By Andrew Stuart
The Animal Kingdom – that category of living things of which we count ourselves members – is staggeringly diverse, and it’s a diversity we’ve only begun to comprehend. There are thought to be more than 7 million animal species on the planet – only about 1.5 million have been described by science.
Much of that diversity escapes the human eye – but microscopic doesn’t mean insignificant. At the turn of the 18th century, naturalists, using early microscopes, identified creatures they named rotifers – “wheel-bearers.” These aquatic beings are the planet’s smallest animals – most are less than half a millimeter long, and many are far smaller. Their heads are topped by a rotating corona, the “wheel” of their name, which they use to sweep organic detritus, algae, dead bacteria and protozoans toward their chitinous jaws. They’re a cornerstone of life in aquatic ecosystems.
Dr. Elizabeth Walsh, of the University of Texas at El Paso, has been researching rotifers in the Chihuahuan Desert for more than two decades. In studying these aquatic creatures in our arid land, she’s shed light on a world of hidden marvels.
“We’ll start with some big rotifers first,” Walsh said. “That makes them easier to see.”
At her UTEP lab, Walsh and her graduate students culture rotifers collected across the Chihuahuan Desert. Walsh has studied rotifers from hundreds of springs, streams, playas and tinajas – from Big Bend to Guadalupe Mountains and White Sands national parks, and deep into Mexico.
Along with microscopic crustaceans called copepods and cladocerans – or “water fleas” – rotifers are “zooplankton.” That means that, while many can swim, they can’t propel themselves against a current.
As resilient and adaptive as they are, you couldn’t call rotifers helpless. But they are prey for… almost everything larger than they are. And in that respect, they’re irreplaceable.
“They’re the base of the animal food web,” Walsh said. “That’s why they’re so important in aquatic systems, because they take energy from bacteria, fungi, protists, convert it into rotifer biomass, and then everything bigger than rotifers eats them – everything from hydra to insect larvae to larval fishes.”
The rotifer world is a microscopic matriarchy. Most rotifers can reproduce through parthenogenesis – female rotifers clone themselves. But some rotifers reproduce both sexually and asexually. In this respect, they’re utterly unique.
“That’s one reason that people like to study rotifers,” Walsh said, “is to look at the evolution of sex – because you have all possibilities within one phylum.”
Male rotifers are short-lived – and sex is their sole task. They’re efficient at it. In some species, males engage in “egg guarding.” Using unknown cues, they detect eggs bearing female young, that are about to hatch, and linger close – so they can inseminate the females as soon as they’re born.
Rotifers thrive in ephemeral waters – and they have remarkable adaptations for dealing with dry periods. Rotifers called bdelloids can survive extended desiccation in a state called anhydrobiosis. Scientists recently found bdelloids that had been frozen in Siberian permafrost for 24,000 years – when they added water, the bdelloids resumed their life cycle.
Other rotifers lay eggs that can endure in a resting state for years, waiting to be watered.
Walsh has done her deepest research at Hueco Tanks State Park and Historic Site, near El Paso, at rock pools there. As ephemeral waters go, these pools are extreme even by desert standards. They’re often dry two days after a rainfall.
When the pools fill, female rotifers emerge from resting eggs. In all previously studied rotifer species, females must clone themselves asexually before producing males. But at Hueco Tanks, there’s no time to waste – a proportion of females here, Walsh found, immediately produce male young. The males in turn fertilize eggs, to complete the process of producing the resting stage before the pool dries.
Walsh’s research at Hueco Tanks has yielded other wonders. There’s one rotifer species there that she’s found nowhere else – indeed, its closest relation appears to be in Australia.
Which raises a larger question – how do rotifers become established in pools, springs and other isolated desert waters, separated from one another by many miles?
Some dispersal surely takes place via other animals – resting eggs are transported on a bird’s foot, or a hiker’s boot.
But in our region, wind may be a major factor. Walsh and her students collected dust – from numerous classic El Paso dust storms – and analyzed the contents.
“Based upon the DNA sequencing, we found all kinds of different taxa in the dust,” Walsh said. “It included rotifers and other micro-invertebrates that live in aquatic habitats. We were able to demonstrate that these resting stages could move not only locally, but regionally through these dust storms.”
In spring windstorms, rotifer eggs are being broadcast across West Texas. By chance or fate, a few will arrive at a water source. The unique Hueco Tanks rotifers are likely the descendants of such wind-blown eggs – which, over time, have adapted to their distinctive habitat, and become a distinct species.
Walsh and her students are probing other rotifer mysteries. Some rotifers form colonies – which may help them avoid predation, or feed and reproduce more efficiently.
Rotifers are indicators of the health of aquatic ecosystems – without them, those systems could collapse. The planet’s smallest animals remind us of the hidden connections on which all life depends.