Oyster

If you ever go to a seashore, you will find several oysters. An oyster is mollusks that have environmental, nutritional, and economic value. People mostly cultivate specific species of oysters for obtaining pearls. Pearls have exceptional economic value in the market. They are also obtained naturally. A major part of the economy of coastal countries is based on oysters.

There are different types of oysters in the world. They are mostly found in saline waters in warm temperate climates. Therefore, mostly, an oyster is found in the coastal waters of all oceans. These oysters differ in their shape, structure, and nutritional value. However, all oysters also share a lot of similarities. All of them belong to the phylum Mollusca and are bivalved in shape. They are filter feeders. The process of making pearls in them is also similar in some steps. Let us look into some of the essential details of oysters.

Etymology of Oysters

Oyster refers to all bivalve mollusks that live in saline or brackish habitat. The word ‘oyster’ originates from the old French word oistre. The word first originated in the English language in the 14th century. The French word is derived from the Latin word Ostrea, meaning bone. The oyster scientific name can be either Ostreidae or Aviculidae, depending on the type.

Types of Oysters

Oyster is a collective term for different families of saline bivalve mollusks. The different types of oysters include:

• True Oysters

The true oysters belong to the family Ostreidae. They first evolved during the Early Triassic epoch. This family also includes edible oysters. Edible oysters are from the genera Ostreola, Ostrea, Crassostrea, Magallana, and Saccostrea. They have been routinely cultivated for food for more than 2000 years. You will also find certain members of the genus Liostrea on living ammonoid shells.

The major species included in this family are Ostrea, Crassostrea, and Pycnodonte. Ostrea species that are commonly found are the European flat oysters and the edible oyster O. Edulis, Olympia oysters O. Frons., and O. Lurida. The Crassostrea species include North American (Virginia) Oyster C. Virginica, Portuguese Oyster C. Angulata, and the Japanese oyster C. Gigas. 

O. edulis is mainly found along the stretch of the Mediterranean sea from Norway to Morocco and also in the Black sea. It shows hermaphroditic properties and can grow to a length of 3 inches (about 8 centimeters). O. Lurida are mainly found on the Pacific coast of North America. They can grow to a size of the same length. The females release around 50,000,000 eggs at a time.

C. virginica is the most commercially important mollusk in North America. They can change their sex, similar to the C. commercialis (Sydney rock oyster). They can be born as males but changes sex later. C. commercialis is the most commercially important Australian edible oyster. C. angulata is mostly found on the coasts of western Europe. C. Gigas is found in the coastal waters of Japan. They are one of the largest mollusks found here, growing to a length of almost 1 foot (30 centimeters).  

• Pearl Oysters

All pearl oysters belong to the family Cichlidae. Most pearl oysters belong to the genus Meleagrina. The genus is also known as Margaritifera or Pinctada. These oysters are not related to the true oysters. 

In reality, almost all mollusks that can bear shells can form and secrete pearls. However, not all pearls are considered to be valuable. Pearls can be produced both in freshwater as well as the saltwater environment. Freshwater mussels can also produce pearls that have commercial values.  

The largest pearl oyster is found in the marine ecosystem. It is called the Pinctada maxima. Its size almost resembles a dinner plate. 

• Other Types of Oysters

Certain other bivalve mollusks are also categorized under the name of oysters. They all have the word ‘oyster’ in their common name. The reason behind such classification is that they can either produce some noticeable pearls or have tasted like edible pearls. Some examples of these bivalve mollusks are Thorny oysters belonging to the genus Spondylus, Saddle oyster (members of the Anomiidae family), Pilgrim oysters, Dimydarian oysters belonging to the Dimyidae family, and windowpane oysters. 

Feeding Anatomy of Oysters

An oyster animal has a bivalved structure. The two valves of the shell differ in shape. They have rough surfaces. Most oysters have dirty gray-colored valves. The upper valve is convex in shape. It is lower around the edges while being higher in the middle. The lower valves are generally fixed to the bottom surface. They are larger than the upper valves. They have smoother edges and are mostly flat. Both valves have smooth inner surfaces that are generally white in color. 

The valves are held together tightly with the help of an elastic ligament at the narrow ends. The closing of the valves is governed by the adductor muscle, which is a large central muscle functioning against the pull of the elastic ligament. 

Oysters are considered filter feeders. They keep their valves slightly open and draw in water over their gill with the help of the cilia, which are tiny hair-like structures. The wavy motion of the cilia helps in drawing the water into the system. Around two to three gallons of water pass through the oyster in one hour. 

Minute food particles and suspended planktons get entrapped in the gills’ mucus and serve as food. From the gill, the food is transferred to the mouth, where the food is digested. The undigested food particles are expelled as pseudofeces or feces. An oyster animal demonstrates most active feeding at a temperature higher than 10 °C.

The Other Organs of Oysters

Along with the gills, the oysters can also perform the exchange of gases through their mantles. Mantles are lined with several thin-walled, small blood vessels. It possesses a three-chambered heart located beneath the adductor muscle. The heart pumps colorless blood to all the body parts. The blood is colorless.

An oyster animal also possesses two kidneys. They are also located beneath the muscles. These kidneys help in the removal of waste products. Oysters have a nervous system including three pairs of ganglia and two pairs of nerve cords. 

Hermaphroditism in Oysters

Like any other bivalve mollusks, oysters generally have both male and female genders. However, some oysters, like the Olympia oyster and the European oyster, also exhibit hermaphroditism. These oysters have both the sperms and eggs as their reproductive organs. Therefore, the oysters can technically fertilize their own eggs. Their gonads are present around their digestive system. The gonads are made up of sex cells, connective tissue, and branching tubules. 

Usually, an oyster animal prefers to breed in summer. After fertilization, the females release millions of eggs. The larvae develop from the fertilized eggs within six hours and are called veligers. They are ciliated larvae that can swim in water for several days before they settle down on any surface. After settling down, they metamorphose to get permanently attached to the site. Maturation to sexual adulthood takes around one year.  

How are Pearls Formed?

The material that lines the oyster shell is called the nacre. Pearl is formed by the accumulation of nacre around any foreign matter that gets entrapped inside the shell. Even an edible oyster can also form pearls. However, these pearls do not have a luster and therefore do not have a commercial value.

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Pearls can be both natural and artificial. The best natural pearls are obtained from a few oriental species of oysters. One such species is Meleagrina vulgaris, which is found to be native in the Persian Gulf. These oysters are found at a depth of 48 to 120 feet.

Artificial or cultured pearls are produced by harnessing the natural process of making pearls. The pearl farmers place a nucleus, sometimes bits of mother pearl, inside the shell of the oysters. The oysters take around three to seven years to form a perfect pearl. Most pearl cultures are carried out in the Australian or Japanese coastal waters. After the advent of artificial pearls, the cultured pearl market has overgrown the natural pearl market.

Oysters as Food

As stated earlier, true oysters are mostly taken in as food. These oysters are shucked in piles. They can be eaten raw, smoked, or cooked. The most popular oyster species for food are Lynnhaven and Bluepoint. Both these oysters are varieties of C. virginica. They are named according to the place from where they are harvested. For example, Lynnhaven oysters are named because they are harvested from Lynnhaven bay. Another form of oyster for food is O. edulis. They are commercialized by the names of Marennes in France and Colchester in Britain. 

Along with humans, oysters are also fed on by snails, sea stars, and birds. Many fishes also eat oysters. For example, the oyster drill snail (Urosalpinx cinerea) can drill a tiny hole through the oyster shell. The name of the snail is based on this phenomenon. Through the hole, the snail can suck out the living tissue. 

Conclusion

An oyster animal can grow in both freshwater and marine ecosystems. They are mostly found in the warm temperate climate, mostly in the coastlines. There are mainly two types of oysters- true oysters and pearl oysters, although some other varieties are also considered in this group. The true oysters are mainly edible. Although many oysters and other mollusks can also form pearls, pearls from pearl oysters are considered commercial value. People also culture oysters for pearls.