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The word "mussel" is most frequently used to mean the edible bivalves of the marine family Mytilidae, most of which live on exposed shores in the intertidal zone, attached by means of their strong byssal threads ("beard") to a firm substrate.
In most marine mussels the shell is longer than it is wide, being wedge-shaped or asymmetrical. The external colour of the shell is often dark blue, blackish, or brown, while the interior is pearly violet or white.
The word "mussel" is also used for many freshwater bivalves, including the freshwater pearl mussels. Freshwater mussel species inhabit lakes, ponds, rivers, creeks, canals, grouped in a different subclass, despite some very superficial similarities in appearance.
Freshwater Zebra mussels and their relatives are not related to previously mentioned groups, even though they resemble many Mytilus species in shape, and live attached to rocks and other hard surfaces in a similar manner, using a byssus. They are classified with the Heterodonta, the taxonomic group which includes most of the bivalves commonly referred to as "clams".
General anatomy
The mussel's external shell is composed of two hinged halves or "valves". The valves are joined together on the outside by a ligament, and are closed when necessary by strong internal muscles. Mussel shells carry out a variety of functions, including support for soft tissues, protection from predators and protection against desiccation.
The shell has three layers. In the pearly mussels there is an inner iridescent layer of nacre (mother-of-pearl) composed of calcium carbinate, which is continuously secreted by the mantle; the prismatic layer, a middle layer of chalky white crystals of calcium carbonate in a protein matrix; and the periostracum, an outer pigmented layer resembling a skin. The periostracum is composed of a protein called conchin, and its function is to protect the prismatic layer from abrasion and dissolution by acids (especially important in freshwater forms where the decay of leaf materials produces acids).
Like most bivalves, mussels have a large organ called a foot. In freshwater mussels, the foot is large, muscular, and generally hatchet-shaped. It is used to pull the animal through the substrate (typically sand, gravel, or silt) in which it lies partially buried. It does this by repeatedly advancing the foot through the substrate, expanding the end so it serves as an anchor, and then pulling the rest of the animal with its shell forward. It also serves as a fleshy anchor when the animal is stationary.
In marine mussels, the foot is smaller, tongue-like in shape, with a groove on the ventral surface which is continuous with the byssus pit. In this pit, a viscous secretion is exuded, entering the groove and hardening gradually upon contact with sea water. This forms extremely tough, strong, elastic, byssus threads that secure the mussel to its substrate. The byssus thread is also sometimes used by mussels as a defensive measure, to tether predatory molluscs that invade mussel beds, immobilising them and thus starving them to death.
In cooking, the byssus of the mussel is known as the "beard" and is removed before the mussels are prepared.
Life habits
Feeding Both marine and freshwater mussels are filter feeders; they feed on plankton and other microscopic sea creatures which are free-floating in seawater.
Marine mussels are usually found clumping together on wave-washed rocks, each attached to the rock by its byssus. The clumping habit helps hold the mussels firm against the force of the waves. At low tide mussels in the middle of a clump will undergo less water loss because of water capture by the other mussels.
Reproduction Both marine and freshwater mussels are gonochoristic, with separate male and female individuals. In marine mussels, fertilization occurs outside the body, with a larval stage that drifts for three weeks to six months, before settling on a hard surface as a young mussel. There, it is capable of moving slowly by means of attaching and detaching byssal threads to attain a better life position.
Freshwater mussels reproduce sexually. Sperm is released by the male directly into the water and enters the female via the incurrent siphon. After fertilization, the eggs develop into a larval stage, which temporarily parasitize fish, attaching themselves to the fish's fins or gills. Prior to their release, the larvae grow in the gills of the female mussel where they are constantly flushed with oxygen-rich water. In some species, release occurs when a fish attempts to attack the mussel's minnow or other mantle flaps shaped like prey.
The Glochidia (larvae) are generally species-specific, and will only live if they find the correct fish host. Once the larval mussels attach to the fish, the fish body reacts to cover them with cells forming a cyst, where the glochidia remain for two to five weeks (depending on temperature). They grow, break free from the host, and drop to the bottom of the water to begin an independent life.
Predators Marine mussels are eaten by humans, starfish, seabirds, and by numerous species of predatory marine gastropods. Freshwater mussels are eaten by otters, raccoons, ducks and geese.
Distribution and habitat
Inshore waters support the native Blue mussel which can reach 9 cm in legnth. Wave-swept rockey shores have beds of the larger California Mussels, which grows to 25 cm in legnth.
Freshwater mussels inhabit permanent lakes, rivers, canals and streams They require a constant source of cool, clean water. They prefer water with a substantial mineral content, using calcium carbonate to build their shells.
As food
Humans have used mussels as food for thousands of years and continue to do so. Tthe most commonly eaten in BC are Mytilus edulis (Blue Mussel) which has smooth outer shell durface, and Mytilus californianus (California Mussel) which has raised radial ribs. Mussels are found in groups, attached to rocks or seaweed on Pacific Ocean beaches. They are easily gathered by hand and there is no minimum size limit. Once harvested, place your catch in a bucket and cover with a damp cloth.
Freshwater mussels nowadays are generally considered to be unpalatable, though the native peoples in North America ate them extensively. During the second World War, mussels were commonly served in diners. This was due to the unavailability of red meat related to
Preparation
Mussels can be smoked, boiled, steamed, roasted, barbecued or fried in butter or vegetable oil. As with all shellfish, except shrimp, mussels should be checked to ensure they are still alive just before they are cooked; enzymes quickly break down the meat and make them unpalatable or poisonous after dying or un cooked. Some mussels might contain toxins. A simple criterion is that live mussels, when in the air, will shut tightly when disturbed. Open, unresponsive mussels are dead, and must be discarded. Unusually heavy, wild caught, closed mussels may be discarded as they may contain only mud or sand. (They can be tested by slightly opening the shell halves.) A thorough rinse in water and removal of "the beard" is suggested. Mussel shells usually open when cooked, revealing the cooked soft parts.
Although mussels are valued as food, mussel poisoning due to toxic planktonic organisms can be a danger along some coastlines. For instance, mussels should be avoided along the west coast during the warmer months. This poisoning is usually due to a bloom of dinoflagellates (red tides), which contain toxins. The dinoflagellates and their toxin are harmless to mussels, even when concentrated by the mussel's filter feeding, but if the mussels are consumed by humans, the concentrated toxins cause serious illness.
Source: Wikipedia