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For most people, I suspect, the word “Daphnia”
brings to mind no clear picture. So let me describe them for you. Daphnia are a
genus of tiny aquatic crustaceans. Like larger crustaceans you may be familiar
with, such as shrimp, they have segmented legs and bodies protected by a rigid
exoskeleton. But even the largest species of Daphnia grow no more than five
millimeters long, and so could easily rest atop a pencil eraser.
Daphnia occupy a wide range of freshwater aquatic
habitats, including the type of wetlands known as ephemeral ponds, which are
pools that form during the wet part of the year and dry up completely sometime
after. That’s where Chris Holmes studies them.
Holmes is a graduate student in the UI Department
of Animal Biology working in the lab of professor Carla Cáceres. Through his Master’s project, he seeks to shed light on a big
ecological question—how diversity influences community assembly dynamics—by
manipulating both species and genetic diversity among Daphnia and other tiny
crustaceans in experimental ponds.
[Photos: Daphnia through a microscope (Chris Holmes); Holmes and project assistant Kelly Hogan discuss sampling strategy (Ryan Smith).]
These
creatures make great subjects for Holmes’ work because they reproduce quickly, which
should enable him to witness evolutionary change over generations in a
relatively short period of time.
Thanks to
a connection his adviser has with Kim Schulz at the State University of New
York - College of Environmental Science and Forestry in Syracuse, Holmes has
the opportunity to pursue his study at an ideal site, a group of recently
created experimental ponds in upstate New York. The site includes 71 experimental
ponds in all, and they vary in terms of structure, depth and the landcover surrounding
them.
In
addition to his advisor, Holmes is collaborating on the project with Schulz.
They began work in 2010, when the ponds were excavated, by stocking them with
different mixes of Daphnia and other tiny freshwater creatures. This past
summer, Holmes, with a team of undergraduates he mentors, lived in Syracuse so
he could get out to sample the ponds once every two weeks.
How do Holmes
and company sample for aquatic zooplankton in a pond? They dip a plastic
pitcher into the water and pour it through a sieve with holes almost too small
for the human eye to see. What’s left in the sieve are hundreds, or even
thousands of tiny organisms.
Back in the lab at Illinois, Holmes uses a
microscope to count the Daphnia and other organisms from his samples and sort
them by species. He points out, “This can be pretty tricky, because on the
smallest ones you’re comparing legs and antenna that are only about a
millimeter long.”
Some of the other research taking place at the
ponds where Holmes is working has fairly straightforward, practical
applications. For example, it is anticipated that research on amphibians there
will provide land managers with answers to questions such as “what’s the best way
to make a pond if you’re trying to create breeding habitat for salamanders and
frogs.
In contrast, Cáceres and Holmes are pursuing more fundamental questions. It is
well understood, they note, that populations can adapt to their environment and
evolve over time. “What scientists don’t understand so well,” says Professor Cáceres, “is how the adaptation of one
species may influence interactions with other species that occupy the same
environment. That’s how Holmes' research is important to ecology.”
