A) brown mussel; B) Atlantic pearl oyster, C) amber pen shell Mollusks, known as bivalves, have two half shells, known as valves hinged together. Also known as pelecypods, they are found mostly in marine habitats but some live in and freshwater. Their shells enclose a fold of the mantle, which in turn surrounds the body and organs. Many are born with a true head but it largely disappears by the time they are adults. They breath through gills on either side of the mantle. The shells of most bivalves close shut to protect the animal inside. Their class name Pelecypida, or “hatchet foot,” is a reference to the wide expandable foot used to burrow and anchor the animal in soft marine sediment.
Bivalves include clams, mussels, oysters and scallops. They vary a great deal in size. The largest, the giant clam, is 2 billion times larger than the smallest. Bivalves like clams, oysters, scallops and mussels are much less mobile than univalves. They foot is a protrusion that is used mainly to pull the animal down into the sand. Most bivalves spend their time in a stationary position. Many live buried in the mud or sand. The most mobile bivalves are scallops..
Bivalves are a diverse group that inhabit virtually the entire world ocean, from the balmy tropics to the sub-zero Arctic, and from the deep ocean to sandy and rocky shorelines. A few have even taken up residence around hydrothermal vents found deep in the Pacific Ocean, below 4,000 meters (13,000 feet). There is some disagreement over the number of species. Wikipedia and NOAA say 9,200. Animal Diversity Web says 15,000. The University of Michigan says “over 20,000 species of bivalves around the world, and probably thousands not yet discovered.”
Websites and Resources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Fishbase fishbase.se ; Encyclopedia of Life eol.org ; Smithsonian Oceans Portal ocean.si.edu/ocean-life-ecosystems
Bivalve mollusks have an external covering that is a two-part hinged shell that contains a soft-bodied invertebrate. Like fish, bivalve mollusks breathe through their gills. As filter feeders, bivalves gather food through their gills. Some bivalves have a pointed, retractable "foot" that protrudes from the shell and digs into the surrounding sediment, effectively enabling the creature to move or burrow. Bivalves even make their own shells. An internal organ called the mantle secretes calcium carbonate so that as the inner invertebrate grows, the outer shell provides a roomier home. [Source: NOAA]
The shells of bivalves grows in annual increments, which makes it possible to determine their age. According to Animal Diversity Web: A bivalve is characterized by possessing two shells secreted by a mantle that extends in a sheet on either side of the body. The oldest part of the shell, the umbo, can be recognized as a large hump on the anterior end of the dorsal side of each shell. The two shells are joined at the dorsal end by a region called the ligament. The ligament is comprised of the tensilium and resilium. Together they open the shells at rest. A bivalve closes its shells by contracting its powerful adductor muscles. Commonly there are two, an anterior and a posterior one, but in some taxa (such as scallops) there is only a single, central one. [Source: : Derek Kellogg and Daphne G. Fautin., Animal Diversity Web (ADW), January 26, 2001]
The body is laterally compressed. The only external structures are the labial palps; in some groups, there are sensory tentacles and photoreceptors at the edge of the mantle. Bivalves also possess two ctenida (in most cases) and a muscular foot. The edges of the mantle are fused in some taxa and prolonged to form tube-like siphons. One siphon carries water to the mantle cavity (the inhalent siphon) and one from it (the exhalent siphon); in many taxa they are fused but the water streams remain separate. A bivalve uses its muscular foot either to attach itself to a substrate or to burrow. Scallops propel themselves through the water by jet propulsion: rapid closing of the valves squirts water out of the mantle cavity, and the animal "swims" in the opposite direction. /=\
The two half shells (valves) of the bivalve are attached to each other by a strong hinge. The tasty past of the animal that people eat is the large muscle, or adductor, attached to the center of each valve. When the muscle contracts, the shell closes to protect the soft part of the animal. The muscle can exert force only to close the shell. To open the shell relies entirely on a little rubbery pad of protein just inside the hinge.
Adam Summers, a professor of bioengineering at the University of California at Irvine, wrote in Natural History magazine, “The rubbery pad gets squashed when the shell closes, but as the closing muscle relaxes, the pad rebounds and pushes the shell back open. That’s why when your shopping for live bivalves for dinner, you want the closed ones: they are manifestly alive because they’re still holding their shells tightly shut.”
Bivalves have very small heads and do not have a radula, the mouthpart that snails and gastropods use use to rasp away at their food. Most bivalves are filter feeders with modified gills designed for straining food, carried to them in water currents, as well as breathing . Water is often drawn in and pushed out with siphons. Bivalves that lie with their shell's open suck water through one end of the mantle cavity and squirt it out through a syphon at the other. Many barely move.
Many bivalves dig deep into mud or sand. At just the right depth they send two tubes up to the surface. One of these tubes is a current siphon for sucking in seawater. Inside the clam’s body this water is finely filtered, removing plankton and tiny floating pieces or organic matter known as detritus before being squirted back out through the second excurrent siphon.
Bivalve Feeding and Predators
According to Animal Diversity Web: Most bivalves are filter feeders, but some are scavengers or even predators. The four main feeding types of bivalves are defined by gill structure. In protobranchs, the ctenida are used only for respiration and food is caught by the labial palps. In filibranchs and lamellibranchs, the ctenida trap the food particles in their mucous coating and transfer the food to the labial palps via ciliary action. These two groups differ in that the branches of the ctenida are connected only by ciliary junctions in filibranchs whereas lamellibranchs have tissue connecting the branches of the ctenida. A septibranch bivalve has a septum across its mantle cavity, which functions to pump in food. Thus, Bivalvia is the only molluscan class characterized by the absence of a radula. [Source: : Derek Kellogg and Daphne G. Fautin., Animal Diversity Web (ADW), January 26, 2001]
Despite the protection of their shells, bivalves are vulnerable to predation by sharks, rays, octopus, starfish and predatory gastropods. Bilvalves are most vulnerable as larvae, because they are eaten by planktivores and are easily swept away from desirable benthic (bottom-dwelling) areas by ocean currents.
With their hard shells that are difficult to pry open when closed, you might think that there would be few predators that could prey on bivalves. But that is not true. A number of animal species have developed means to get around their defenses. Some birds and fish have teeth and bills that are able to crack or split open the shells. Octopuses can pull the shells open with their suckers. Sea otters cradle the shells on their chests and crack the shells open with rocks. Conches, snails and other gastropods drill through the shells with their radula.
Bivalve Reproduction and Development
Bivalves are oviparous (young are hatched from eggs) and iteroparous (offspring are produced in groups). Reproduction is external, meaning the male’s sperm fertilizes the female’s egg outside her body. All bivalves are broadcast spawners, which means eggs and sperm are released by individuals in sea and may combine with those of animals of the same species
Spawning can be dependent on a number of factors which including salinity, currents, air exposure, and temperature. As broadcast spawners, there is no parental investment . Young develop independently in the water column, drifting as plankton. [Source: Albert Gamez, Animal Diversity Web (ADW) /=]
Most marine bivalves go through a trochophore stage before turning into a free-swimming veliger larva. This type of larva looks like a miniature bivalve with a row of cilia along the edge of the mantle. Freshwater species lack these stages. Instead, some go through a larval stage known as the glochidium. Rather than being free-swimming, a glochidium attaches to fish or other objects that will not be swept downstream. Glochidia can be serious pests of freshwater fish. /=\
Bivalves, Their Ecosystem Roles and Humans
Many bivalve species play important roles in aquatic and marine ecosystems by filtering the water and serving as habitat and prey for a variety of sea life. Bivalves such as clams, mussels and scallops are important food sources for many creatures. Because they feed directly on abundant material in sea water they can form colonies of incredible size and density, especially in sheltered inner bays, where the sand and mud substrate they love tend to collect.
Bivalves influence phytoplankton concentrations through “top-down” grazer control. According to Animal Diversity Web: This action reduces particle density within the water and increases the amount of light which can reach benthic (bottom-dwelling) organisms. Bivalve waste can be assimilated as food for phytoplankton growth. Furthermore, the bivalve beds form a sheltering hard-substrate habitat, housing numerous invertebrate species, and the oysters themselves are food for higher-order carnivores (animals that mainly eat meat or animal parts). [Source: Albert Gamez, Animal Diversity Web (ADW) /=]
According to Animal Diversity Web (ADW): Many bivalves (such as clams or oysters) are used as food in places all over the world. Pearl oysters are used for commercial production of pearls. Bivalves can also cause economic damage. The glochidia larvae of some freshwater mussels can be serious parasites of fish, and some marine bivalves bore through wood, causing damage to wooden ships, pilings, and other wood structures. NOAA estimated the 2011 economic value of commercial bivalve mollusk harvesting at about $1 billion annually in the U.S., and the weight of the harvest was estimated at 70 million kilograms (153.6 million pounds).
Mussel is the common name used for members of edible bivalve molluscs from the marine family Mytilidae. Mussels grow fast and have high reproduction rates. They first mature as males, then later develop female reproductive capabilities. Each blue mussel female can produce between 50 and 200 million eggs during a spawning event.. Mussel shells are longer than they are wide and are wedge-shaped or asymmetrical. The external color of the shell is often dark blue, blackish, or brown, while the interior is silvery and nacreous. There are 412 mussel species. They include the blue mussel, Asian green mussel, Mediterranean mussel, Ribbed mussel and
Mussels are good scavengers. They remove many pollutants from the water. They also produce a strong glue that scientists are studying because it bonds well even in cold water. Mussels use the glue to secure themselves to rocks or other hard surfaces and are able to maintain a firm grip even under strong waves and currents. They often grow in large clusters and sometimes present problems to ships and power plants by clogging up intake valves and cooling systems. Mussels are easily raised in aquiculture systems. Some species live in fresh water.
The glue used by saltwater mussels to secure themselves to rock is made of proteins fortified with iron filtered from sea water. The glue is administered in dabs by the foot and is strong enough to allow the shell to cling to Teflon in crashing waves. Automakers use a compound based on blue mussel glue as an adhesive for paint. The glue is also being studied for use as a sutureless wound closure and dental fixative.
Blue mussels (Scientific name: Mytilus edulis) are also known as edible mussels. They are five to ten centimeters (two to four inches) at maturity and can grow up to 20 centimeters (eight inches in length. Blue mussels are economically and environmentally important filter-feeding bivalves. They are commonly harvested for food throughout the world, from both wild and farmed sources. In the U.S. they can be found off Alaska, New England and Mid-Atlantic, Southeast, West Coast [Source: NOAA]
The blue mussels shell is black, blue-black or brown, tear-drop shaped, and has concentric lines marking the outside; the inner shell is white. The ‘beard’ is the byssal threads allowing the mussel to attach to substrate. Like oysters, clams and scallops they are bivalve mollusks, and have a hinged shell. Adults are sessile (fixed in one place) and inhabit both intertidal and subtidal areas.
Mediterranean mussels (Mytilus galloprovincialis) have shells that are black–violet in color and elongated and asymmetrical, compared with the shells of other edible bivalves such as scallops and clams, which are often more or less rounded or oval. The Mediterranean mussel is native to the Mediterranean coastline, but is an invasive species found in many other parts of the world due to unintentional transport. They are also known as the black mussel because the shell can be dark blue or brown to an almost black color. Mediterranean mussels are large and have a smooth shell and can grow up to 15 centimeters (six inches) but is typically between five to eight centimeters (2-3 inches) in length. Both shells of one individual two shells are equal, each with a rounded and a slightly bent edge, almost four-sided in shape. [Source: Nicholas Argent, Citrus Reef]
Zebra mussels — thumb-nail mollusk from the Black Sea and Caspian, Sea — are an invasive species in the U.S. They have invaded the Great Lakes, clogging intake valves and pipes, causing light to dim, rudders to freeze up and causing $3 billion worth of damage to factories and utilities as of the early 2000s. The invaders were carried into Lake St. Claire Ontario in 1988. They liked it there very much and attached themselves to hard surfaces and accumulated in great numbers, with as many as 70,000 accumulating on one square foot. Getting rid of them have proved to be impossible and business lost $149 million because of the creatures every year. [Source: National Geographic]
Image Sources: Wikimedia Commons, NOAA
Text Sources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.
Last Updated April 2023