Bivalvia

The bivalves (Bivalvia; bi: 'two', valvia: 'valve' or 'plate '), lamellibranchs (Lamellibranchia) or pelecypods (Pelecypoda) are living beings classified biologically in the phylum (type of organization) Mollusca (from molluscum). There are approximately 13,000 species of bivalve mollusks, most of them marine.^{[citation needed]} They have a shell with two lateral valves, which are closed by the action of one or two muscles adductors. These are symmetrical, usually joined by a hinge and ligaments.

They are found buried in soft bottoms (infauna), as fixed inhabitants of rigid or free surfaces and structures on the bottom (epifauna). Some species bore into the substrate (rock or wood) and some more are commensals or parasites.^{[citation needed]}

Features

They lack a differentiated head and tentacles. They do not have maxillae or radula. The mouth presents fleshy labial palps. They breathe through one or two pairs of gills, usually laminar. They are invertebrates, that is, they do not have bones.

In bivalve shells, there is a great variety of sizes, shapes, colors, and sculpted patterns on the surface. The size fluctuates from tiny shells (2 mm) to species that can reach 15 dm in length and a weight of 250 kg.^{[citation needed]} Among the best-known bivalve molluscs we can name: oyster, clam, razor, mussel, ship joke, coquina, etc.

There are three folds on the front edge of the mantle: inner, middle, and outer. The internal fold is muscular, the middle one stands out for its sensory function and the external one is related to the secretion of the shell. The shell is divided into two valves joined dorsally at the hinge, which consists of an elastic ligament formed by conchiolin and secreted by the mantle; it is not highly calcified, so it remains flexible and elastic. The muscle or "foot" characteristic of molluscs, in bivalves it can be modified or very reduced depending on the habit of the different species.

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  Internal view of a loaf.

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  1: Symmetry plan. 2: Growth lines. 3: League. 4: Umbo.

Natural history

All the representatives of this class are aquatic, both marine and freshwater, and can be found from the upper limits of the high tide to the abyssal zones. Shell protection allows some specialized species to withstand the conditions of the intertidal shoreline.

Typically, species that live on soft substrates such as mud and sand, have an ax-shaped foot that allows them to dig. Sessile species remain attached to the substrate, either by cementation, like oysters, or by secreting a series of filaments that make up the byssus (mussels).

Those bivalves that live under the sand (soft soil), feed by filtering small amounts of water during high tides, from which they extract food. They do not usually travel far, since their organs are adapted to filter water through the valves of their shell instead of the bivalve itself moving to introduce water inside. Whenever it moves, it uses its foot, a muscle that allows it to bury itself in the sand.

Ecology and Paleontology

Bivalvia are essentially aquatic mollusks, mostly marine, that preferentially inhabit the sublittoral benthic region, although they are also found down to the abyssal depths. Salinity and temperature are among the factors that most influence its distribution. They are of great value in paleoenvironmental analyzes due to their close relationship with the type of substrate, their good presence in the fossil record since the Cambrian, their great diversity and abundance in various paleoenvironments, and their excellent representation in current aquatic environments. These mollusks live in a wide range of water temperatures; in general the size and thickness of the shell decrease with decreasing temperature. They are more common in well-oxygenated waters, although some opportunistic or specially adapted species can live in oxygen-poor to completely anoxic environments. The range of salinity they support is also wide, from fresh continental waters to hypersaline seas, but they are much more diverse under normal marine salinity conditions.

Salinity and temperature have little influence on life habits, on the other hand, predation significantly conditions some general aspects of the functional morphology of bivalves, such as byssus secretion or burrowing, although less is known about its influence on the development of specific morphological variants to protect themselves from predators (cementation, siphonal spines). The most important factors affecting the way of life of bivalves are the availability of food, the type of substrate and the movement of water. The functional morphology of the valves is closely related to the character of the substrate. Regarding the turbulence of the water, it can be said that in general they prefer environments with moderate water movement; in turbulent conditions cementers, nesters, perforators and fast excavators predominate.

The study of the morphological diversity of bivalves in relation to different lifestyle strategies has shown that there are recurrent adaptive morphologies, each typified by specific sets of characters. Many bivalves (especially the infaunals) live with their commissure oriented perpendicular to the interface between the substrate and the water, but there are some (common among epifaunals, although there are also infaunals) that do so with one of the valves against the sea. substrate or so that the commissure is approximately parallel or oblique to the interface. These last bivalves are called pleurothetic, and they are recognized for being inequivalve to some degree (right and left valves with different convexity, thickness, ornamentation, or even coloration), they may have twisted valves whose commissure is not in one plane and, in the In the case of epifaunal pleurothetics, they are almost always monomyal. Pleurothetics include all cementing bivalves, most supported bivalves, many bivalves, and some burrowers.

However, these habits are not exercised exclusively, presenting frequent combinations of them. Some burrowing species (especially arcids) employ a weak byssus to increase stability in soft strata. Other species that live fixed by the byssus, belonging to the genera Isognomun and Barbatia, live introduced in the manner of a wedge between colonies of corals or rocks, like some free nesting species. Many swimming forms, such as scallops, spend a good part of their lives resting on the bottom or fixed by the byssus. Petricola pholadiformis normally lives as a burrower in soft sediments or a borer in hard substrates. In the Beagle Channel, Hiatella solida is fixed by the byssus to rocky substrata and to the cachiyuyo, in lower latitudes it is a borer in hard sands or encrusting bivalves and gastropods, and in Brazil it lives fixed on rocks, starfish and bryozoans or between polychaete tubes.

These variations in habit can occur throughout ontogeny, as is sometimes deduced from morphological changes recorded in the shell.

The diversity of shell forms correlates with the diversity of habitats occupied and the variety of lifestyles adopted, and thus phylogenetically poorly related groups of bivalves may develop shells with similar characters.