You may have seen or even eaten an oyster and probably know them by the name of shellfish. The truth is that these little creatures are called bivalves . A bivalve is an animal belonging to the class Bivalvia . The word “bivalve” comes from the Latin bis which means two and valvae which means the leaves of a door . This name is given because all bivalve animals have two shells that can open and close like doors.

What are bivalves?

They are a type of mollusk that were known as pelecipods and that inhabit fresh or salty waters, and whose body is inside a shell formed by two valves that are joined by a hinge.

Bivalves are aquatic animals , and can be found in fresh and salt water . They usually live on the seafloor , on the seabed, or cling to the sides of hard surfaces , such as rocks or artificial pipes. These animals are abundant throughout the world, and due to their wide availability, they are a very common way to eat shellfish.

  • Characteristics of bivalves
  • Types of bivalves
  • Feeding
  • Habitat
  • Reproduction of bivalves
  • Fossils
  • Examples of bivalve mollusk species

Characteristics of bivalves

The main characteristics of bivalves are the following:

  • They can have globular or elongated shells .
  • Its size is quite varied, some measure just millimeters and others can reach a meter in length, although these are very rare.
  • The shell and the mantle of the bivalves are united.
  • They have a mantle that secretes material to form the shell.
  • Its shell is made of calcite or aragonite .
  • They have ligaments in the shells made of keratin proteins.
  • They have well-defined brains that control the mantle cavity and other organs.
  • Your heart has three chambers, two atria, and a single ventricle.
  • They have gills to capture oxygen.
  • Its circulation travels to all the organs of the bivalve.
  • Its body is flattened laterally.

Types of bivalves

Four of bivalves are known. These are the following:

  • Heterodonts : They are marine and the size of their valves is the same, they have few cardinal teeth on the inside of their shell. For example, clams and cockles.
  • Paleoheterodonts : This group is already extinct. They had valves of the same size and a row of teeth.
  • Protobranchs : They live in deep water and have several small teeth.
  • Pteriomorphs : They only inhabit the sea, they have a very small foot and their gills are large. They have eyes and we can mention mussels and oysters in this group.


Virtually all bivalves feed by filtering the waters. From this filtration they manage to capture the food in the form of small particles of phytoplankton . It has two siphons that are the place where the filtered water enters and exits. Some of them, the species that are carnivorous, have a better digestive system to be able to ingest their prey. They have an esophagus, stomach, and intestines.

They have food grooves in the gill plate where the particles arrive and are retained by the cilia , then a selection of the food is made. In this way, the small particles pass into the mouth and the large ones are eliminated.

Some of them live attached to the substrate that feeds them as in the case of mussels by means of filaments known by the name of byssus .


This type of mollusk can be found in practically all aquatic environments. Most of the time they are buried in the sediments of the seabed and are inhabited mainly in the tropics , although they can inhabit places where the water temperature is extreme, such as in the Arctic.

They are sedentary and also live in areas where there is water when the tide is high and that, although it goes down, the humidity is always maintained, some live on rocky surfaces .

Reproduction of bivalves

They manage to complete their reproduction in a sexual way and males can be differentiated from females. Hermaphrodites can also be found within this group of animals. They are oviparous and the young must go through a larval period before reaching adulthood. On some occasions the eggs may incubate in the gill cavity and then, the fertilization in this case is internal.


In bivalves, they can only fossilize the valves , or rather the shells , which are what make up the exoskeleton , and which are composed primarily of calcite and aragonite, although during fossilization it transforms into calcite . Some of the shells have an organic outer layer known as the periostraque , which cannot be fossilized.

Examples of bivalve mollusk species

Some examples of bivalves are the following:

  • Oyster : it has round valves. They filter the water to feed on plankton. They are very persecuted for the gastronomy area.
  • Mussel : filters water to feed and can only live in salt water. Its shell has three different layers and is highly prized for food.
  • Viera : its shell is convex and they are known as fan shells.

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