Page 14 - The Freshwater Mussels of Oklahoma

ZOOGEOGRAPHY

Although there is a lot of data on the distributions of unionids, many of their
distributions are difficult to explain. Strayer (1987) summarized generalizations made by
Ortmann (1913) and gave some of his own opinions primarily on the Hudson River basin
of northeastern United States. Ortmann’s conclusions (in Strayer, 1987)include:
1. The Appalachian Mountains have formed an effective barrier for unionids
and the result are distinct Atlantic Slope and Interior Basin faunas.
2. The Atlantic Slope fauna is depauperate and derived mostly from Interior
Basin species. Very little evolutionary radiation on the Atlantic Slope.
Most unionids apparently dispersed to the Atlantic Slope around the
northern and southern ends of the Appalachians.

In general, species richness tends to diminish from lower to upper (or headwater)
areas of drainage systems. Large river species then tend to be widespread. That is, river
systems like the Mississippi have numerous large tributaries that are spread over a large
area and species adapted to those large rivers can disperse through those continuous
waterways. On the other hand, headwater species tend to be fairly well isolated from the
headwaters of other tributaries. The large rivers themselves may function as barriers to
these species.
Unionids are perhaps victims of their own ecological niche and reproductive
processes. By their very nature, they are relatively immobile. The fact that they are
mostly dioecious means that a male can only reproduce if there is a female downstream
to receive sperm and a female can only reproduce if there is a male sperm source
upstream. If it weren’t for the parasitic stage on relatively mobile fish hosts, the
population would, through time march irreversibly downstream. Fish parasitism is
obviously a great solution to this problem and many fish hosts for the glochidia tend to be
anadromous to some extent. This allows a continuous re-seeding of a population
upstream as far as habitats suitable for the mussel species or its fish host. There is a great
deal to learn about the fish host/mussel glochidium relationship. It appears that in many
cases hosts are fairly specific. This then ties the mussel to the successes or failures of its
host fish. If a fish is capable of dispersing from river system to river system, headwater
to headwater, over mountain barriers, through saline barriers, etc. then its parasitic
hitchhiker also has a chance of making the trip. However, if a mussel is tied to an
endemic headwater fish species (like a darter) then it may have little or no chance of
dispersal.
These factors together with the complicated changes in patterns of stream flow
through geological time can really paint a complex picture. Stream capture in headwater
areas and confluence and separation in coastal areas due to tectonic processes of rising
and sinking land masses or increases and decreases in sea level, also due to tectonic
forces or climatic changes (glacial and interglacial periods) make it extremely difficult to
decipher current distributions.
The following is an example of a problem in the Oklahoma and Texas region (see
Map 1). Quadrula apiculata occurs though much of Texas and it has its variant in the

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