Bivalves: Bivalvia

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BIVALVES: Bivalvia

BLACK-LIPPED PEARL OYSTER (Pinctada margaritifera): SPECIES ACCOUNTS
ZEBRA MUSSEL (Dreissena polymorpha): SPECIES ACCOUNTS
COQUINA CLAM (Donax variabilis): SPECIES ACCOUNTS
QUEEN SCALLOP (Aequipecten opercularis or Chlamys opercularis): SPECIES ACCOUNTS
GIANT CLAM (Tridacna gigas): SPECIES ACCOUNTS

PHYSICAL CHARACTERISTICS

Bivalves have bodies that are flattened from side to side and completely surrounded by two shells called valves. Each valve is made up of a hard mineral called calcium carbonate and is joined to the other by a hinge on the back. Interlocking teeth or sockets form the hinge on the valves, which are then held together by an elastic ligament or fiberlike tissue made up mostly of protein. Powerful muscles contract ligaments to keep the valves closed and relax to open them. The bulge near the hinge is called the umbo and is the oldest part of the shell. The ridges that form around the umbo trace the growth of the valve. The valves are usually similar to one another in size, shape, color, and texture. The outer surface is usually plain, but some species have distinctive colors and patterns.

Inside the valves is the body surrounded by the mantle. The mantle makes the calcium carbonate that forms the valves. In some bivalves, the rear edge of the mantle forms special tubelike openings called siphons (SAI-fens), which take in water carrying bits of food and expel waste, eggs, and sperm into the water. The mantle holds digestive and reproductive organs and a muscular foot. The inside lining of the stomach is tough enough to grind up food. The foot can stretch outward and attach the bivalve to rocks, wood, and other hard surfaces. The very small head lacks the eyes, tentacles, and radula found in most other mollusks. The radula (RAE-jeh-leh) is part of the mouth that is thick or ribbonlike and has rows of teeth. Light-sensitive organs called eye spots may be found on other parts of the body.

GEOGRAPHIC RANGE

Bivalves are found worldwide in freshwater and ocean habitats.

HABITAT

All bivalves need fresh or sea water to breathe, reproduce, and feed. Ocean-dwelling species are found from the seashore to deep-sea habitats. However, the Australian Enigmonia lives on mangrove leaves or seawalls beyond high tide and gets its moisture from sea spray. Most species live on the bottom or burrow into mud and sand. Others attach themselves to rocks, wood, and other solid objects. A few burrow into rock and wood or live on the bodies of other animals.

DIET

Most bivalves eat bits of plants and animals floating in the water. A few species collect food from the bottom. Others absorb nutrients directly into their bodies or capture small crustaceans and worms by grabbing them with a special intake siphon.

BEHAVIOR AND REPRODUCTION

Most bivalves stay in the same place for much of their lives, but others are able to move around. Burrowers move up and down through mud and sand by extending their foot. Then they expand the tip of their foot to anchor themselves and pull their shelled bodies up or down in the burrow. Others "swim" through the water by clapping their valves together.

HOW THE GEODUCK GOT ITS NAME

The geoduck (gooey-duck) is the common name for the clam Panopea abrupta. It burrows in the sandy beaches along the North American shore of the Pacific Ocean from Alaska to California. The Nisqaully tribe of southern Puget Sound hunted and ate this clam and called it gweduc, which means "dig deep." The first European settlers in the region changed the name to gooeyduck or goeduck. In time, through countless misspellings, the bivalve became known as a geoduck.

Bivalves usually require both males and females to reproduce, although some species individuals either have the organs of both sexes or start out as males and later become females. Bivalve eggs and sperm are usually released into the water, where fertilization takes place. The eggs hatch into veligers (VEL-ih-jerz), or young, that live among and eat other plankton. Plankton is made up of microscopic plants and animals that drift about on ocean currents. Eventually, the veligers settle on rocks, wood, or the ocean bottom and begin to develop their valves.

BIVALVES AND PEOPLE

Clams, mussels, oysters, and scallops are raised commercially in the ocean for food. Oysters are sources of natural pearls and mother-of-pearl shell. For more than fifty years, cultured pearls have increased in both quantity and quality through advanced techniques in oyster culturing perfected by the Japanese.

Some bivalves are considered pests. They may concentrate bacteria, viruses, harmful chemicals and other pollutants in their bodies and can cause sickness and spread disease when eaten by humans. Shipworms burrow into and damage or destroy wooden structures, such as boats and piers. In the United States, introduced freshwater zebra mussels clog pipes of water treatment plants and irrigation systems, which cost millions to repair.

CONSERVATION STATUS

Habitat pollution and alteration are the greatest threat to freshwater bivalves, but the introduction of exotic species, such as zebra mussels, can also be disastrous. Nearly one hundred species of freshwater pearl mussels in Eastern North America are already extinct. Most of the remaining species are protected and officially listed by the government as either threatened or endangered. Over two hundred bivalve species are listed by the World Conservation Union (IUCN). Only eight, all giant clams, live in the ocean.

BLACK-LIPPED PEARL OYSTER (Pinctada margaritifera): SPECIES ACCOUNTS

Physical characteristics: Outside blackish valves with white to green spots are round and flat. The inner valve surfaces can be blue, gray, green, pink, and yellow. The valves measure 6 to 10 inches (150 to 250 millimeters) across. The mantle is orange, while the foot is gray or black.

Geographic range: This species naturally occurs in the Indian Ocean and the western to central Pacific, including the Hawaiian Islands. It is also raised commercially in French Polynesia, Cook Islands, Gilbert Islands, Marshall Islands, Solomon Islands, southern China, northern and western Australia, Seychelles, and the Sudan.

Habitat: Black-lipped pearl oysters live at depths of 3 to 130 feet (1 to 40 meters) attached to hard surfaces in and around coral reefs. This species prefers calm, clear waters often poor in nutrients.

Diet: They eat bits of plant and animal plankton.

Behavior and reproduction: Foreign particles or parasites stuck between the valve and the body are encased in hard, shiny layers of calcium carbonate forming a pearl.

Hermaphroditic adults first develop into males, then females. Eggs and sperm released into the water are fertilized there.

Black-lipped pearl oysters and people: This species is the most important source of mother-of-pearl used for carvings and inlays, as well as Tahitian black pearls.

Conservation status: Black-lipped pearl oysters are not considered threatened or endangered. ∎

ZEBRA MUSSEL (Dreissena polymorpha): SPECIES ACCOUNTS

Physical characteristics: The valves are narrowly triangular in shape and pointed at the front end. At the rear, the valves are smooth and appear swollen. They are alternately banded with dark brown and cream, suggesting a "zebra" pattern. Mature adults have valves about 2 inches (50 millimeters) long.

Geographic range: This species was first known from the Black and Caspian Seas. It has since been introduced into the canals and inland waterways of Western Europe. In the past twenty years it has also become established in the Great Lakes, on the Mississippi River, and in other major river systems in the United States.

Habitat: Zebra mussels lives on the bottom and attaches to rocks, wood, boats, and other hard surfaces at depths down to 195 feet (60 meters). Populations may contain one hundred thousand individuals per square yard (meter).

Diet: They eat plankton.

Behavior and reproduction: Smaller mussels can detach themselves and move around, but larger individuals cannot. This species is introduced to new bodies of water by ships releasing water containing mussel larvae.

Both males and females are required for reproduction. A single female can release as many as forty thousand to one million eggs into the water each season. The veligers live among plankton, but eventually settle on hard surfaces after eighteen to twenty-eight days. Total life span is about two years.

Zebra mussels and people: Introduction of this species into waterways in Europe and North America has clogged the pipes of power plants and irrigation systems. They also threaten native bivalve populations by eating all of the available food and taking up living space.

Conservation status: Zebra mussels are not considered threatened or endangered. ∎

COQUINA CLAM (Donax variabilis): SPECIES ACCOUNTS

Physical characteristics: The triangular valves are sculptured with ridges radiating out from center and come in white, yellow, orange, pink, purple, and blue. The inside surface of the valves is not pearly and is usually purple. The margins of the valves are finely toothed. Mature individuals grow up to 1 inch (25 millimeters) long.

Geographic range: They are found along the Eastern coast of North America from Chesapeake Bay to Florida and around the Gulf of Mexico to Yucatan.

Habitat: They burrow in sandy beaches that have wave action.

Diet: Coquina clams eat tiny plants and animals floating in the water.

Behavior and reproduction: They use their muscular foot to rebury themselves after being exposed by the waves and move up and down in the sand and along the beach.

Both males and females are required for reproduction. Eggs and sperm are released into the water, where fertilization takes place. They live up to two years.

Coquina clams and people: They are eaten in "coquina broth," and their shells are used for decorating gardens.

Conservation status: Coquina clams are not considered threatened or endangered. ∎

QUEEN SCALLOP (Aequipecten opercularis or Chlamys opercularis): SPECIES ACCOUNTS

Physical characteristics: The valves are flat and round and have about 20 ribs extending out from the umbo. On either side of the umbo is a pair of small, winglike extensions that are slightly unequal in size. The colors of the outer surface are variable, spotted or solid, and can be white, red, or orange. The right valve is lighter in color than the left. The inside of each valve is white. The margin of the mantle has lots of sensitive tentacles with eyes.

Geographic range: They are found in the Mediterranean Sea and eastern Atlantic coast from Norway to the Cape Verde Islands, the Azores and the North Sea. They are also raised on experimental farms in Spain, France, and the United Kingdom.

Habitat: Queen scallops live on all bottoms, except those covered with rocks, at depths down to 1,312 feet (400 meters), but are most common at about 130 feet (40 meters).

Diet: They eat tiny plants and animals floating in the water.

Behavior and reproduction: They escape danger by "swimming."

Individuals have the reproductive organs of both males and females at the same time. Eggs and sperm are released into the water, where fertilization takes place.

Queen scallops and people: People eat the entire body or just the muscle that closes the valves. The valves are sometimes worn as jewelry.

Conservation status: The species is not considered threatened or endangered. ∎

GIANT CLAM (Tridacna gigas): SPECIES ACCOUNTS

Physical characteristics: This largest of all living bivalves measures up to 53.9 inches (1,369 millimeters) long and weighs up to 579.5 pounds (262.8 kilograms). The whitish valves are thick, heavy, and have four to six distinct folds. The inside surfaces of the valves are white and smooth. The mantle is brightly colored, ranging from yellowish brown to olive green with shiny blue green spots. The openings to the siphons are quite distinctive.

Geographic range: They are found in the Southwestern Pacific from Philippines to Micronesia.

Habitat: Giant clams live at depths of 6 to 66 feet (2 to 20 meters) on coral reefs, partially buried in sand or rubble.

Diet: Giant clams rely on the nutrients made by algae that live only in the tissues of the clam's mantle. Giant clams supplement this diet with tiny bits of plants and animals floating in the water.

Behavior and reproduction: Adults move about coral reefs with their valves hinge-down. The valves remain open unless the clam is threatened. Larger individuals cannot completely close their valves.

Giant clams develop first as males and later become females. Eggs and sperm are released into the water, where they are fertilized. The life span is unknown but is estimated to range from decades to one hundred years.

Giant clams and people: People living on Pacific Islands harvest giant clams and eat the muscle that closes the valves. Giant clam shells have long been used to make mallets, hoes, scrapers, and wash basins. They are also raised to sell to people that keep them in salt water aquariums.

Conservation status: Giant clams are considered Vulnerable by the IUCN, which means they face a high risk of extinction in the wild. ∎

FOR MORE INFORMATION

Books:

Brusca, Richard C., and Gary J. Brusca. Invertebrates. Sunderland, MA: Sinauer Associates, 2003.

Gordon, D. G. Field Guide to the Geoduck. The Secret Life of the World's Biggest Burrowing Clam—from Northern California to Southeast Alaska. Seattle, WA: Sasquatch Books, 1996.

Gordon, D. G., N. E. Blanton, and T. Y. Nosho. Heaven on the Half Shell. The Story of the Northwest's Love Affair with the Oyster. Portland, OR: West Winds Press, 2001.

Nalepa, Thomas F., and Donald W. Schloesser, eds. Zebra Mussels: Biology, Impacts, and Control. Boca Raton, FL: Lewis Publishers, 1993.

Rehder, H. A. National Audubon Society Field Guide to North American Seashells. New York: Alfred A. Knopf, 1997.