Bivalves (Bivalvia) are a class belonging to the phylum of the Molluscs, which as its name indicates, have a shell formed by two shells. They are the second class of Molluscs, if we stick to the number of species belonging to the class of bivalves (13,000 living species). Within the bivalves we find species well known by all, such as mussels, clams, scallops and oysters. Approximately 80% of bivalve species are marine species. We can find both sessile species, which are anchored to rigid surfaces by means of filaments called byssus, and digger species which are buried at different depths in the sandy/muddy bottom, and even species with a certain swimming ability.
The body of the bivalves is flattened laterally, with no cephalization and characterized by two lobes of the mantle that cover the internal organs and are responsible for secreting two shells that have a hinge between them on the dorsal part. Both shells are additionally held together by abductor muscles and ligaments. In some marine bivalves, the mantle forms siphon structures that allow the bivalves to expel water through them to be buried in the sand or mud. Many species generate, through the mantle, mother-of-pearl as a defense mechanism. With this, they are able to isolate foreign particles that have entered the animal's body by means of a coating. By means of this mechanism, and either by accidental and natural form, or by artificial means, it is as the pearls are obtained. Bivalves have a very muscular foot, that they use mainly to bury themselves in the bottom, but that they can use to make limited movements. Some species of clams and scallops are capable by performing repetitive and sudden movements of closing/opening the shells, to obtain some jet propulsion. The size of the bivalves is very varied, with species that have a length of a few millimeters (1-2 mm), up to the giant clams of the Pacific (family Tridacna) that can reach more than 1 meter in length and weigh up to 225 kg.
Most bivalves feed by filtering the suspended particles in the water with their gills. Other forms of feeding present in some species of bivalves are the feeding of the deposits present in the bottoms, the predation of small invertebrates and Crustaceans, feeding from the decomposition of the wood, and there are even species that have established symbiotic relationships with zooxanthellae algaes, which give them total or partial energy requirements.
They are a class of species of great importance for the feeding of many species, including humans. Due to their feeding form, mainly filtering, they have the capacity to accumulate contaminating substances, bacteria, virus or toxins, if these are present in the water. This can cause certain intoxications after their ingestion, if the waters in which they were found were not pristine. On the other hand, this fact makes bivalves an excellent bioindicator of the quality of the water in which they live.
Their calcium carbonate shells have made bivalves the best represented class of animal in the fossil record.