Mussel

Mussel
	Mussels
Scientific classification
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Bivalvia
Subclasses
	Pteriomorpha (marine mussels); Palaeoheterodonta (freshwater mussels); Heterodonta (zebra mussels)

The common name mussel is used for members of several different families of clams (bivalve molluscs) from both saltwater and freshwater habitats. "Mussel" is a loose and inaccurate term, but it has historically been applied to those families of clams where the shell is longer than it is wide, being wedge-shaped or assymetrical-looking, and where the external color of the shell is dark blue or brown, as opposed to the more globular lighter-colored families of bivalves.

Marine mussel species live in intertidal and subtidal areas along coastlines worldwide. Freshwater mussel species inhabit lakes, ponds, rivers, creeks, canals, and similar habitats.

Freshwater mussels (several allied families, the largest being the Unionidae) and saltwater mussels (family Mytilidae) are not closely related at all. They are taxonomically grouped in different subclasses, despite some superficial similarities in appearance. For example, the freshwater Zebra mussels and their relatives in the family Dreissenidae somewhat ressemble marine mussels such as Mytilus species, and they can live attached to rocks and other hard surfaces in a manner similar to marine mussels, but they are classified with the Heterodonta, the taxonomic group which includes most of the bivalves that are commmonly referred to as "clams".

Anatomy

The mussel's external shell is composed of two hinged halves or "valves", which protect it from predators and desiccation. 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 dessication.

The shell is made of three layers. In the pearly mussels there is a nacreous layer, an inner iridescent layer of nacre (mother-of-pearl) composed of calcium carbonate that 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 ressembling 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 decay of leaf materials produce acids).

Like most bivalves, mussels have a large organ referred to as 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 not moving.

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 poured out which enters the groove and hardens gradually when it comes into 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.

Feeding

Blue mussels in the intertidal zone in Cornwall, England.

Both marine and freshwater mussels are filter feeders; they feed on plankton and other microscopic sea creatures which are free-floating in seawater. A mussel draws water in through its incurrent siphon. The water is then brought into the branchial chamber by the actions of the cilia located on the gills for cilliary-mucus feeding. The wastewater exits out through the excurrent siphon. The labial palps finally funnel the food into the mouth, where digestion can start.

Marine mussels are usually found clumping together on the wave-washed rocks, each one attached to the rock surface by its byssus. The clumping habit helps hold the mussels firm against the force of the waves. And at low tide those mussels found 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, and there is a larval stage that drifts for a period of three weeks to six months before settling down 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 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—an unconscious action that forms a cyst, where the glochidia remain for two to five weeks (depending on the temperature). They grow and then break free from the host and drop to the bottom of the water. If they land in a place that suits their needs, they will continue their development and begin their independent life.

Reproduction in the Dreissenidae (zebra mussels and their relatives) is similar to that of the marine mussels.

Distribution

Marine mussels are abundant in the low and mid intertidal zone in temperate seas all over the world, in both northern and southern hemispheres.

Other species of mussel do live in the tropical intertidal areas, but not in the same huge numbers as they do in the temperate zones.

Certain species of marine mussels prefer to live in salt marshes or quiet bays, while others thrive in pounding surf, completely covering exposed wave-washed rocks. In addition, some species have colonized abyssal depths in the vicinity of hydrothermal vents.

Freshwater mussels inhabit permanent lakes, rivers, canals and streams throughout the world in all but 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, because they need the calcium carbonate to build their shells.

Cultivation

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.

Intertidal growth technique, or bouchot technique: pilings, known in French as bouchots, are planted at sea; ropes, on which the mussels grow, are tied in a spiral on the pilings; some mesh netting prevents the mussels from falling away. This method needs an extended tidal zone.

One country in which mussels are cultivated extensively is New Zealand. The most common method in that country is to attach mussels to ropes which are hung off a rope back-bone supported by large plastic floats. The most common species cultivated in New Zealand is the New Zealand green-lipped mussel.

Mussels as food

Archaeologically, there is much evidence for humans having utilised mussels as a source of food for thousands of years. Nowadays marine mussels are still a popular seafood item, especially in Belgium and the Netherlands, where they are consumed with french fries ("mosselen met friet" or "moules frites"). In Italy they are a popular dish, often mixed with other sea food, or eaten with pasta. In Turkey mussels are either covered with flour and fried on shishs ('midye tava') or filled with rice and served cold ('midye dolma'). Mussels are usually consumed with alcohol (mostly with raki or beer). In France the Éclade des Moules is a mussel bake popular along the beaches of the Bay of Biscay. In Cantonese cuisine, mussels are cooked in a broth of garlic and fermented black bean. In New Zealand they are commonly served in a chili based vinaigrette. During the Second World War in the United States, mussels were commonly served in diners. This was due to the unavailability of red meat related to wartime rationing.^[1]. In Ireland and among the Irish Community in the West of Scotland they are a popular food: boiled and seasoned with vinegar, with the "bray" or boiling water as a supplementary hot drink.

Mussels can be smoked, boiled or steamed. As for all shellfish, mussels should be alive just before they are cooked because they quickly become toxic after they die. A simple criterion is that live mussels, when in the air, will tightly shut when disturbed; open unresponsive mussels are dead and should be discarded. Also closed mussel shells that are unusually heavy should be discarded as well, because usually contain only mud and can be tested by slightly moving the two shells away from each other. Mussel shells open by themselves when the mussels are cooked, revealingly the cooked soft parts.

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.^[2]

Although mussels are widely valued as a food item, mussel poisoning due to toxic planktonic organisms can be a potential 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 do not harm the mussels, even when they are 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 United States 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.

Inter-tidal herbivorous shellfish such as mussels and clams can help people reach a healthy balance of omega-3 and omega-6 fats in their diets, instead of the current (Western diets) 1:17. ³⁷ (Omega-3:6 ratio and human health)

References

^ Alton Brown, Good Eats
^ 'Jeannie Bastian'. The Mussels from Brussels. Accessed November 15 2006.

37. Robson, A. 2006. "Shellfish view of omega-3 and sustainable fisheries." Nature 444, 1002 Shellfish in NATURE