Mussel

Mussel is the common name, which is used for members of several families of bivalve molluscs. They come from saltwater and freshwater habitats. The common thing in these groups is a shell with an elongated outline. This is different from other edible clams which are more or less rounded or oval. The word "mussel" is frequently used to define the bivalves of the marine family of Mytilidae, which includes many different types of mussels. Most of them live on exposed shores in the intertidal zone, attached by means of their strong byssal threads. 

These threads in turn form a firm substrate. A few species that belong in the genus Bathymodiolus have colonised extreme conditions like the hydrothermal vents. These hydrothermal vents are associated with deep ocean ridges. The most common examples of this species that people generally know are the blue mussel. 

General Characteristics of Mussel

In most of the marine mussels, the shell is longer than its width. And it is in wedge-shaped or asymmetrical form. The external colour of the mussel shell can range from dark blue to blackish, or brown. The interior of the mussel shell is silvery and somewhat nacreous which is lustrous rainbow colours. As mentioned, the common name "mussel" is used for many freshwater bivalves. It also includes freshwater pearl mussels. Freshwater mussels are generally known as muckets and are generally found in lakes, ponds, rivers, creeks, and canals. They are classified in a different subclass of bivalves, although there are some very superficial similarities in appearance.

Freshwater zebra mussels and their relatives in the family Dreissenidae are not related to previously mentioned groups. These zebra mussels may resemble many Mytilus species in shape, and also live attached to rocks and other hard surfaces using a byssus. They are instead classified with the Heterodonta, which is a taxonomic group that includes most of the bivalves commonly referred to as clams.

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The Making of Mussels

The external mussel shell is made up of two hinged halves which are known as "valves''. The valves are joined together on the outside by a ligament. The valves are closed as per the necessity of the strong internal muscles. These internal muscles are the anterior and posterior adductor muscles. The mussel shell carries out diverse sets of functions. These functions include providing support for soft tissues, protection from predators and protection against desiccation.

The shell of the mussel has three layers. In the pearly mussels, there is an inner iridescent layer of nacre, known as mother-of-pearl. It is composed of calcium carbonate, which is continuously secreted by the mantle. The middle layer is the prismatic layer, composed of chalky white crystals made up of calcium carbonate in a protein matrix. The outer layer is the periostracum, which is a pigmented layer resembling skin. The periostracum is composed of a protein called conchin. The function of the protein is to protect the prismatic layer from wearing or erosion such as scratch. It also protects the layer from dissolution by acids. This is especially important in freshwater forms. The decay of leaf materials produces acids which can cause damage to the layer.

Like most bivalves, mussels have a large organ called a foot. For freshwater mussels or the muckets, this foot is large, muscular, and generally hatchet-shaped. It is used to pull the animal through the substrate ( sand, gravel, or silt) in which it lies buried. It does this by repeatedly advancing the foot through the substrate. Then by 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.

On the other hand, In marine mussels, the foot is smaller, tongue-like in shape. It has a groove on the ventral surface that is continuous with the byssus pit. In this pit, a very bad secretion is released. It further enters the grooves and hardens slowly as it comes into contact with seawater. Because of this, there is a formation of extremely tough, strong, elastic, byssal threads. These byssal threads then attach the mussel to its substrate. As a result, the mussel remains sessile in areas of high flow. These byssal threads are also sometimes used for defence by the mussels. It does so by tethering the predatory molluscs like the dog whelks which invade the mussel beds. Thus, this immobilizes the molluscs and they then starve to death. In culinary art, the byssus of the mussel is typically known as the "beard". At the end of the cooking process once the mussel is opened and the byssus is removed. The inner anatomy of the mussel is visible in the given picture.

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How Do Mussels Live and Where are They Found?

Both mussels - freshwater and marine are known to be filter feeders. They typically feed on planktons and other microscopic organisms in the sea that are found floating freely. A mussel takes in water through its incurrent siphon which is an inhalant. Following this, the water is brought into the branchial chamber by the cilia which are located on the gills for ciliary-mucus feeding. Then the wastewater is removed through the excurrent siphon which is an exhalant. The food is taken by the mouth through the funnel-shaped labial palps, where the digestion begins. 

Mussels that are found in marine ecosystems are found clumped together on wave-washed rocks, where each one is attached to the rock via byssus. Due to the clumping habit, mussels are held firm against the force of the waves. When there is low tide, the mussels clump together. The mussels in the middle of the clump will then undergo less water loss because of the water capture by the other mussels. 

The mussels that are present in the marine and freshwater ecosystems are gonochoristic. They have separate male individuals and female individuals. In the marine mussels, fertilisation takes place outside the body. The larval stage usually drifts for a period of three weeks to six months. It then settles on the hard surface as the young mussel. Once settled on the hard surface the mussel is capable of moving slowly by the process of attachment and detachment of the byssal threads. This is to attain a better life position. 

The muckets have a sexual mode of reproduction. The males release the sperm directly into the water. It then enters the female through the incurrent siphon which is used for inhalation. The fertilised egg then develops and attains a larval stage which is known as glochidium. It then leads to the temporary paralysis of the fish as it attaches itself to the fish's fins or the gills. Before the release, the glochidia then grow in the gills of the female mussel. They are then constantly flushed with oxygen-rich water. In some of the species, the glochidia are released when a fish tries to attack the mussel's minnow or the other mantle flaps which are shaped like prey. An example of such behaviour is aggressive mimicry. 

The glochidia are normally species-specific. Hence, their chances of survival are dependent on the correct mussel fish host. As the larvae of the mussels get attached to the fish, the fish body reacts by covering them with the cells forming a cyst. The glochidia remain in the cyst for a period of two to five weeks which depends on the temperature. When they grow, after a period of two to five weeks they break free from the host. They drop to the bottom of the water to begin an independent life. 

Marine mussels are edible and hence are eaten by humans. Other predators that feed on marine mussels include starfish, seabirds, and numerous species such as marine gastropods belonging to the family of Muricidae, the dog whelk species, Nucella lapillus. Also, the freshwater mussels are eaten by the muskrats, otters, raccoons, ducks, baboons, humans and geese. 

In the world, marine mussels are mostly found in low and mid intertidal zones of temperate seas. Some other species that belong to the marine mussels live in tropical intertidal areas but fewer numbers. Some of the species of marine mussels are found in salt marshes or quiet bays. Others thrive in pounding surf that completely covers the wave-washed rocks. Some other species are found in the very deep areas near hydrothermal vents. Some of the examples include the South African white mussel. It is an exception in the sense that it does not bind itself to the rocks but is burrowed inside the sandy beaches. They extend two tubes above the surface of the sand for ingesting food, and water and for removing wastes. 

The freshwater mussels are known to permanently live in the lakes, rivers, canals and streams throughout the world. The only exception is the polar regions where they are typically absent. They can be usually found in water that has a significant amount of minerals such as calcium carbonate which they use to build their shells. 

Aquaculture of Mussels and Other Environmental Applications

Freshwater mussels are normally used as host animals for the cultivation of pearls that are found in freshwater bodies. Some of the fresh mussels are cultivated as food sources. Examples include the blue mussel, Mytilus edulis, commonly known as the blue mussel, and the New Zealand green-lipped mussel, Perna canaliculus.

In some areas, the mussel farmers gather the naturally available marine mussel seeds and transfer them to better growing areas. In some cases like the North American mussel farmers, they rely on the seed produced from the hatching. The people who grow mussels usually take the seed after it has reached at least about 1mm in size. Or they buy it after the seed has been nursed in upwellings for an additional 3-6 weeks, by which time the size is 2-3mm. After obtaining the seed is reared in a nursery environment. For the rearing, it is transferred to the material having an appropriate surface for the relocation to the growing area in the later stage. After the end of three months, the mussel seed in the nursery is soaked by placing it in a tube-like mesh material. They are then hung on the longlines or rafts for growing out. In need of access to food sources that are present in the water column the mussels migrate to the outside of the sock. Within two years the mussels grow and get ready for being harvested. In comparison with other bivalves, mussels use the byssus threads for attaching themselves to any form of the substrate which is further suitable for quite a number of cultural methods. During this period of growth, the mussels reach marketable size. The methods that are used for the harvesting are usually dependent on the grow-out area and the rearing method in use. In some cases, dredges are also currently used for the on-bottom culture. 

Mussels that are grown on the wooden poles are harvested by hand or along with a hydraulic powered system. For the raft longline methods of culture, a platform is used and is lowered below the mussel lines. They are then cut from the system and then are brought to the surface. They are then dumped into containers on a nearby vessel. Before they are made available for marketing the mussels they are typically placed in the seawater tanks in order to get rid of any impurities.

The byssal threads that are used for anchoring the mussels to the substrates are known as superior bonding agents. Hence, a number of studies have been performed to find out the applications of mussel glues in industry and surgery. They are also studies for providing knowledge related to the construction of artificial tendons.

The mussels are normally used as bio-indicators across the world. They are used as bio-indicators for monitoring the health of any aquatic environments which include both freshwater environments and aquatic environments. Their worldwide presence makes them useful. Their different characteristics make sure that they represent their sampling or placing environment. The status of an ecosystem involving water can be known from the population status, their structure, physiology, behaviour or the level of contamination. 

Mussel as Food Source and Its Conservation

For dietary purposes, mussels can be prepared by smoking, boiling, steaming, roasting, barbecuing or frying in butter and vegetable oil. Like the shellfish, the mussels have to be checked that they are alive just before they are cooked. This is because the enzymes otherwise would break down the meat and make them poisonous after dying or if it remains uncooked. They are too carefully selected and prepared as they might contain some toxins. A simple way to identify the live ones is that the live mussels will shut slightly on disturbance when kept in the open air. The ones that remain open and are unresponsive are the dead ones. The unusually heavy wild mussels which are closed may be discarded because they contain only mud or sand. 

Mussels have been a part of savoury dishes along with many coastal areas. But precautions should be kept as the mussels have been known to cause poisoning. For example, the mussels along the west coast of the US during the warmer months are particularly poisonous. This is because of the bloom of the red tides known as dinoflagellates. The dinoflagellates do not harm the mussels but their toxins can get concentrated inside the mussels and in turn that could cause harm to humans. An example of this is paralytic shellfish poisoning which can also be caused due to the ingestion of toxic mussels. Hence, during specific times, the mussels along a particular coastal area should be avoided. Thus, caution is needed to be maintained while eating mussels and one should as much as possible avoid any wild mussels. 

Mussels are known to be excellent sources of vitamin B12 and the mineral selenium which is required for the synthesis of selenocysteine amino acid. Along with that, the mussels are also good sources of zinc and folate. They also provide proteins, carbohydrates, fibre, calories, fats, etc. 

Out of 297 known species, almost 72% i.e. 213 species are included in the endangered and threatened category. They are all of special concern. The main reasons that have led to the decline of the freshwater mussels are because of the destruction from the dams, increased siltation, modification of the channels, etc. Another reason is the introduction of invasive species like the Zebra mussel.