Rosemarie Arbur, Oregon, US
ciliate that swims smoothly and rests quietly in place for moments at a
time, Spirostomum rewards its observer by being always interesting.
About a week after I brought home water and many tufts of filamentous green
algae (Stigeoclonium) from the sunny shallows of a lake, Spirostomum
began to appear, joining the Stentor and Paramecium that
abounded in my jar of water. Not having seen Spirostomum for at
least six months, and after having succeeded in getting one from jar to
slide, I decided on a thorough reacquaintance.
simplicity and clarity, my drawings leave out body cilia and, after the
first, Spirostomum's macronucleus; I've also omitted the myonemes
visible in the anterior two-thirds of the body and the membranes surrounding
food vacuoles where they're not relevant. Colors here are just a bit exaggerated;
the white spots are very small, bright granules that with the even smaller,
darker ones give Spirostomum's cytoplasm its characteristic visual
Spirostomum is about 450 µm long, with a peristome (left)
that does not reach even the midpoint of its body. (Spirostomum ambiguum,
the "representative" species, exceeds 1 mm and has a longer peristome,
three-quarters of its body length.) The water-expelling vesicle or contractile
vacuole (right) is relatively long, with a very long canal, and its transparency
makes the myonemes adjacent to it easily visible as longitudinal lines.
This Spirostomum species, most likely S. teres, has a single,
oval macronucleus near the center of its body; as I adjusted focus, I couldn't
always see it. But I did see food vacuoles, large and small: the darker
bodies in cytoplasm.
would hardly be a Spirostomum if it didn't perform its most famous
behavior: contracting itself faster than any other living entity or cell.
Like others I've seen, this specimen makes itself almost diamond-shaped
when it pulls its front and rear ends toward each other.
seconds, it resumes its previous length and leisurely cruises about, happily
(for me) staying in same general area. I notice a suspicious-looking clump,
as big as a Cyclidium (25 µm diam), near its rear end.
Spirostomum seems to be towing it.
their moderately soft, flexible pellicles, swimming Spirostomum
often bend around bits of algae and other obstacles they encounter. This
Spirostomum is bending for no discernible reason, folding its anterior
half almost back on itself. There are two suspicious-looking clumps now.
its acrobatics, Spirostomum resumes its normal shape. Then it empties
its water-expelling vesicle, twisting the posterior third of its body and
presenting this curious profile. I've seen other Spirostomum empty
this vesicle by compressing it from "top" (as seen in preceding drawing)
to "bottom," without much alteration of "top" edge of pellicle. The view
here suggests that this Spirostomum is compressing the vesicle in
three dimensions (it's shortening the vesicle slightly, and it's decreasing
the diameter of a cylinder, not just the width of a rectangular oval).
The clumps seem to dissolve as they're left behind; but new ones, not so
finally occurs to me that the suspicious-looking clumps are feces being
expelled at Spirostomum's posterior end. Well, I think, watching
food vacuoles form is an educational exercise for microscopists; why not
watch the rest of digestive process?
forms food vacuoles that are quite large initially. Ingested material passes
through the mouth or cytostome (at the end of the peristome) into a membranous
sac; when enough has been ingested or when the sac reaches a sufficient
size, the membrane pinches shut to form a discrete vacuole. Digestion occurs
while the vacuole moves about, circulating with the cell's other contents.
As digestion proceeds, the food vacuole becomes smaller.
I could see, the large food vacuoles don't move into the vicinity of the
water-expelling vesicle; they move as the arrows show, rather slowly, and
may make several circuits of the anterior two-thirds of Spirostomum's
body. Digestion and absorption of the contents take several minutes before
the vacuole reaches the size and location of the small round one above.
digestion of its contents is complete and the shrunken food vacuole approaches
the front end of the water-expelling vesicle, the movement of the vacuole
speeds up quite noticeably.
moves along and beyond the water-expelling vesicle, from near its anterior
end (left) to the organism's posterior extremity (right), in less than
a minute, the membrane that surrounded the contents of the food vacuole
most of the undigested matter is expelled through an anal pore.
minutes later, Spirostomum curls about itself, this time more tightly
than before; its anterior tip pushes hard against the end of its peristome,
and the folded-over portion of its body flattens. The large food vacuole
remains near the fold. Possibly, it does not move toward the rear because
the organism's body is kinked like a garden hose. Spirostomum holds
this posture for about five minutes.
all tension in the body relaxes, and the organism is dead. Both water-
and food vacuoles seem larger than before, the latter appearing yellowish.
Because the extreme folding seems to have prevented that food vacuole from
moving on, the way a "twisted gut" can bring a horse's great digestive
system to a fatal standstill, I wonder if this Spirostomum died
of colic. Putting that grim humor aside (seeing micro-critters die upsets
me), I also wonder about Spirostomum's myonemes. Dead, its body
is shorter than when it was alive; can it be that the (proto-"muscular")
tension in the myonemes not only contracts the organism periodically to
a fraction of its usual length but also, as a normal, most-of-the-time
process, actively stretches Spirostomum to its characteristic size?
All comments to the author
Comments to the author sent via our contacts page quoting page url plus : ('rarbur','')">Rosemarie
Arbur are welcomed.
Also see Wim van Egmond's photoessay
'Spirostomum: The fastest contraction
in the micro-world'.
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Published in August 2001 edition
of Micscape Magazine.
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