Rotifer Diversity
Testudinella patina (Photo by T. Schröder, R.L. Wallace, E. Walsh)
Collotheca ornata (Photo by T. Schröder, R.L. Wallace, E. Walsh)
Asplanchna brightwelli (Photo by T. Schröder, R.L. Wallace, E. Walsh)
By Megan Wilde
Wherever water punctuates our arid region, there’s a universe of invisible life. Every pristine spring, mucky cattle tank, and rain-filled hueco is home to a diverse array of microscopic animals. Who are these creatures? And why does the Chihuahuan Desert have a uniquely varied assortment of them?
Liz Walsh is a biology professor at the University of Texas at El Paso. She’s spent years surveying tiny aquatic animals called zooplankton in the Chihuahuan Desert.
Walsh: Although many people haven’t heard of them, they’re very important in maintaining the integrity of freshwater habitats. They do a lot of nutrient cycling, and they’re also components of the aquatic food web. So basically insect larvae, fish, everybody eats zooplankton.
Rotifers are one group of zooplankton that she’s studied particularly closely. Their name means “wheel-bearer” and refers to a ring of whirling cilia around their mouths.
Walsh: Some of them are predators and have quite scary-looking jaws, and they can extend them out of their body and grab prey items like little tweezers. Some of them are perfectly flat like a little round pancake swimming around. Some of them have really super long toes.
They can be found wherever there’s water, from spring-fed pools and ephemeral playas to water drops on soil particles. They thrive in so many environments thanks partly to how they reproduce.
Rotifers can clone themselves as well as produce eggs sexually. These eggs are in a state of suspended animation and won’t hatch until a specific set of environmental conditions have been met, like a certain day length or water temperature. They’re patient too, with some capable of waiting more than 100 years to hatch.
Walsh: And that’s how these zooplankton get dispersed among these aquatic habitats in the desert. Because you can imagine that these habitats are surrounded by a vast, dry landscape, where no small, essentially microscopic animals could travel very easily. But they form resting eggs which can be carried by water or wind or by animals.
Once they hatch, some species only live about four days, one of the fastest life cycles of any animal. This is an asset in the desert, where watery habitats can quickly dry up.
When it comes to aquatic creatures, most people don’t think of the desert as a biodiversity hotspot. But Walsh has found that our region’s hodgepodge of water sources harbors an exceptionally diverse range of zooplankton.
For example, Walsh says the rotifer diversity at Hueco Tanks State Park and Historic Site near El Paso alone is higher than in several of the world’s other deserts combined. And of the more than 2,000 rotifer species known to science, over 250 are found in the Chihuahuan Desert.
Walsh cautions these figures may be partly due to the fact that few scientists study desert rotifers. Still, it’s safe to say our region’s zooplankton diversity is extraordinary. And Walsh thinks the theory of island biogeography explains why.
Desert zooplankton essentially live in islands of water. And island animals typically must pick mates who share their insular habitat. This reproductive isolation is thought to ultimately lead to the formation of new species. It’s the same theory that explains the Galapagos Islands’ legendary diversity.
Such diversity is irreplaceable. Every time a desert spring goes dry or becomes polluted, the invisible universe of microscopic creatures depending upon it is lost.
Walsh: So, I just hope everybody will conserve water and think carefully about that there’s this whole interesting, diverse group of animals using that resource as their habitat, and the important roles they play in maintaining our freshwater resources.
Have a question or comment about this episode? Contact Nature Notes Coordinator Megan Wilde at mwilde [at] cdri [dot] org. Or discuss this episode on Nature Notes’ Facebook page.
References & Resources for Educators
- Hueco Tanks State Park and Historic Site: Texas Parks and Wildlife Department
- Liz Walsh: research and publications (University of Texas at El Paso)
- Introduction to the Rotifera: University of California Museum of Paleontology
- “Sea Soup: Zooplankton” by Mary M. Cerullo and Bill Curtsinger (Tilbury House Publishers, 2001)
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Nature Notes is sponsored by the Meadows Foundation and the Dixon Water Foundation and is produced in cooperation with Marfa Public Radio.
