scallop Volachlamys tranquebaria in the Vellar estuary, southeast India is reported ith photographs.