In biology, bivalvia (from Latin bi, doubled , and valvia double-leaf door ) formerly Pelecypoda and Lamellibranchia , refers to the class of the phylum mollusca that includes aquatic animals popularly called bivalves.
Characteristics
These organisms are characterized by the presence of a carbonate shell formed by two valves.
This shell protects the body of the mollusk, among them are the muscular foot and the inhalation and exhalation siphons for the entry and exit of water, which brings oxygen, and is then absorbed by direct diffusion through the gill plates.
The group appeared in the Cambrian and is currently very diversified, with around 15,000 species. The separation of the different subclasses is made by the type of structure of the gills of living organisms and by the characteristics of the valves of fossil bivalves.
Bivalves are a class of mollusks that are characterized by having a two-part hinged shell. Here are some key characteristics of bivalves:
- Shell structure: Bivalves have a two-part shell that is composed of calcium carbonate. The shell is typically symmetrical along a central axis and is divided into two halves or valves. These valves are connected by a hinge and can be opened and closed by powerful muscles.
- Filter feeders: Bivalves are primarily filter feeders, meaning they obtain their food by filtering particles from the surrounding water. They have specialized structures called gills that help them filter out plankton and organic matter. As water passes through their gills, food particles are trapped and then transported to their mouth for ingestion.
- Sedentary lifestyle: Most bivalves are sedentary and live in marine or freshwater habitats. They typically burrow into the substrate or attach themselves to rocks or other surfaces using a muscular foot. Once attached, they remain in the same location for extended periods of time, filtering food from the water that passes by.
- Reproduction: Bivalves have separate sexes, meaning they can be either male or female. They reproduce by releasing eggs and sperm into the water, where fertilization takes place externally. The fertilized eggs develop into larvae, which then go through a metamorphosis before settling and developing into adult bivalves.
- Economic importance: Bivalves have significant economic importance. Many species are commercially harvested for food, such as clams, mussels, and oysters. Bivalves are also important ecologically as they help filter and purify water, and their shells provide habitats for other organisms.
It is worth mentioning that bivalves exhibit a wide range of diversity in terms of size, shape, and habitat. Some examples of bivalves include the Pacific oyster, the giant clam, and the freshwater mussel. Understanding the characteristics of bivalves is important for studying their ecology, evolution, and their role in aquatic ecosystems.
Examples
Mussels, clams and shellfish are popular examples of bivalves that serve as food for humans. Oysters, which contain pearls, also belong to this classification.
Shell morphology
The shell of bivalves is similar to that of brachiopods, since it is made up of two valves. The main difference lies in the fact that, in brachiopods, the two valves are unequal (inequivalve) but symmetrical with respect to an imaginary median plane. On the other hand, in bivalves, the valves are equal but inequilateral, whose plane of symmetry is parallel.
The two valves are joined by ligaments and adductor muscles. The ligament, composed of resilium and tensilium, controls the opening of the leaflets that occurs when this structure is at rest. The bivalve closes the shell through the contraction of the adductor muscles, which may be one or two, depending on the species.
The geometry and distribution of the scars of the adductor muscles inside the valve is an important criterion for the classification of current and fossil bivalves. The oldest area of the valve is the umbo, a protuberance located in the anterodorsal area of each valve, around which there are radial growth lines. .
The valves are joined by the hinge, a set of cardinal and/or lateral teeth, which are carbonate expansions, and respective pits, which fit into the opposite valve.
Types of hinges
- Taxodont: numerous teeth of equivalent size, located perpendicular to the hinge or in an arch.
- Disodont: Small protuberances next to the umbo. Example: mussel.
- Schizodont: two or three voluminous teeth, with orthogonal folds.
- Heterodont: two or three cardinal teeth or two lateral teeth. Example: clam.
- Desmodont: It is a spoon-shaped tooth, called a chondrophore.
The characteristic mantle of mollusks is the bivalves completely told to the shell. The pallial line of the loose valves shows the limit of extension of the soft body of the organism.
Ecology
Bivalves are a fairly diversified group. They are exclusively aquatic animals, but they can be found in environments of varying salinity such as salt, fresh or brackish water. Most species are benthic and live on the bottom.
Some bivalves are sessile organisms that attach to the substrate through the byssus, a fibrous segregation, while other species live buried in sandy bottoms. Species that live loose can move by propulsion achieved by the expulsion of water under pressure.
Bivalves are the only mollusks that do not have a radula, and they feed by filtering through siphons, the only soft body parts that these organisms manage to remove from the shell.
Generally there are two siphons, one is to suck in the water loaded with small organisms and algae and the other is to expel it. The siphons, when collected, occupy a cavity in the valve called the pallial sinus.
In fossil species, the relative size of the pallial sinus is an indicator of the organism’s way of life. When large, this structure indicates that the bivalve lived buried since it needed large siphons.
Geological history
Bivalves appeared in the Cambrian , approximately 500 million years ago, in a marine environment and became very common in the Silurian . The first freshwater forms appeared in the Early Devonian and some groups served to define the stratigraphy of Carboniferous coal deposits .
Bivalves suffered a sharp reduction in the Permotriassic extinction , but the groups that survived quickly recolonized the various ecological niches.
In the Middle Jurassic, the rudists appeared, a group of bivalves and reef builders that dominated the Mesozoic seas and disappeared with the extinction of the KT. In the Cenozoic , bivalves become quite abundant and diversified and remain so to the present day.