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FEATURE ARTICLE

Tardigrades

These ambling, eight-legged microscopic “bears of the moss” are cute, ubiquitous, all but indestructible and a model organism for education

William R. Miller

Finding a New One

2011-09MillerF8.jpgClick to Enlarge ImageLast summer, the student who inquired at my office, Rachael Schulte, became an intern working on our National Science Foundation grant under the Research at Undergraduate Institutions (RUI) program designed to teach research by exploring and expanding the biodiversity of the phylum Tardigrada in North America.

After a couple of weeks of practice on lichen from local trees, Rachael had become proficient with the tools of the tardigrade trade—the dissecting scope, the wire Irwin loop, slide preparation, imaging, record keeping and identification to the level of genus. She was ready to work on actual research material, so we set her up with samples collected a couple of years before on a transect from more than 9,000 feet up in the Sierra Nevada Mountains down to Fresno, California.

Just a week later, she came to me with a finely made slide.

I think it is a Pseudechinsicus but it has many small, plates across its back, lateral filaments on the edge of each segment, and a toothed collar on the last legs, she says.
This was a teaching and a learning moment. I put the slide on the stage of our computer-imaging microscope to take a look.
Do you have other specimens?
Eight, all from the same sample, she replies.
I have seen this before, it is Pseudechinsicus because of this pseudo, or false, segmental plate. But it appears to also have a pseudosegmental plate on each segment. Let’s see how it keys out in Ramazzotti and Maucci, I say.

In 1983, Giuseppe Ramazzotti and Walter Maucci published the monograph The Phylum Tardigrada. It was translated from Italian into English by Clark Beasley in 1985. It is now 27 years out of date and includes only half of the described species. But it remains the reference of first resort. We started with the genus Pseudechinsicus. As I read the diagnostic questions in the key, Rachel worked the microscope to answer them.

The animal looked like Pseudechinsicus raneyi, as described by Gragrick, Michelic, and Schuster in 1964. We pulled up a copy of the paper from the files (we have PDF files of 95 percent of all tardigrade papers) and read. The description matched our animal. We then looked at the 1994 list of species, along with the relevant research papers and geographic distributions, prepared by McInnes. There were only two listings for our species—the original description from California and Schuster and Gragrick’s entry in their 1965 classic work on the western North American tardigrades, which added Oregon to Pseudechinsicus raneyi’s known range. Searching through the more recent literature in our database, Rachael discovered that I had also found the creature in Montana during my master’s work at the University of Montana, Missoula, in the late 1960s, although I did not publish the record until 2006. Now after 40 years we had a fourth record and a new location for an uncommon regional animal.

During our literature review, we learned that the genus was described by Gustav Thulin in 1911, who gave high taxonomic value to the presence of the pseudosegmental plate. Then in 1987, Kristensen revised the family Echinsicidae, redescribed the existing genera and added four new ones to the list. Because this occurred after our creature was described, we needed to confirm the genus assignment by reviewing its characteristics against the amended, more detailed description.

We started down the list of characteristics under the genus Pseudechiniscus I read the first line:

Echiniscidae with black eyes: rigid buccal canal, stylet supports may be present, but very tiny and located close to the margin of the pharyngeal bulb.
Looking through the microscope, under oil immersion at 1,000-times magnification, Rachael says:
Black eyes, yes, but the buccal canal is long and bent. Flexible. No visible stylets.
I continue, … unpaired scapular plate. Typical tiny basal secondary spurs.
No, it is paired. And the spurs are not tiny, nor basal, she says.
I look up and say, No mention of the extra pseudosegmental plate.

Our specimens did not match the description of the genus Pseudechinicus. So we checked the other generic descriptions within the family the same way and concluded that our specimens matched none of them. We now thought there were enough significant deviations from the existing descriptions to merit describing and naming a new genus.

Over the next several months we borrowed the original type specimen of Pseudechiniscus raneyi from the Bohart Museum at the University of California at Davis and confirmed that it was the same as our specimens. Rachael and I made images of the slides, measured multiple characteristics on each specimen and developed a comparative table. We checked and double checked our specimens. As we started to pass the draft of a manuscript back and forth, I asked Rachael whether she wanted to be a coauthor describing the new animal or to have it named after her.

I get to choose?
You found it, helped detect the differences, and contributed to the new description, I say.
What is the difference? she asks.
Both are a bit of immortality. But you can’t be an author and namee, I say. Normally, we would name a new genus after some unique characteristic of the organism, or to commemorate a fellow researcher. We could name the genus after you, but the species would remain raneyi because our specimens match the already described Pseudechiniscus raneyi. We are simply moving an existing species into a new genus, so the species name does not change, I explain. But if you are an author of the paper describing the new genus, your name goes after the genus name, before the date, every time the genus is listed, I say.
So what do we name it? she asks.
What is the most distinctive feature of the critter?
The elongated buccal tube? she says.
True, but that is very difficult to see. What else have you seen?
The extra pseudosegmental plates? she suggests.
Okay, how about Multi pseudechiniscus? I ask.

Rachael presented a poster about the discovery at the November 2010 Sigma Xi International Meeting and Student Research Conference in Raleigh, North Carolina, with 250 other undergraduate researchers. The new genus of water bear is shown in Figure 8. Our manuscript reporting the find is under review at a peer-reviewed journal.

Bibliography

  • Glime, J. M. 2010. Bryophyte Ecology. Online monograph in two volumes. Chapter 5: Tardigrades. Sponsored by Michigan Technological University and the International Association of Bryologists. www.bryoecol.mtu.edu.
  • Guil, N., S. Snachex-Moreno and A. Machordom. 2009. Local biodiversity patterns in micrometazoans: Are tardigrades everywhere? Systematics and Biodiversity 7:259–268.
  • Kinchin, I. M. 1994. The Biology of Tardigrades. London and Chapel Hill, N.C.: Portland Press.
  • Kristensen, M. A., et al. 2011. Survival in extreme environments—on the current knowledge of adaptations in tardigrades. Acta Physiologica 202:409–420.
  • Kristensen, M. A. 1987. Generic revision of the Echiniscidae (Heterotardigrada), with a discussion of the origin of the family. In Biology of Tardigrades, R. Bertolani (ed.). Selected Symposia and Monographs. Union Zoologia Italia, Mucchi Modena. 1:261–335.
  • McInnes, S. J. 1994. Zoogeographical distribution of terrestrial/freshwater tardigrades from current literature. Journal of Natural History 28:257–52.
  • Miller, W. R. 1997. Tardigrades: Bears of the moss. The Kansas School Naturalist. Emporia State University. 43:1–16.
  • Persson, D., et al. 2011. Extreme stress tolerance in tardigrades: Surviving space conditions in low earth orbit. Journal of Zoological Systematics and Evolutionary Research 49(suppl 1):90–97.
  • Welnicz, W., et al. 2011. Anhydrobiosis in tardigrades—the last decade. Journal of Insect Physiology 57:577–583.








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