Bivalvia

Tridacna gigas.001 - Aquarium Finisterrae

Ensis phaxoides-V

Margariiera-margariiera-Anatomy

Valve-InternalView

Valve-DorsalView collored

Bivalvia, commonly known as bivalves, is a class within the Mollusca phylum, which includes some of the most familiar marine and freshwater organisms. These creatures are characterized by their two-part hinged shells, which enclose the soft-bodied animal inside. Bivalves are a diverse group, including oysters, clams, mussels, and scallops, among others. They play significant roles in aquatic ecosystems and have considerable economic importance to humans.

Anatomy and Physiology

Bivalves possess a bilaterally symmetrical body that is enclosed within a shell consisting of two valves connected by a flexible ligament. The shell is secreted by the mantle, a significant part of the bivalve's anatomy that also functions in respiration and excretion. The gills in bivalves are large and not only assist in gas exchange but also in feeding. Bivalves are filter feeders; they draw water in through an incurrent siphon, filter out the food particles with their gills, and expel the filtered water through an excurrent siphon.

The nervous system of bivalves is relatively simple, with three pairs of ganglia connected by nerve cords. Despite their simple nervous system, bivalves can respond to environmental stimuli, such as changes in light and water chemistry. They have a heart that pumps blood through an open circulatory system, and they reproduce either by releasing sperm and eggs into the water or, in some species, by brooding their young.

Ecology and Life History

Bivalves inhabit a wide range of aquatic habitats, from the deep sea to freshwater rivers and lakes. They are particularly abundant in coastal areas, where they often form dense beds that provide important habitats for other marine organisms. Bivalves play a crucial role in the aquatic food chain and are a primary food source for many species of birds, fish, and invertebrates.

The life cycle of bivalves varies among species. Most bivalves are sexually reproductive, with separate male and female individuals, although some species are hermaphroditic. Fertilization is usually external, with larvae developing in the plankton before settling to the bottom and metamorphosing into juveniles.

Economic Importance

Bivalves are of significant economic value to humans. Many species, such as oysters, clams, and mussels, are cultivated for food. Bivalve aquaculture is a growing industry, providing sustainable seafood options. In addition to their value as food, bivalves are also harvested for their pearls (especially certain species of oysters) and shells, which are used in jewelry, decoration, and as a source of lime.

Conservation

Despite their abundance, many bivalve species are threatened by overfishing, habitat destruction, pollution, and the impacts of climate change, such as ocean acidification. Conservation efforts are underway to protect bivalve habitats and manage fisheries sustainably.