A wide variety of Unionacea can be found in North America.
Superfamily: Unionacea
Major families: Margaritiferidae and Unionidae
Native Range: All species are native to North America
Habitat: Burrowed in the sediment (infaunal) of freshwater lakes, streams, rivers, ponds
Size: Shell length may range from 4-30 cm, with an average length of 15 cm
Life span: Long-lived (5+ years)
Reproductive cycle:
· Dioecious and hermaphroditic species; internal fertilization.
· Ovoviviparous (i.e., brooding embryos throughout early development stages on gill).
· Produce parasitic larvae, called glochidia, which attach to specific hosts, usually fish; glochidiae are NOT free-swimming although they are found in the plankton immediately after release from an adult.
· Juveniles drop off the host and burrow into the substrate.
Distinguishing characteristics:
Internal
· Pseudocardinal teeth (internal dorsal shell structures that aid in shell valve articulation) present.
· Lateral teeth (internal dorsal shell structures that aid in shell valve articulation) present.
In many species of bivalves the two portions of the shell are joined together by large and often distinct teeth that aid in holding the shell pieces together.
· Gills adapted for brooding of larvae.
Whereas larvae of Dreissenidae are completely pelagic (i.e., free-floating in the water column), other freshwater bivalves (e.g., Corbiculidea, Unionidae, Sphaeriidea) brood their larvae through early development stages in marsupial sacs on the gill. Shelled juveniles (arrows) of a fingernail clam, Musculium sp. (Sphaeriidae) can be seen in this scanning electron micrograph.
External
· Large, robust shell valves.
· Infaunal (burrows into sediments).
· Umbone located dorsally.
· Byssal threads absent.