| Unionidae | |
|---|---|
| The shell of a duck mussel, Anodonta anatina | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Mollusca |
| Class: | Bivalvia |
| Subclass: | Paleoheterodonta |
| Order: | Unionoida |
| Family: | Unionidae |
| Genera | |
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See text |
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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.
Contents |
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.
Some of the species in the freshwater mussel family, Unionidae, commonly known as pocketbook mussels have evolved a remarkable reproductive strategy. The edge of the female's body that protrudes from the valves of the shell develops into an imitation of a small fish complete with markings and false eyes. This decoy moves in the current and attracts the attention of real fish. Some fish see the decoy as prey, while others see a conspecific. Whatever they see, they approach for a closer look and the mussel releases huge numbers of larvae from her gills, dousing the inquisitive fish with her tiny, parasitic young. These glochidia larvae are drawn into the fish's gills where they attach and trigger a tissue response that forms a small cyst in which the young mussel resides. It feeds by breaking down and digesting the tissue of the fish within the cyst.[4]
Widespread
Africa
Central America and Mexico
Eastern Asia
Europe
India
Middle East
New Guinea
North America
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.[5] For example, in the Dinosaur Park Formation, fossil hadrosaur eggshell is rare.[5] This is because the breakdown of tannins from local coniferous vegetation would have caused the ancient waters to become acidic.[5] Eggshell fragments are present in only two microfossil sites, both of which are predominated by the preserved shells of invertebrate life, including unionids.[5] 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.[5]