Environmental Almanac: Studying tiny creatures to answer big questions

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 5 millimeters long and could easily rest atop a pencil eraser.

By means of specialized swimming appendages, Daphnia propel themselves through the water in short bursts that remind some observers of the hopping of fleas, so even though they are not insects, they are sometimes called water fleas.

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 University of Illinois Department of Animal Biology, working in the lab of Professor Carla Caceres. 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.

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 Caceres has with Kim Schulz at the State University of New York — College of Environmental Science and Forestry (SUNY-ESF) 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 landcover surrounding them.

In addition to his adviser, Holmes is collaborating on the project with Schulz. They began work in 2010, when the ponds were excavated, by stocking them with different levels of genetic diversity and species diversity of freshwater zooplankton, including Daphnia.

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 Daphnia 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. Since they want to get samples from the various sub-habitats within the pond, they affix the pitcher to a pole to access the water in the middle without creating too much disturbance.

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. "This is tricky," he said, "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 shape and depth for a pond if you're trying to create breeding habitat for salamanders and frogs?"

In contrast, Caceres 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," said Caceres, "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."

Environmental Almanac is a service of the UI School of Earth, Society and Environment, where Rob Kanter is communications coordinator. Environmental Almanac can be heard on WILL-AM 580 at 4:45 and 6:45 p.m. on Thursdays.

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