habitats, but we were surprised to find how unique
each individual population of crayfish actually is,”
said Helms.
“We discovered that the two species that seemed
outwardly so similar,” said Santos, “actually
experienced a very different evolutionary history in
the Tallapoosa River Basin.”
Even between populations of the same species
there were such extreme genetic differences that
Helms and Santos believe the Tallapoosa Crayfish
and the Slackwater Crayfish may actually have
“species” within the “species,” or “cryptic diversity.”
antennae; however, color differentiation isn’t
always evident, especially since once crayfish are
preserved, the color changes. The most evident
physical differentiation is the second tail segment,
or pleuron, which in the Tallapoosa Crayfish is
angled and in the Slackwater Crayfish it is curved.
Helms noted that perhaps the most important
difference between the two species is their
preferred habitat: the Tallapoosa Crayfish seems to
prefer gravel and cobbles in swift-flowing waters,
whereas the Slackwater Crayfish is generally
more common in undercut banks and root wads
found in slower moving water. The difference is
significant because geographic separation between
populations mirrored the genetic differences and
similarities Santos discovered between populations
of both species.
“Going into this study, we had ideas on how
the two species would be related based on their
Researchers discovered another unexpected
variation; Tallapoosa Crayfish and Slackwater
Crayfish populations in the same geographic region
are typically more closely related to one another
than either is to members of the same species in
other locations.
“When we saw these genetic
differences, we kind of
scratched our heads and
wondered how it happened,
so much genetic variation.
We are kind of like forensic
scientists – we just look at
how things unfold, at what
nature has done,” said Santos.
Scientists can only speculate about how the
two species of crayfish came to be so genetically
divergent.
“The Tallapoosa River region in Alabama has
experienced some interesting events geologically,
like higher sea levels breaking up populations,
then reconnecting after the sea level dropped,” said
Santos. “Whatever caused the genetic differences
between populations, we know the event wasn’t
recent because DNA mutations happen at a
predictable rate. We know it was something that
happened hundreds of thousands of years ago. It
was most likely sea levels changing.”
A better understanding of crayfish provides a better
understanding of freshwater systems in general,
according to Santos and Helms, due to their unique
behaviors, abundance, size, feeding habits, and role
as a significant food source for many fish and birds.
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Journey/Fall:
Journey/Fall: 2015
2015