In the coastal areas of the world, there lives an exotic group of animals called “oysters,” bivalve mollusks (with two valves or shells), famous for their ability to create a wonder of nature within their bodies, the pearl!
For over 2000 years, this amazing genus from the family “Ostreidae” has been considered one of the main seafoods; moreover, its ecological importance stands out for acting as a filter for seawater, improving its quality.
Characters of novels, legends, and beliefs that extol their supernatural power, oysters possess extraordinary qualities. Here I present to you their most outstanding characteristics, what they eat, how they reproduce, and the types of oysters that exist!
What are oysters?
“Oysters” are a group of marine animals, belonging to bivalve mollusks, meaning their body develops inside two valves (shells) of different sizes, and are part of the Order “Ostreoida,” which includes the Family “Ostreidae” and the Genus “Ostrea.”
Characteristics
External Anatomy
Soft body, covered by a shell formed by two parts (valves): a left or bottom one, sunken, of larger size, which serves to attach to the substrate, and a right or top one, smaller and flatter.
In turn, each valve is divided into three layers: an internal, shiny one (nacre or mother of pearl), a middle one (calcium carbonate), and an external (membranous) one, and their size varies between 7 and 15 centimeters. On the outside, its texture is rough, greenish-gray in color; on the inside, it is smooth and white.
Both pieces (valves) fit together, joined at one end (umbo) and sealed by a firm muscular structure in their center (adductor muscle).
Internal Anatomy
The body is covered by a film or “mantle” that secretes the material that forms the shell (calcium carbonate), with free edges.
Digestive System
Composed of a mouth surrounded by four appendages (labial palps), esophagus, stomach, intestine, and anus. At the base of the stomach, it has a groove containing a rod (crystalline style); it also has a digestive gland.
Nervous System
They do not possess a brain; they have a simple nervous system formed by two pairs of ganglia (cords) connected with nerve cells distributed throughout the body.
Circulatory System
The circulatory system is open, with a heart that connects with tiny, thin-walled blood vessels, veins, and arteries; the blood is colorless (hemolymph).
Respiratory System
Composed of gills, beige lamellar structures in the shape of a basket, with thin hairs (cilia) useful for breathing and filtering food.
Reproductive System
Differentiated reproductive organs (gonads); the female has ovaries and the male, testicles, consisting of a series of small tubes on each side of the body.
Muscular System
The oyster has a central adductor muscle, made up of smooth and striated muscle fibers, responsible for controlling the opening and closing of the shells or valves.
Did you know…? The average lifespan of oysters is 6 years; however, some species can live up to 50 years.
Types of Oysters
There are two types of oysters: flat and cupped, differentiated by the shape of their valves (shells) and the habitat they occupy.
Flat Oysters
Their valves are like two flat, rounded lids, with irregular edges; they inhabit clean waters, with little sediment and high salinity. In this group, we find the common or European oyster, Mediterranean oyster, Olympia oyster…
Cupped Oysters
Named so because their lower part is concave (sunken) to attach to hard and sandy bottoms associated with estuaries (the mouth of a river into the sea), where the sediment is high and the salinity variable.
Among its representative species are: Portuguese oyster, American oyster, Japanese or Pacific oyster…
How do oysters reproduce?
Fertilization is external and varies depending on the type of oyster.
In flat oysters, the female releases eggs and retains them in the gill cavity; the male releases sperm into the sea water; fertilization occurs when water is filtered by the oyster during the feeding process. In cupped oysters, both eggs and sperm are released into the water.
After a week, the eggs transform into a tiny larva (veliger), which swims freely until it finds a surface to attach to and begin its sedentary (immobile) stage and its metamorphosis (transformation) into an adult.
There are females (C.virginica) that can release up to 50,000,000 eggs at once!
Did you know…? Certain oyster species (Ostrea edulis) live as males and every three years, if the temperature is favorable (16º Celsius), they can regularly change sex! Becoming females, and then males again during cold periods.
Where do oysters live?
They inhabit the coastal areas of all the seas of the world, attached to rocks or an artificial equivalent they can adhere to, at depths not exceeding 20 meters; or on sea floors (at a maximum depth of 90 meters).
In their initial stage (larva), they live freely in the water and sand; until they find a firm substrate on which to settle definitively. There are some reefs formed by groups of oysters, which become an ideal habitat for other species (sea sponges, seahorses).
What do oysters eat?
As the oyster remains immobile, to feed, it filters the water from its habitat; from there, it obtains its food, mainly phytoplankton, microscopic algae, and very small organic particles (detritus) from the decomposition of animals and plants.
How does an oyster feed?
Through its gills (respiratory organs), the oyster filters and retains the food particles in suspension present in the water, using small filaments or hairs (cilia) that carry the filtered particles to the labial palps (appendages near the mouth).
Thanks to the labial palps, the particles suitable for digestion are selected, and they are transported to the mouth, to pass into the stomach; the rest is discarded dispersing the remaining water in the shell.
Did you know…? Adult oysters are capable of filtering about 15 liters of water per day in addition to sediments and algae, contributing to a healthier marine environment for the rest of the species.
Oysters in Alice in Wonderland
A moving scene takes place in the 1951 Disney animated film “Alice in Wonderland,” when a family of juvenile oysters are deceived and devoured by a walrus.
The segment of “the story of the curious oysters” or “the walrus and the carpenter,” shows a group of small oysters in their habitat, who are enticed by two characters, the walrus and the carpenter, who offer them a walk.
Despite the warnings of an adult oyster (for some, their mom; for others, an older sister), that it was not the time to leave their nursery, the young oysters accept the invitation, unaware of the cunning plan both had: to cook them in a tasty sauce to devour them!
The plan is fulfilled with one difference, the selfish walrus eats all the oysters and lies to the carpenter by serving the empty shells.
Two curious facts are associated with the scene: a red “R” marked on the calendar, related to the months in which oysters are consumed, and the appearance of the little oysters in a subsequent scene, behind other characters.
Do oysters die when the pearl is removed?
When some irritating element enters the interior of the oyster (a grain of sand, a parasite…), it tries to expel it; if it fails, it goes to the bottom of the lower valve (shell), causing a wound.
Immediately, the oyster activates a defense mechanism: the mantle (membrane that lines its body) secretes a substance (nacre) that coats the invader; this mixture solidifies over time, thus originating a “pearl.”
The longer that pearl remains inside the oyster, more layers of nacre will be deposited on it. Their shapes and colors can be the most diverse, depending on the foreign element and the characteristics of the oyster within its habitat.
Thus, it is possible to find pearls of white, blue, black, green, red, gray, among other colors…
Unfortunately, during the process of pearl extraction, oysters feel pain and can die, because when it is not possible to open them with a scalpel, they are completely split open.
Did you know…? The Pearl of Lao Tzu or Pearl of Allah is recognized as the largest in the world, discovered in the Philippines, with a diameter of 24 centimeters (9.45 inches) and 6.4 kilograms.
However, in 2016, an even larger one appeared, measuring 70 centimeters in length and more than 30 in width, and weighing more than the Pearl of Allah, found by a fisherman also in the Philippines!