Nature Obscura. Kelly Brenner
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tardigrades will lay eggs on moss or in water and leave them, but others will cleverly lay them as they shed their cuticle, or skin, leaving the eggs inside a protective shell until they hatch. During my time searching through these microscopic worlds, I found many empty cuticles, also called exuvium, but had less success finding any eggs.
But in Dr. Jenny Tenlen’s lab at Seattle Pacific University, tardigrade eggs are easy to find. This isn’t Jenny’s first time teaching in Seattle; before earning her PhD she taught various sciences at a local high school. After time spent at the University of North Carolina–Chapel Hill as a postdoctoral research associate and teaching fellow, she returned to Seattle as an associate professor of biology. Now she teaches undergrad students biology and also studies tardigrades to learn more about germline development. This has to do with the development of cell lineage that leads to reproductive cells. Studying this cell development in mice and other traditional lab-study organisms is challenging because it happens within a few days of conception, and even with dissection, it’s hard to see. But tardigrades—which are small and transparent, allowing their egg development to be easily observed under the microscope—are the ideal study animals for Dr. Tenlen’s work.
When I visited Jenny’s lab, she placed a dish of tardigrades under the microscope so I could see her tiny water bears. My view was filled with them, and there, right in the center, was a cuticle filled with large, round eggs.
Jenny told me that despite their ability to survive in space or when exposed to radiation, tardigrades generally make lousy lab subjects. Many species of tardigrades have been tried in labs, but they survive no longer than one or two generations before dying out. Jenny was impressed I had managed to keep my tardigrades alive for a full month in a water-filled glass dish on my desk.
Still, one species has been grown in labs for three decades now, started by a hobbyist who now sends specimens to researchers. This species, Hypsibius exemplaris, is not the one that was sent to space, and Jenny calls it “wimpy” because it doesn’t form tuns well. But whether that’s because it’s a temperate species or has lived so long in labs, she doesn’t know. It’s a cosmopolitan species that can be found around the world, including the Pacific Northwest.
Dr. Tenlen’s lab is one of just a handful around the world that studies tardigrades. She said she could count the number in the United States on two hands; but there are other labs in Poland, Germany, Japan, and a few other countries. Many of those labs, however, aren’t solely focused on tardigrades. As a result, little is known about the creatures’ life histories. Typically, science funding is directed to studies of organisms that are either beneficial or harmful to humans, and tardigrades have very little influence on human life. Now that their survival skills have become renowned, most of the funding for studies involving them goes toward studying their stress responses. Only recently have tardigrades been used in studies on human health with an interest in cryopreservation of human cells.
During our conversation, Jenny rattled off a list of things we still don’t know about water bears, including basic facts about their life history. How long do they live? We don’t know because it’s often impossible to determine how much time they’ve spent as tuns. Tardigrades are hard to study. Many species don’t survive long in labs, and they also live accelerated lives—an egg transforms into a sexually mature adult in only two weeks. We also don’t have complete information about where they live. Some habitats have been explored, but we don’t have a full picture, nor do we have a good grasp on population densities in different habitats.
Like many invertebrates, tardigrades molt out of their skin as they grow. But they don’t only molt their skin; they also shed their mouthpart, called a stylet. Why do they do this? And how do they have the ability to regrow an entirely new mouth on a weekly basis? When they lay eggs, it’s usually between one and ten, but why does the number vary? Is it environmental or caloric? All of these questions just go to show that despite becoming famous, the tardigrade is still very much an animal of mystery. And even when we think we do know something, we can be thrown a curveball once in a while.
At the time of my visit, Dr. Tenlen had recently finished a protocol and was awaiting her paper’s publication. But there was just one little problem. Scientists at a lab in Poland had their own paper in the pipeline regarding Dr. Tenlen’s lab species, which at that time was called Hypsibius dujardini. The question they were asking was this: Was Hypsibius dujardini actually Hypsibius dujardini? There may have been a translation error from the original French species description to Italian, causing a potential breakdown in the species identification. It was possible the Hypsibius dujardini species currently in labs around the world could actually have been an entirely different species. Such a simple question of identity, but with huge implications. Eventually, the lab in Poland found that, based on some anatomical differences, Hypsibius dujardini is actually Hypsibius exemplaris. Dr. Tenlen’s article sat in purgatory for over a year until the question was resolved—all because of a tardigrade we thought we knew.
After observing eggs in Dr. Tenlen’s lab, I returned home to set up a new dish of pet tardigrades to study. I wanted to find one carrying eggs and began by collecting moss from the bottom of our back stairs, scraping it off the concrete surface. I didn’t search long before finding my first specimen, and I examined it closely. It was clear and I could easily see through it, and I thought I detected spherical objects inside. But surely, that was much too lucky? I turned up the power of the microscope, looked closer, and discovered that there were definitely eggs inside. They were enormous compared to the water bear’s body, and I couldn’t fathom how they all fit inside—and I’m not alone; scientists still don’t understand this about tardigrade development either. It was hard to get an accurate count because the tardigrade was constantly in motion, and as her body moved from side to side, the eggs squished back and forth.
I cleared a space in the debris around the water bear so I could see her more easily, but I was soon distracted by a much larger nematode thrashing off to the side. Easily ten times the length of the tardigrade, it soon moved into the center of my view. The unfortunate gravid tardigrade was bumped and thrown around by the larger animal, but her legs never stopped moving, and at one point she managed to grab onto the nematode, holding on like a cowgirl on a bucking bronco. After a few good bucks she was thrown off, and I used a tool to pull the nematode away from her so she could continue in peace.
Somehow the tardigrade ended up on her back with her little legs clawing at the water until she found a patch of debris and was able to climb, although with eight legs it looked more like gliding. But as her head lifted up toward me, she fell back again. Was the burden of those eggs too much for her? She spent some time scrambling on her back again, as if unable to right herself. I wondered if she could sense me. Every bump of the glass dish sent her tumbling, my touch having an earthquake-like effect on this tiny world. Even my breath rippled the water, and I had to hold my hand in front of my face to not disturb it.
I flipped the switch on the microscope, turning the backlight off and the overhead light on. Suddenly the little water bear turned from transparent to a translucent white color and I could see a patch of yellow on her. The yellow looked shiny, almost like she’d ingested tiny flecks of gold. I had a hunch and flipped the switch again. The gold appeared to be in the same place as her eggs—were the eggs gold? As I watched this endearing creature with her golden eggs, I thought with guilt about her living in the moss at the bottom of the stairs, being stepped on by us giant Homo sapiens. I suppose that’s nothing compared to a voyage to space, but I apologized to her anyway.
The Crow Roosts at Night
They come from the north. Every night as the sky darkens and the sun touches the horizon, hundreds of them fly over our house in one long, continuous line, a river that may change course to the east or west of us but always within view. They are the commuters of the sky, unimpeded by traffic lights and not limited by the routes of the road. They fly together on the same plane, but once in a while an individual will dive or swoop over a tree or over nothing at all, seemingly just for fun. They fly through the rain, the wind, the fading sunset. Many nights I stop what I’m doing to watch them. The half-closed curtains are still gripped in my hands when my mind