Freshwater bivalve
In contrast, one of the largest species of freshwater bivalves is the swan mussel from the family Unionidae; it can grow to a length of 20 cm (7.9 in), and usually lives in lakes or slow-flowing rivers.
[1] Freshwater bivalves can thrive in many different types of habitats, ranging from small ditches and ponds to large lakes, rivers, canals and wetlands.
In North America, for instance, many freshwater mussel species have gone extinct, and of those remaining, 65 percent are rated as endangered, threatened or vulnerable.
[2][3] Freshwater bivalves, as their name implies, have a protective exoskeleton composed of two half-shells or "valves", connected via a soft ligament along a hinge.
[4] The third and innermost layer is also the thickest and is most commonly referred to as the mother of pearl--- a widely harvested source for the production of ornamental buttons.
[4] Valve surface appearances can range from smooth to dramatically sculpted, showcasing ornamental pustules, pimples, grooves, and ridges.
As expected, these gills mainly act as a respiratory structure, performing gas exchange but can also facilitate filter feeding.
Water enters the mantle cavity via an incurrent siphon, and passes over the gills where food particles are trapped by secreted mucus.
[4] Anterior and posterior adductor muscles connect the left and right valves, facilitating shell opening and closure.
[4] The less major anterior and posterior retractor muscles extend from the shell and attach the body to a structure called the foot.
The study of freshwater bivalves predates Aristotle and has since been in a state of constant flux and dispute, regarding their identification and classification.
[5] As time has progressed, so too have different techniques and technologies that allow scientists to more comprehensively study the assemblages of organisms in the natural world, freshwater bivalves included.
[5] The most commonly used method of identification/classification utilizes an exceptionally diverse set of ever-expanding morphological features, ranging from shell anatomy, variations in internal soft tissue, degrees of mantle fusion, to larval stage development.
[8] The order Unionidae have an obligate parasitic larval stage where the larvae are attached to the gills, fins or the body of a particular host fish.
[5] Freshwater bivalves are also important in the process of nutrient cycling because they deposit organic matter in the sediment through biodeposition created from the fine particles they filter in.
[14] When introduced to freshwater ecosystems, dreissenids lead to a decline in indigenous marine animal populations and are also known for causing benthic algae and cyanobacterial blooms.
[15] For the Neotropics and Australasia biogeographic realms, ecosystem modification has the largest impact on freshwater bivalve species.
[15] Hydropower plants and dams are two examples of human ecosystem modification which contributes to loss of habitat as well as changes to channel morphology, river and floodplain connectivity and nutrient limitation.
