Freshwater Molluscan Shells
The Unioninae of Eastern and Central North America are widespread and colorful. Several genera have well-developed sexual dimorphism. There are also several thin-shelled Anodonta species in lakes and rivers of the far West (see below).
Cyclonaias tuberculata (Rafinesque,1820)
"Purple Wartyback" Widespread, American interior.
Eliptio complanata (Lightfoot,
1786) "Eastern Elliptio" Wide ranging,
from Georgia to Ontario, Canada on the
Atlantic drainage. Nacre may be purple,
pink, white, or salmon.
Elliptio dariensis (Lea, 1842) "Georgia Elephant Ear" Southeastern United States, Atlantic drainage.
Of the 19 species of Elliptio listed by Burch (1975), only two are widespread in the American interior west of the Appalacians. The remainder are largely endemic to small areas on the Atlantic slope drainages, some Gulf drainages of the American southeast, and Florida. This is in contrast to North American unionaceans an general, whose greatest diversity is in the interior.
Regarding their origins and dispersal across the Appalacians (Great Eastern Divide, in part), good arguments can be made for either: 1) colonization of the Atlantic slope rivers from interior species, with subsequent radiation into vacant niches on the Atlantic slope only, or 2) colonization of the American interior by two members of an older diversified Atlantic slope fauna. Until the clams that did it are captured and interviewed, we will probably not be sure. Please see http://www.biosci.ohio-state.edu/~molluscs/Elliptio/index.htm for comprehensive information on this group.
Elliptio sp., central Georgia. Elliptio shepardiana (Lea, 1834)
"Altamaha Lance". Georgia rivers.
Alasmidonta arcula (Lea, 1838) "Altamaha Arc-mussel" Georgia. Right specimen is back-lit with incandescant light.
Anodonta suborbiculata Say, 1831.
"Flat Floater" American interior lowlands.
Anodonta (Pyganodon) gibbosa
Say, 1824, "Inflated Floater" Southeast
Atlantic drainage of the United States.
The widespread Anodonta (Pyganodon)
grandis Say, 1829 of the North American
interior is similar, but usually lacks the
Lasmigona complanata (Barnes, 1823) "White Heelsplitter" Widespread in American and Canadian interior.
Arcidens confragosus (Say, 1829)
Rock Pocketbook. American Midwest lowlands.
Lasmigona complanata, Potamilus ohioensis, Potamilus alatus, most Leptodea species (see below), and several others are more or less alate or winged. This characteristic has evolved in unionacean mussels more than once, in Africa, Asia, South America, and North America. The "wings" are extensions of composite shell material above the ligament, connecting the two valves at a sharp fold. This material is elastic and semi-flexible when wet and, like the ligament, tensioned against the adductor muscles to open the shell. The wings are best developed in young individuals, tending to break and frazzle as the animal matures. They become much more brittle when dry. For alate members of the family unionidae, any attempt to divide the two halves of the shell causes them to crack unevenly, like a chicken wishbone. Alate species tend to be thin-shelled, but many thin-shelled species are not. The function of the wing is unknown, but may be to protect the ligament, alter the flow of water over the siphon, or protect the siphon as the animal pulls itself forward through the substrate. It also causes the valves to press open more forcefully when the adductor muscles relax, an effect that could help the animal stay open as it moves. See http://members.aol.com/savetheclams/AsiaUnios.html (this site) for some more extreme examples.
Scanned image (left) of Potamilus alatus wing.
Sketch below is interpretative, showing what you
might see if you cut through a dried specimen more
or less across the ligament, out of view in the scan.
The small cross-membranes may not be present,
and tend to run parallel to the ligament or curve
down to meet it. The ligament thins to become
Potamilus purpuratus (Lamarck, 1819)
"Bluefer" Widespread, lowlands of the American
interior, and detail of hinge teeth, left.
Leptodea fragilis (Rafinesque, 1820)
"Fragile papershell" American interior.
Actinonaias pectorosa (Conrad, 1834)
"Pheasantshell" Mid-south rivers.
Ligumia recta (Lamarck, 1819)
"Black Sand Shell" Widespread, American interior.
Obliquaria reflexa Rafinesque,
1820 "Threehorn" American interior.
Lampsilis dolabraeformis (Lea, 1838)
"Altamaha pocketbook" Georgia.
Lampsilis splendida (Lea, 1838)
"Rayed pink fatmucket" Georgia.
Lampsilis siliquoidea (Barnes, 1823)
"Rayed fatmucket" Northern and western
Lampsilis teres (Rafinesque, 1820) "Yellow Sandshell" American south and interior.
Rafinesque, 1820. "Deertoe"
Epioblasma triquetra (Rafinesque, 1820) "Snuffbox"
Female on top, large male shell below. Members of this
genus have been among the hardest hit by environmental
degradation during the Twentieth Century. American interior.
Ptychobranchus subtentum (Say, 1825)
"Fluted Kidneyshell" American mid-south.
Toxolasma parvus (Barnes, 1823) "Lilliput"
Widespread in American interior and south.
North American far West (including mussels not within the subfamily Unioninae)
Burch (1975) lists only eight unionacean species in the Pacific drainages of the far West, most in the larger rivers of the Pacific Northwest.
See http://18.104.22.168/search?q=cache:7z1lpCGtwIQJ:columbiariver.fws.gov/pdfdocs/habitat/Proceedings.pdf&hl=en&ie=UTF-8 (http://columbiariver.fws.gov/pdfdocs/habitat/Proceedings.pdf) for recent work on western United States unionaceans.
Margaritifera falcata (see http://members.aol.com/savetheclams/Margaritiferidae.html this site.
A. dejecta (probably extinct)
Gonidea angulata (Lea, 1838) Western ridgemussel.
Northwestern rivers. Hinge teeth are vestigial or lacking,
periostracum is greenish in transmitted light. The "ridge"
is said to become better developed the farther north you
go, almost lacking in individuals towards the southern end
of their range.in central California. This specimen is from
the Oregon Coast Ranges
Anodonta kennerlyi Lea, 1860
Pacific Northwest. These are from the large lakes
in the vicinity of Seattle. Most Anodonta species
seem to change shape as they mature. An
immature individual (left) from the same location,
is substantially rounder and flatter.
Anodonta nuttalliana Lea, 1838. Rivers near Portland. Cracking is typical over time as the thin shell, high in
organic material, dries and shrinks unevenly.