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Feature Articles
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
The young woman in my office doorway is
inquiring about the summer internship I
am offering.
What’s a tardigrade? she asks.
T
ardigrades, I reply, are microscopic, aquatic animals found just
about everywhere on Earth. Terrestrial
species live in the interior dampness of
moss, lichen, leaf litter and soil; other
species are found in fresh or salt water. They are commonly known as water bears, a name derived from their
resemblance to eight-legged pandas.
Some call them moss piglets and they
have also been compared to pygmy
rhinoceroses and armadillos. On seeing them, most people say tardigrades
are the cutest invertebrate.
At one time water bears were candidates to be the main model organism for studies of development. That
role is now held most prominently by
the roundworm Caenorhabditis elegans,
the object of study for the many distinguished researchers following in the
trail opened by Nobel Prize laureate
Sydney Brenner, who began working
on C. elegans in 1974. Water bears offer the same virtues that have made C.
elegans so valuable for developmental
studies: physiological simplicity, a fast
breeding cycle and a precise, highly
patterned development plan. Some
species may, like C. elegans, be eutelic,
William R. “Randy” Miller is director of biological
research and an assistant professor of biology at
Baker University in Kansas. He received his bachelor of arts and master’s degrees from the University of Montana and his Ph.D. from the University
of New England. Address: Mulvane Science Hall,
Baker University, Baldwin City, Kansas 66006.
E-mail: [email protected]
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American Scientist, Volume 99
meaning that the organisms retain the
same number of cells through their
development. Tardigrades have somewhere over 1,000 cells. I and others use
water bears as a model educational organism to teach a wide range of principles in life science.
Tardigrades are nearly translucent
and they average about half a millimeter (500 micrometers) in length,
about the size of the period at the end
of this sentence. In the right light you
can actually see them with the naked
eye. But researchers who work with
tardigrades see them as they appear
through a dissecting microscope of 20to 30-power magnification—as charismatic miniature animals.
Most tiny invertebrates dart about
frantically. Tardigrades move slowly as
they clamber around on bits of debris.
They were first named tardigrada in
Italian from the Latin meaning “slow
walker.” Tardigrades walk on short,
stubby legs located under their bodies, not sticking out to the sides. These
stout legs propel them unhurriedly
and deliberately about their habitat.
Tardigrades have five body sections,
a well-defined head and four body
segments, each of which has a pair
of legs fitted with claws. The claws
vary in different species from familiarly bearlike to strangely medieval
fistfuls of hooked weaponry. The hindmost legs are attached backwards, in a
configuration unlike that of any other
animal. These legs are used for grasping and slow-motion acrobatics rather
than for walking.
Inside these tiny beasts we find anatomy and physiology similar to that of
larger animals, including a full alimentary canal and digestive system. Mouth
parts and a sucking pharynx lead to an
esophagus, stomach, intestine and anus.
There are well-developed muscles but
only a single gonad. Tardigrades have
a dorsal brain atop a paired ventral nervous system. (Humans have a dorsal
brain and a single dorsal nervous system.) The body cavity of tardigrades is
an open hemocoel that touches every
cell, allowing efficient nutrition and gas
exchange with no need for circulatory
or respiratory systems.
Taxonomists divide life on Earth
into three domains: Bacteria, Archaea
(an ancient line of bacterialike cells
without nuclei that are likely closer in
evolutionary terms to organisms with
nucleated cells than to bacteria), and
Eukarya. Eukarya is divided into four
kingdoms: Protista, Plantae, Fungi and
Animalia. Phylum Tardigrada is one
of the 36 phyla (roughly, depending
on whom one asks) within Animalia—
making water bears a significantly distinctive branch on the tree of life.
Tardigrades are encased in a rugged
but flexible cuticle that must be shed as
the organism grows. Thus they have
been placed among the phyla on the
ecdysozoa line of evolution between
animals such as nematodes and arthropods that also shed their cuticles
to grow.
I can see in my drop-in student’s eyes
that she remembers the ecdysozoan way of
being from introductory biology but she
does not really understand it.
Animals grow in either of two ways,
by adding more cells or by making each
cell larger. Tardigrades generally do the
latter. If an animal has a hard cuticle or
exoskeleton, it must break out of that
shell in order to grow. For example, in
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Eye of Science/Photo Researchers
Figure 1. In this colorized electron micrograph (EM), which has the feel of a museum diorama, a tardigrade emerges from under a moss leaf to
hunt for food or a companion. EMs are produced by layering a molecular film of metal on a sample. The technology gives a false sense of the
“hide” of this tardigrade. In actuality, tardigrades are translucent and display a variety of colors—white, green, orange, red. In the microenvironments made by water that coheres in the fissures of mosses and lichens due to surface tension, tardigrades thrive by feeding on smaller
organisms and by sucking contents out of plant cells. Their moist realm is transient, and in response tardigrades have evolved an array of
strategies based on induced cryptobiosis—the suspension of metabolism by drying or freezing. In their cryptobiotic state, desiccated or frozen,
they are astonishingly durable. These organisms survive extreme conditions—of temperature, pressure and radiation—to a degree unparalleled in nature.
summer in many parts of the world,
one encounters the shed exoskeletons
of locusts on trees everywhere.
Tardigrades are divided into two
classes, Eutardigrada and Heterotardigrada. As a general rule, the members of Eutardigrada have a naked or
smooth cuticle without plates, whereas
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the Heterotardigrada boast a cuticle
armored with plates.
Tough Customers
Tardigrades’ best-known feature is
their brute, dogged ability to survive
spectacularly extreme conditions. A
few years ago, the Discovery network
show Animal Planet aired a countdown
story about the most rugged creatures
on Earth. Tardigrades were crowned
the “Most Extreme” survivor, topping
penguins in the Antarctic cold, camels in the dry oven of the desert, tube
worms in the abyss and even the legendarily persistent cockroach.
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2011 September–October
385
salivary gland
eyespot
sacklike ovary
immature eggs
malpighian tubule
esophagus
excretory gland
brain
Dr. David J. Patterson/Photo Researchers
oviduct
stylet
tubular sucking
pharanx pharynx
buccal apparatus
rectum
stomach
claw
gland
ventral
ganglion
cloaca
.05 millimeters
Figure 2. Tardigrades’ appearance is not their only aspect that is reminiscent of macrofauna. A light-microscope image of the anterior end of a
tardigrade (left) shows the mouth; stylets, strutures that help them feed; the buccal apparatus, part of the digestive tract; and the pharynx at the top
of the alimentary system. A cross section of a generic water bear (right) shows the relative positions of the organ systems. Lacking are circulatory
and respiratory systems. At this tiny scale, an open hemocoel cavity is sufficient to distribute oxygen and nutrients through the organism.
But extreme survivorship applies
only to some species of terrestrial tardigrades. Marine and aquatic tardigrades did not evolve these characteristics because their environments are
stable. It appears that the extravagant
survival adaptations have been selected in direct response to rapidly changing terrestrial microenvironments of
damp flora subject to rapid drying and
extreme weather.
Terrestrial tardigrades have three basic states of being: active, anoxybiosis and
cryptobiosis. In the active state, they eat,
grow, fight, reproduce, move and enact
the normal routines of life. Anoxybiosis
occurs in response to low oxygen. Tardigrades are quite sensitive to oxygen
tension. Prolonged asphyxia results in
failure of the osmoregulatory controls
that regulate body water, causing the
Figure 3. Tardigrades (far left) were for a
time considered competitors with the round
worm Caenorhabditis elegans (near left) and
the fruit fly Drosophila melangaster as major
invertebrate model organisms. Tardigrades
have played that role less over the years, but
research attention is increasing as new genetic research tools allow deeper inspection
of their extreme durability and adaptivity in
response to changing environmental conditions. Tardigrades are predators of nematode
worms such as C. elegans. Under the microscope, tardigrade researchers occasionally
encounter a water bear grabbing a nematode
around the middle. The nematode wriggles
furiously all over the dish, with the tardigrade hanging on like a bronco rider, until
the drained nematode surrenders. (Photograph courtesy of Bob Goldstein and Vicky
Madden of the University of North Carolina
at Chapel Hill.)
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American Scientist, Volume 99
tardigrade to puff up like the Michelin
Man and float around for a few days
until its habitat dries out and it can
resume active life.
Cryptobiosis is a reversible
ametabolic state—the suspension of
metabolism—that has inevitably been
compared to death and resurrection.
In cryptobiosis, brought on by extreme desiccation, metabolic activity is
paralyzed due to the absence of liquid
water. Terrestrial water bears are only
limnoterrestrial—aquatic animals living within a film of water found in their
terrestrial habitats. Moss and lichens
provide spongelike habitats featuring a
myriad of small pockets of water and,
like sponges, these habitats dry out
slowly. As its surroundings lose water,
the tardigrade desiccates with them.
It has no choice. The creature loses up
to 97 percent of its body moisture and
shrivels into a structure about one-third
its original size, called a tun. In this
state, a form of cryptobiosis called anhydrobiosis—meaning life without water—the animal can survive just about
anything.
Tardigrades have been experimentally subjected to temperatures of
0.05 kelvins (–272.95 degrees Celsius
or functional absolute zero) for 20
hours, then warmed, rehydrated and
returned to active life. They have been
stored at –200 degrees Celsius for 20
months and have survived. They have
been exposed to 150 Celsius, far above
the boiling point of water, and have
been revived. They have been subjected to more than 40,000 kilopascals of
pressure and excess concentrations of
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anoxybiosis
oxygen deficit disrupts osmoregulation,
causing swelling and turgidity
encystment (cyst)
active
eat, grow, move and reproduce
in unfavorable conditions, organism
retracts within cuticle, forming new
cuticular layers around dormant body
cryobiosis (tun)
osmobiosis (tun)
induced proteins disrupt ice
crystallization as tun forms
rarely, osmotic effects of
extreme salinity are countered
by tun formation
anhydrobiosis (tun)
in dessicating conditions, slow surrender
of water leads to shriveled, dry tun
tardigrades in cryptobiosis are capable of surviving:
• 20 hours at –273°C (–459°F) • 20 months at –200°C (–328°F) • +150°C (+302° F)
• 6,000 atmospheres of pressure • pure vacuum • excessive concentrations of carbon monoxide, carbon dioxide,
nitrogen and sulfur dioxide
• x-ray and ultraviolet radiation • over 125 years (possibly)
Figure 4. Tardigrades have evolved a suite of survival tactics to escape the vagaries of their localized and vulnerable environments. Anoxybiosis and encystment, described in the upper part of this figure, are responses one might see in a variety of organisms. The bottom half of the
chart shows three states of cryptobiosis, in which metabolism is suspended—an act usually diagnostic of death. Cryobiosis occurs in response
to freezing, and anhydrobiosis in response to drying. During the latter, an organism surrenders its internal water to become a desiccated pellet.
Both result in the formation of a durable shrunken state called a tun. More rarely, a tun is created to resist osmotic assault, which requires water.
In the tun state, tardigrades can survive for many years, impervious to extremes far beyond those encountered in their natural environments.
suffocating gasses (carbon monoxide,
carbon dioxide, nitrogen, sulfur dioxide), and still they returned to active
life. In the cryptobiotic state, the animals even survived the burning ultraviolet radiation of space.
Challenging student scientists to
ponder the astonishing durability of
tardigrades brings their understanding
of physics, chemistry and biology into
play. They recall that water expands as
it approaches the freezing point, which
is why ice floats. At 4 degrees celsius
the expansion of water exerts sufficient
force to split boulders, rupture metal
containers and explode living cells. A
cell is more than 95 percent water. The
rupturing forces and icy microshards
that form in frozen cells are the same
that cause frost bite.
How can water bears survive all
that? my new student and, perhaps, future colleague asks.
Really, deep down, we’re still puzzled about a lot of it, I respond.
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The survival attributes of tardigrades are in fact quite appropriate
for an organism that makes its home
in mosses and lichens (bryophytes),
which provide them with just a thin
layer of protection. Bryophytes are
subject to the environmental extremes
experienced on a planet bathed in solar
radiation. They may receive varying
periods of direct ultraviolet exposure
and are never far from drying out as
ambient conditions change.
Improvise, Adapt and Overcome
Tardigrades exhibit distinctly different
responses, grouped under the general
name of cryptobiosis, to different sources of stress. Anhydrobiosis and cryobiosis lead to the formation of tuns, but they
are not equivalent—they are different
mechanisms for protection against different environmental assaults.
Anhydrobiosis—metabolic suspension brought on by nearly complete
desiccation—is a common state for tardigrades, which they may enter several
times a year. To survive the transition,
water bears must dry out very slowly.
The tun forms as the animal retracts
its legs and head and curls into a ball,
which minimizes surface area. When
nearly all of its internal water has been
surrendered, the tardigrade is in anabiosis, a dry state of suspended animation.
It is almost as if the animal preserves
itself by becoming a powder comprised
of the ingredients of life. When rehydrated by dew, rain or melting snow,
tardigrades can return to their active
state in a few minutes to a few hours.
In cryobiosis, another form of
cryptobiosis, the animal undergoes
freezing yet can be revived. Any temperature below the cell cytoplasm’s
freezing point suppresses molecular
mobility and therefore suspends metabolism. Deep-freeze temperatures
could be expected to cause additional
structural disruptions, yet tardigrades,
as noted above, have survived the
most drastic chills. It seems likely that
survival is conferred by the release or
synthesis of cryoprotectants. These
agents may manipulate tissue freez-
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2011 September–October
387
Eye of Science/Photo Researchers
Figure 5. Eutardigrades lack armor, which appears to have done little to inhibit their evolutionary success. Larger eutardigrades—such as those of the genus Macrobiotus (shown above
in active form and tun state)—are found in many habitats, where they consume smaller tardigrades as well as nematode worms and rotifers. Their large appetite for nematodes (they
may consume many per day), and their resulting controlling role on the nematode population,
indicates a significant role in the food web for tardigrades at the micro scale.
ing temperature, slowing the process
and allowing an orderly transition into
cryobiosis, and they may suppress the
nucleation of ice crystals, resulting in
an ice-crystal form that is favorable for
subsequent revival with thawing.
Osmobiosis is a response to extreme
salinity, which can cause destructive
osmotic swelling. Some tardigrades
exhibit strikingly effective osmoregulation, maintaining stasis in the face of
steep osmotic gradients. Some others
escape via formation of a tun that is
impervious to osmotic transfer.
In 2007, tardigrades became the first
multicellular animal to survive exposure to the lethal environs of outer
space. Researchers in Europe launched
an experiment on the European Space
Agency’s BIOPAN 6/Foton-M3 mission
that exposed cryptobiotic tardigrades
directly to solar radiation, heat and the
vacuum of space. While the experimental vessel orbited 260 kilometers above
the Earth, the researchers triggered the
opening of a container with tardigrade
tuns inside and exposed them to the
Sun. When the tuns were returned
to Earth and rehydrated, the animals
moved, ate, grew, shed and reproduced.
They had survived. In summer of 2011,
Project Biokis, sponsored by the Italian
Space Agency, ferried tardigrades into
space on the U.S. space shuttle Endeavor.
Colonies of tardigrades were exposed
to different levels of ionizing radiation.
The damage is now being assayed to
Figure 6. The armored Heterotardigrade of the genus Echiniscus in the active state (left) and in the cryptobiotic tun state (right). The armor of
these tiny predators contains chitin, the same material incorporated in the cuticle of insects. The armor may slow the process of drying. In drier
environments, heterotardigrades are predictably represented in larger numbers than are naked species. The armor plates may supply some
degree of protection to the vulnerable active form. (Images courtesy of the author.)
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American Scientist, Volume 99
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Figure 7. These images of tardigrade claws are magnified 3,000 to 5,000 times. Even at so fine a scale, structures have developed that are distinctive
to each genus, suggesting adaptations for different lifestyles. Tiny hooks suitable for spearing tiny hors d’oeurves contrast with bristling claws
seemingly optimized for a raking, tearing attack. Little studied, tardigrades are far from understood. The diversity of claw types may have roles
in mating, tun formation and other tardigrade activities that have not yet been discovered. (Images courtesy of the author and Clark W. Beasley
of McMurry University.)
learn more about how cells react to radiation and, perhaps, how tardigrade
cells fend off its damage.
My student’s mind is turning.
How far is the nearest solar system? Could cryptobiotic tardigrades make it to Earth? she asks.
I saw a post recently on a listserv
that says it might be only about 10
light years away, I answer. So if tardigrade tuns were transported here
on a meteor or asteroid at just one
tenth the speed of light, they could
conceivably make the trip within
the known survival capability of
the animal. Theoretically. But the
likelihood is awfully small. And
think of the poignancy of arriving
after that great journey but having
no way to make it through the fiery
descent through an atmosphere.
Even tardigrades can’t survive that.
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Surviving intense radiation suggests
an especially effective DNA repair system in an active organism. Effective
osmoregulation in extreme salinity implies a vigorous metabolism—osmoregulation in the face of high environmental salinity is energetically extremely
expensive as metabolic transactions
go, requiring the pumping of ions
against steep osmotic and ionic gradients. Thus, we see in tardigrades two
opposing responses to environmental
extremes: the passive response of dormancy in the form of cryptobiosis, balanced by the hyperactive responses of
impressive DNA repair and high-performance osmoregulation. As practitioners of adaptive evolution, tardigrades
are virtuosos.
Getting Around
Tardigrades have been discovered just
about everywhere that anyone has
looked, from the Arctic to the equa-
tor, from intertidal zones to the deep
ocean, and even at the top of forest
canopies. Their ubiquity is intimately
linked to their survivorship. I am often asked how tardigrades manage to
find their way to the canopy of towering trees. Most likely, wind carries
them. In the tun state they are barely
distinguishable from dust particles.
But like spores, pollen and seeds,
the tuns have a preference for where
they land. Many microenvironments
will be unsuitable habitats for freshly
arrived tardigrades. Yet an unhappily placed tun can simply wait for
a change in precipitation or perhaps
a change in season. When conditions
improve, life can begin again.
Contributing to their success as travelers is the fact that many tardigrades
of moss, lichen and leaf litter are parthenogenetic, able to produce eggs
without mating, and in a few cases are
hermaphroditic, able to self-fertilize. A
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2011 September–October
389
Figure 8. A light-microscope image reveals the dorsal plates and cirri, cuticular extensions, of what one day could be known officially as Multipseudechiniscus raneyi. While working with the author, Baker University undergraduate Rachael Schulte found the organism in samples her teacher
had collected in California. They have submitted a paper describing the organism for publication. (Image courtesy of the author and Rachael Schulte.)
lone tardigrade on an ill wind—active,
tun or egg—may be able to establish a
population where it lands if the habitat
is suitable. We may be under tardigrade rain right now.
At present there are about 1,100 described species of water bears, but not
all are valid. Some descriptions are repeats and some are just plain flawed.
Around 1,000 species have been properly identified and described. We have
about 300 marine, 100 freshwater and
600 terrestrial species. But the land
species are much easier to find and
have been pursued by many more researchers over many more years. Still,
my students have discovered and described four new species so far, and
we are working to confirm another
half dozen, including one found on the
campus of Baker University in Kansas,
where I am a faculty member. We believe there is an abundance of species
yet to be discovered, especially in the
nonterrestrial environments.
I might discover a new species?
she asks.
Yes, sitting at a microscope, you
might observe an animal nobody
in the world has ever seen before.
That is pure exploration. In the
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blink of an eye, you might find a
clue to the evolution of the phylum or identify the animal that
holds the cure for cancer, I say.
Then again, you might not. It
took me 16 years to find my first
new species.
Finding a New One
Last 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 computerimaging 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, Rachael
worked the microscope to answer them.
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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.
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.
O k a y, h o w a b o u t
pseudechiniscus? I ask.
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What is the most distinctive feature of the critter?
The elongated buccal tube? she
says.
I look up and say, No mention of
the extra pseudosegmental plate.
True, but that is very difficult to
see. What else have you seen?
Multi
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 peerreviewed journal.
So what do we name it? she asks.
No, it is paired. And the spurs are
not tiny, nor basal, she says.
www.americanscientist.org
The extra pseudosegmental
plates? she suggests.
© 2011 Sigma Xi, The Scientific Research Society. Reproduction
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