Freshwater bivalve

Freshwater bivalves, as their name implies, are composed of two halves, or a left and right valve, connected via a soft ligament along a hinge.[1] These two valves are non-living, composed of both organic and inorganic substances that make up three major valve layers.[1] The first, outermost layer is the thin epidermis (periostracum), followed by a second prismatic layer, containing calcium carbonate.[1] 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.[1] The external appearances of these shells can be extremely variable when comparing members of different families, genus, etc., as well as intraspecies.[1] Valve surface appearances can range from smooth to dramatically sculpted, showcasing ornamental pustules, pimples, grooves, and ridges.[1] The overall shape of the valve can also vary drastically, from laterally compressed and narrow, to wide and globular.[1]

The mantle is a multifunctional, generally thin and fragile structure that line bivalve interiors and encloses their bodies. This structure secretes the shell, contains respiratory organs (facilitates respiration), and facilitates feeding. The cavity that exists between the mantle and other soft tissues is aptly named the mantle cavity.[1]

Within the mantle cavity on either side of the foot are the gills. The inner gills are adjacent to the foot and the outer gills nearest to the mantle and shell. As expected, these gills mainly act as a respiratory structure, facilitating gas exchange but can also facilitate in feeding. Water can enter the mantle cavity via an incurrent siphon, and pass over the gills where food and other particles are trapped by secreted mucus.[1]

Anterior and posterior adductor muscles connect the left and right valves, facilitating shell opening and closure.[1] The less major anterior and posterior retractor muscles extend from the shell and attach the body to a structure called the foot.[1] This muscular foot is typical of most bivalves, extending anteriorly between the valves (via an anterior protractor muscle) and aiding in locomotion, digging and anchoring (holdfast).[1]

• Arcoida (Order)
  • Arcidae (Family)
• Mytiloida
  • Mytilidae
• Unionida
  • Etheriidae
  • Hyriidae
  • Iridinidae
  • Margaritiferidae
  • Mycetopodidae
  • Unionidae
• Veneroida
  • Cardiidae
  • Corbiculidae
  • Sphaeriidae
  • Dreissenidae
  • Solenidae
  • Donacidae
  • Navavulidae
• Myoida
  • Corbulidae
  • Erodonidae
  • Teridinidae
• Anomalodemata
  • Lyonsiidae

The Unionida, of worldwide distribution, are the pearly freshwater mussels. All reproduce by means of a larval stage that is parasitic on a fish or salamander. Many species are utilized as sources of mother-of-pearl.

Families:

• Margaritiferidae
• Unionidae
• Hyriidae
• Etheriidae
• Mutelidae
• Mycetopodidae
• Iridinidae

• Order Unionida: pearly freshwater mussels
• 
  Family Margaritiferidae, a shell of Margaritifera auricularia
• 
  Family Unionidae, Quadrula metanevra, the monkeyface mussel
• 
  Family Unionidae, Anodonta cygnea

The Veneroida is a large group of bivalve "clams", most of which are marine. However, several families occur in fresh and brackish waters.

Families:

• Corbiculidae
• Sphaeriidae
• Dreissenidae

• Order Veneroida
• 
  Family Corbiculidae, shell of Corbicula fluminea
• 
  Family Sphaeriidae, Sphaerium corneum, one of the small fingernail clams.
• 
  Family Dreissenidae, Dreissena polymorpha

Freshwater Bivalves can utilize either ovoviviparous or viviparous reproduction strategies. Ovoviviparous meaning embryos develop and grow inside eggs inside the female until they are ready to be released. The embryos get nutrition from egg yolk, but are not connected to the mother by a placental connection. Viviparous meaning embryos develop inside the body of a female and usually gain nutrition by a placental connection. Females tend to have a single reproductive spawning period (when the ova moves to the gills) while males tend to have two spawning periods (a release of sperm into the water column).[4] Females have highest ova, or mature female reproductive cells, during September with a gradual decrease until December. Males spawn between September and December with a second spawning period between May and July.[4] Males release their sperm into the water column for females to accept. Females take up the sperm along with water through their circulatory system and have the potential to become fertilized when the sperm meets the ova. In contrast to Marine Bivalves, most female freshwater bivalve species hold the fertilized embryos until they develop into larvae when they are released into the water.[5]

Once larvae are released into the water column they become semi-parasitic and attach to the gills of freshwater fish. They attach to a host where they grow into juvenile adults while doing little to no damage to the fish host.[5] 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.[2]

Microbial water composition and sediment composition are important in larval nutrition.[6]

Example of Dreissena polymorpha (Zebra Mussel).

Dreissenidae are a family of freshwater mollusks considered to be an invasive species found across Eurasia and North America.[11] The most common types of dreissenids are Dreissena polymorpha (Zebra mussel) and Dreissena rostriformis (Quagga mussel).[11] These mussels damage both ecological systems and human infrastructure. In North America, biofouling caused by dreissenids created 267 million dollars’ worth of damage between 1989 and 2004.[11] 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.[11] The total impact of dreissenids on freshwater ecosystems is still unknown.

Pollution, human disturbance, invasive species, and ecosystem modification are the main threats to freshwater bivalves.[12] In North America freshwater bivalves are extremely threatened, with 202 of 300 species considered critical, possibly extinct, or extinct.[12] Of the dangers facing freshwater bivalves, eighty-five percent of them are considered to be “ongoing threats”.[12] Ecosystem modification and pollution are currently the two biggest threats to molluscs and gastropods in Palearctic and Nearctic ecozones.[12] Pollution is the dominant issue for these animals in the Afrotropic and Indomalayan biogeographic realms.[12] For the Neotropics and Australasia biogeographic realms, ecosystem modification has the largest impact on freshwater bivalve species.[12] 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.[13] Rates of extinction among freshwater bivalves are higher than those of terrestrial groups which share the same ecosystem.[12] Among those bivalves, freshwater gastropods are the most highly threatened due to smaller species distribution.[12] Freshwater bivalves are at a heightened risk for endangerment and extinction because of the connectivity of river systems.[12] Anthropogenic impacts on rivers spread throughout the whole ecosystem.[12]