Strobilidium

I’ve always regarded the small ciliate, Strobilidium, as the world’s fastest organism. It's absolute speed may not eclipse that of a cheetah or perigrine falcon, but relative to its size, Strobilidium leaves those two record book veterans in the dust.

Strobilidium

Strobilidium seems to literally zoom across the microscope field of view. It gets its phenomenal acceleration and blinding speed using a fan of powerful cilia. Normally it is attached to an object by an invisible (to the light microscope) tether line. But when disturbed it lets go of the tether and zips away like an airplane engine cut loose from the plane. Having observed this amazing behavior for years I came to know Strobilitium as the unchallenged champion of speed, until recent observations suggested there may be other contenders for the gold medal of micro sprints.

Observing a community of protists living in and around a pin-head size clump of Oscillatoria (the filamentous cyanobacteria that gives swampy areas their characteristic odor) I noticed an occasional streak zipping through the field of view. One of the zips terminated with the organism becoming trapped in a tangle of Oscillatoria strands, offering a chance for a closer look. The small cell had a number of long stiff bristle-like cilia emerging from a cleft that divided the cell. About half of these bent forward and the others backward with a few projecting straight out to the sides. There were also some short projections from the front of the cell. Then the cell flipped up offering an end-on view. In this aspect the cilia appear as a sunburst with each cilium flicking slowly as though the cilia motors had been set to idle. Then, as I began shooting video, the organism simply vanished from the viewfinder. Playing the videotape frame by frame, I observed the organism in one frame and completely missing in the next frame — it had "jumped" out of the frame in less than 1/30th of a second.

It didn’t take long to find a picture of the organism in an identification key. Its name is Mesodinium pulex. (pulex means "to jump").

Mesodinium

These observations suggest that not only is Mesodinium pulex an exceptionally fast swimmer, but it’s also capable of remarkable acceleration — clearly giving speedsters such as Strobilidium competition in the fastest organism contest. From an evolutionary standpoint, this rapid expenditure of energy suggests that there must be a survival pay-off, possibly an aid in escaping from the many kinds of predatory protists that prey on ciliates in Mesodinium’s size range.

Another escape artist that is similarly equipped with isolated stiff cilia is Halteria, a common little ciliate often seen bouncing around in drops of pond water. I have seen Halteria drawn into by a Stentor’s powerful feeding current, only to turn on the power just short of the predator’s mouth and shoot away to safety.

Halteria

All three speedsters, Strobilidium, Halteria, and Mesodinium are small, roughly 20-30 micrometers across, and each one is equipped with it own brand of specialized cilia motors (or should they be called paddles or oars?). The mechanism behind such a powerful cilia stroke may be different than normal ciliary action of the kind seen in the thousands of cilia that cover Paramecium. Should someone examine a section through the long, stiff cilia of Halteria, or Mesodinium, using a transmission electron microscope, it would be interesting to see if they have the typical nine double microtubules surrounding two central microtubules. Or would a TEM show something different that might give some clue as to the remarkable acceleration and speed these motors produce?

Now, imagine the physics involved in these sprints. For organisms in this very small size range, the resistance of water is great. Were we to shrink to their size, it would be like swimming through molasses. How do these three little speedsters achieve such high speeds traveling through such a gooey (for them) medium? What tricks of fluid dynamics have they perfected? What might be the survival value of such sprints? What kinds of predators might be successful at capturing such speedsters? How might a protistan speedster have evolved? How closely related are the three speedsters? Are the long bristle-like cilia seen in Halteria and Mesodinium homologous structures or did they evolve separately? How might that be decided? What might be the function of Mesodinium’s five front-facing rods?

So, wether you are a student of biology, or of physics, these little speedsters pose some fascinating questions for speculation and research.