The common name mussel is used for members of several different families of clams or bivalve molluscs, from both saltwater and freshwater habitats. The one thing that these different groups have in common, is that they have a shell whose outline is somewhat elongated and asymmetrical compared with that of many other edible clams, the shells of which are often more or less rounded or oval in shape.
The word "mussel" is most frequently used to mean edible bivalves of the marine family Mytilidae, most of which live on exposed shores in the intertidal zone, attached by means of strong byssal threads ("beard") to a firm substrate. A few species (in the genus Bathymodiolus) have colonized hydrothermal vents associated with deep ocean ridges.
In most marine mussels the shell is longer than it is wide, being wedge-shaped or asymmetrical. The external color of the shell is often dark blue, blackish, or brown, while the interior is silvery and somewhat nacreous.
The word "mussel" is also used for many freshwater bivalves, including the freshwater pearl mussels. Freshwater mussel species inhabit lakes, ponds, rivers, creeks, canals, and similar habitats. These bivalves belong to several allied families, the largest family being the Unionidae. They are not closely related to saltwater mussels; they are taxonomically grouped in a different subclass, despite some very superficial similarities in appearance.
The freshwater Zebra mussels and their relatives in the family Dreissenidae are not related to either of the 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".
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 is made of three layers. In the pearly mussels there is an inner iridescent layer of nacre (mother-of-pearl) composed of calcium carbonate, 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 an extremely tough byssus thread that secures the mussel to its substrate. The byssus thread is also used by mussels as a defensive measure to tether predatory molluscs, such as dog whelks, that invade mussel beds, immobilising and starving them to death.
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.
Freshwater mussels also reproduce sexually. Sperm released by the male directly into the water enters the female via the incurrent siphon. After fertilization, the eggs develop into the larval stage called glochidia. The glochidia grow in the gills of the female where they are constantly flushed with oxygen-rich water. For a time, these glochidia are parasitic on fish, attaching themselves to the fish's fins or gills. Glochidia 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. If they land in a suitable location, they will continue development and begin an independent life.
Marine mussels are abundant in the low and mid intertidal zone in temperate seas globally.
Other species of marine mussel live in tropical intertidal areas, but not in the same huge numbers as in temperate zones.
Certain species of marine mussels prefer salt marshes or quiet bays, while others thrive in pounding surf, completely covering wave-washed rocks. Some species have colonized abyssal depths near hydrothermal vents.
Freshwater mussels inhabit permanent lakes, rivers, canals and streams throughout the world except in the polar regions. They require a constant source of cool, clean water, with bottoms that are not muddy. They prefer water with a substantial mineral content, using calcium carbonate to build their shells.
In 2005, China accounted for 40 per cent of the global mussel catch according to a FAO study. Within Europe, Spain remained the industry leader.
Freshwater mussels are used as host animals for the cultivation of freshwater pearls. Some species of marine mussel, including the Blue Mussel (Mytilus edulis) and the New Zealand green-lipped mussel (Perna canaliculus), are also cultivated as a source of food.
There are a variety of techniques for growing mussels.
In India mussels are popular in Kerala, Bhatkal, and Goa. They are either prepared with drumsticks, breadfruit or other vegetables, or filled with rice and coconut paste with spices and served hot. Mussels can be smoked, boiled, steamed or fried in batter.
As with all shellfish, mussels should be checked to make sure they are still alive just before they are cooked; they quickly become toxic after they die. 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. Closed mussel shells that are unusually heavy should be discarded as well, because they usually contain only mud. They can be tested so see if they are full of mud or sand by slightly moving the two shells away from each other.
A thorough rinse in water and removal of "the beard" is suggested. Mussel shells will open by themselves when cooked, revealing the cooked soft parts. (Any unopened shells should be discarded.)
In Belgium, mussels are often served with fresh herbs and flavorful vegetables in a stock of butter and white wine. Frites/Frieten and Belgian beer are popular accompaniments. Months with an "R" in their name (September to April) are said to be the "in" season for mussels.
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 of the United States 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, such as paralytic shellfish poisoning. Usually the U.S. government monitors the levels of toxins throughout the year at fishing sites. See Red Tide.
Freshwater mussels nowadays are generally considered to be unpalatable, though the native peoples in North America utilized them extensively.
Mussels are not a source of omega-3 fatty acids. Reference http://www.truestarhealth.com/Notes/1839009.html