The trophi of bdelloid rotifers and the dental formula
basic morphology of bdelloids: the mastax
The mastax is the organ in the digestive system of all bdelloid rotifers which reduces the diet to small pieces. Aside from muscles, nerve cells and digestive glands this grinding is accomplished by some chitinous hard parts called trophi.
Artificially colored image of a compressed specimen of the very common rotifer Rotaria macrura. Bdelloid rotifers only have a single type of trophi. The image shows the ramate trophi in cephalic view (view from the anterior part); so the pharynx (P) yields the mastax with the trophi from the ventral side (the ventral side (V) is bottom, the dorsal side (D) is top). The colors characterize the different parts of the trophi. Red: manubria; blue: unci; green: rami.
The comb-like strucure of the unci is comrpised of chitinous teeth. The 4 arrows point to the major teeth, the number of which is a species-specific trait. The number of major teeth is therefore expressed as the so-called dental formula (DF) which is 2/2 for this specimen/ species.
Two more examples. Right image: Scepanotrocha semotecta with dental formula 10 / 10. Although the major teeth get less visible top to bottom in this image they can be distinguished from the minor teeth which are below the resolution of this microscope lens in this image. Right image: aside from the major teeth (marked by arrows) in the trophi of some species there are teeth in the size between major and minor teeth (marked by arrowheads). This is expressed in the dental formula which is: 1+2 / 2+1 in the case of the species Macrotrachela magna presented here.
