frontal aspects of bdellod rotifers
Mniobia magna; frontal view

It is of course rather banal to point out that the name of the "wheel animalcules" is derived from the "wheel-like" appearance of their front part. Although I have been dealing with this creatures for quite some time now, there are still certain phenomena that are fascinating for me, and there are also some which I still don't really understand. The readers of this should also know that the rotifers do not really turn anything, so this perception is an optical illusion. In the bdelloid rotifers in focus here there are in principle 3 groups of cilia on the columns at the front: 1. Here illustrated by an image of Didymodactylos carnosus, a quite common rotifer, which occurs in moss:

1. in the very front there is a wreath of long cilia, the so-called trochus (Tr) 2. posterior to the trochus is a field of short cilia, which extend posteriorly to the pharynx (circumapical band (CaB)). 3. behind it is another wreath of long cilia, which is incomplete (cingulum, Ci).

The axis of the "rotation" of the apparent rotational metachonal movement of the trochus is parallel to the longitudinal axis of the body; the apparent rotational metachronal movement is therefore perpendicular to it. How do these cilia move? One could now assume that the cilia also move in the direction of "rotation". However, this is not the case, but rather the cilia move perpendicular to it, i.e. parallel to the longitudinal axis of the body (double arrow).

This can be seen here with the rotifer Rotaria macrura. The double arrow marks the movement of the cilia; the arrowhead labelled "ES" marks the direction of the effective stroke; the arrowhead labelled "RS" marks the direction of the return stroke. The curved arrows indicate the direction of ther metachronal movement:

It is therefore also a laeo- or dexioplectic movement (which is known also from ciliates); the zoologist Zelinka has shown this (1893) in the following drawing for one of the trochal disks (which can be seen here frontally):
Zelinka also provided the template for the next figure, which visualizes the water flow caused by the cilia movement of the bdelloid rotifer Mniobia magna. The water flow was made visible in the preparation by carmine granules (shown here in carmine red). The turquoise arrows mark the direction of flow:
I admit that although I have been using a microscope for quite a long time and have seen at least the aquatic rotifers like Rotaria rotatoria or Rotaria macrura hundreds of times, I have only now become aware of some findings or questions (which I could have come up with earlier).

Considering the dominance of our mechanistic world view it is obvious to assume that these "gears" of the bdelloid rotifers would behave like two interlocking gears, i.e. they would move in opposite directions. Often we assume that when an object appears symmetrical, parts that exist in pairs are mirror-symmetrical to each other. Also in two- or four-engined propeller airplanes, half of the propellers rotate in the opposite direction to the others.

The turquoise arrows in the picture above also suggest this symmetry.

But this is not so. The apparent direction of movement of the cilia runs in the same direction for both trochi. So the "direction of rotation" is asymmetrical. Zelinka investigated this at that time on the rotifer Mniobia magna; this species is also quite often found in dry moss, which is watered before. These animals are then quite greedy for food and are not disturbed by the light of the microscope and vibrations. Furthermore their body is often in vertical position when they are whirling, so that in this moment the plane of the trochi is in focus. The following image shows the trochal discs of Mniobia magna in frontal view:

Based on the above findings, of course, further questions arise immediately:

1. in which direction do the "wheels" seem to run?
2. can the rotifer also work the other way around?

As can be seen from Zelinka's drawing above, the cilia movement in M. magna is always such that the impression of a counter-clockwise rotation is created when looking at the critters from the front. I have not seen any other "direction of rotation" in this species. This is of course immediately followed by the question, how is this with other bdelloids: in the bdelloid species I have observed, the "direction of rotation" has also always been counterclockwise.

Nowadays there is of course the possibility to document the direction of rotation on video. But for this you have to be aware that with these rotary movements, even if they are only apparent, the Shannon's sampling theorem has to be taken into account. We know this from old Western films where the carriage moves to the right, but the wheels turn counterclockwise because the frame rate of the film camera does not capture the rotation frequency of the wheels accurately enough (aliasing). My first attempts with a DSLR with 30 fps resulted in a fottage which showed the reverse direction of what I could see live. So at least the "sampling frequenxy of 30 fps is too low. I have recorded some videos for documentation purposes with 180 fps (Panasonic GH5), it seems to work there.

Still images taken with a flash of approx. 100µs result in images with groups of cilia apperaring roughly "C-shaped" like this:

... and not like this:
So it has further to be confirmed if a C-shaped pattern of the cilia may be looked upon as a documentation for the counter-clockwise ciliary movement of a certain bdelloid species.

The following images show frontal aspects of bdelloid rotifers, although the first two images of Adineta and Ceratotrocha do not show the "typical" appearance of a bdelloid corona, the other images show the corona of bdelloids of the order Philodinida:

Adineta steineri
Ceratotrocha cornigera
Habrotrocha ligula
Habrotrocha curvicollis
Macrotrachela brachysoma
Macrotrachela brachysoma
Macrotrachela ehrenbergi
Macrotrachela plicata
Macrotrachela quadricornifera
Otostephanos regalis
Otostephanos torquatus
Philodina flaviceps
Philodina roseola
Philodina rugosa
Philodina vorax
Philodina megalotrocha
Pleuretra lineata
Rotaria macrura
Rotaria macrura
Rotaria socialis
The question about the asymmetry of the direction of the metacronalcilia movements can be extended to other groups of organisms: how does the "rotational movement" behave for example in the other group of monogonont rotifers, like Floscularia, Filinia, Hexarthra, to name a few examples of species with a distinct corona. So here is an interesting example of a corona from Conochilus hippocrepis:.
In contrast to all other rotifers observed up to now the apparent movement of the cilia of this species (genus??) is clockwise (arrows). What causes this different direction is not known.
freshwater life
marine life