Unionidae is a family of freshwater mussels, the largest in the order Unionoida, the bivalve mollusks sometimes known as river mussels, naiads, or simply as unionids.

The range of distribution for this family is world-wide. It is at its most diverse in North America, with about 297 recognised taxa^[1][2][3], but China and Southeast Asia also support very diverse faunas.

Freshwater mussels occupy a wide range of habitats, but most often occupy lotic waters.

Life habits

Unionidae burrow into the substrate, with their posterior margins exposed. They pump water through the incurrent aperture, obtaining oxygen and food.

Unionidae are distinguished by a unique and complex life cycle. Most Unionids are of separate sex (although some species, such as Elliptio complanata, are known to be hermaphroditic). The sperm is ejected from the mantle cavity through the male’s excurrent aperture and taken into the female's mantle cavity through the incurrent aperture. Fertilised eggs move from the gonads to the gills (marsupia) where they further ripen and metamorph into glochidia, the first larval stage. Mature glochidia are released by the female and then attach to the gills, fins or skin of a host fish. A cyst is quickly formed around the glochidia, and they stay on the fish for several weeks or months before they fall off as juvenile mussels which then bury themselves in the sediment.

Genera

Genera within the family Unionidae include:

• Genus Actinonaias
• Genus Alasmidonta
• Genus Amblema
• Genus Anodonta
• Genus Cucumerunio Iredale, 1934
  • Cucumerunio websteri delli, McMichael & Hiscock, 1958
  • Cucumerunio websteri websteri, Simpson, 1902
• Genus Elliptio
• Genus Epioblasma
• Genus Fusconaia
• Genus Hamiota
• Genus Hyridella Swainson, 1840
  • New Zealand freshwater mussel, Hyridella menziesi, Gray, 1843
• Genus Lampsilis
• Genus Lasmigona
• Genus Leptodea
• Genus Ligumia
• Genus Obliquaria
• Genus Plethobasus
• Genus Pleurobema
• Genus Pseudanodonta
  • Depressed river mussel, Pseudanodonta complanata
• Genus Ptychobranchus
• Genus Pyganodon
• Genus Quadrula
• Genus Quincuncina
• Genus Sinanodonta
  • Sinanodonta woodiana
• Genus Strophitus
• Genus Truncilla
• Genus Unio
• Genus Utterbackia
• Genus Villosa

Taphonomic implications

In large enough quantities, unionid shells can have enough of an impact on environmental conditions to effect the ability of organic remains in the local environment to fossilize.^[4] For example, in the Dinosaur Park Formation, fossil hadrosaur eggshell is rare.^[4] This is because the breakdown of tannins from local coniferous vegetation would have caused the ancient waters to become acidic.^[4] Eggshell fragments are present in only two microfossil sites, both of which are predominated by the preserved shells of invertebrate life, including unionids.^[4] It was the slow dissolution of these shells releasing calcium carbonate into the water that raised the water's pH high enough to prevent the eggshell fragments from dissolving before they could be fossilized.^[4]

References