ELECTRIC ELLIPTIO LAND
in North America, North of Mexico
The freshwater mussel (Unionidae) genus Elliptio has baffled malacologists for centuries. Native to North America and northern Central America, most species occur along the Atlantic Coastal Plain and Gulf drainages. Exactly how many species are involved is certainly debatable -- estimates range from a handfull to many dozens of species. And there is no wanting for names. Isaac Lea and the Wrights named numerous taxa, the identity of many remaining uncertain.
Given the topography of the Atlantic Coastal Plain and Gulf drainages, it is not surprising that we may expect a high level of endemism. Most of the drainages run parallel to the sea, offering a chance for speciation during marine transgressions. Sepkowski and Rex (1974, Systematic Zoology 23:165-188) argued that mussel taxa moved between these rivers on anadromous fish hosts but this idea has not gained popularity. More likely, the taxa have been exchanged between systems during periods of headwater capture. An examination of the Atlantic Coastal Plain mussels shows that most are related to headwater Ohio River species. Lacking are representatives of the big river fauna -- Quadrula, Amblema, etc., and only a single Fusconaia and no Pleurobema. Obviously, the Atlantic Coastal Plain fauna is derived from stream capture between the headwaters of the Ohio River system and those creeks to the east. To the south, the Mobile River system has commingled with the Ohio River system as well, but not just at the headwaters, resulting in many big river taxa.
If this is the case, then the Atlantic Coastal Plain and Gulf Elliptios are derived from a very few interior ancestors. Only two species occur in the Mississippi River system -- Elliptio dilatata and E. crassidens. Both have obvious representatives in the Atlantic Coastal Plain and Gulf drainages. But most southeastern Elliptios were probably derived from each other once the initial "seeding" occurred.
This overview is based on shell material because 99% of the material in collections are shells. Although genetic approaches have been tried on a very few of these taxa, a genetic resolution to this problem is a long way off. I propose here a scheme based on years of study of shell morphology and patterns of zoogeography.
Everyone has their own opinion on this group -- this is mine. We are indebted to Dr. Henry McCullagh and Bob Butler for the numerous specimens, although the views presented here are strictly our own.
Species are shown in groups of superficially similar species. The groups do not necessarily reflect phylogenetic relationships. The site is divided into the following species groups:
Arctata Group
Buckleyi Group
Complanata Group
Crassidens Group
Dilatata Group
Icterinia Group
Shepardiana Group