Rotifers in the River Ehen

I promise that this is the final post on the River Ehen for a while. There is often quite a lot of the aquatic moss Fontinalis antipyretica at one of the sites we visit and it proved too tempting a subject for my new underwater camera. However, the real surprise came when I put a couple of leaves under the microscope on my return. I thought I should check that this really was F. antipyretica and not the related F. squamosa, and this necessitated a quick check of the leaf structure. What I saw when I looked down was that every leaf had several tiny rotifers attached to the surface, busily whirring their cilia to suck food particles into their gullets.


Fontinalis antipyretica in the River Ehen, March 2014.
Rotifers really are beautiful organisms to watch under the microscope. The ones I was looking at were mostly about 0.1-0.2 mm long (it is hard to give exact dimensions as their shapes were constantly changing). They were attached by their “foot end” to the leaf surface whilst the other end bears a simple mouth surrounded by a ring of cilia. These beat in synchronous waves to sweep food particles into the mouth; the impression to the viewer is of a rotating ring though, in fact, the cilia remain stationary. The rotifers I was looking at, Bdelloidea, actually possess two of these rotating “wheels” of cilia.
I wanted to take some photographs, and even videos, of these rotifers, because the visual effect of these beating rings of cilia is quite mesmerising. However, the rotifers were constantly in motion, moving through three dimensions making it impossible to keep the wheels in focus for long enough. There are ways of slowing rotifers down but all require experimentation to get it right. I tried adding alcohol and then some gum arabic, which should have made the water just viscous enough to slow down the cilia. Neither worked for me but it is probably just a matter of time and experience to get the concentrations and exposure times right. There is also a commercial preparation, ProtoSlow, which has much the same effect. In the end, I resorted to my pencils and paintbrushes though this does not really capture the full glory of the spinning wheels of cilia around the rotifer’s mouth.


A sketch of a Bdelloid rotifer feeding on the leaf of Fontinalis antipyretica in the River Ehen, March 2014. The main picture shows the rotifer in an upright position, extracting food particles from the water; the smaller picture, bottom right, shows the rotifer bent over to “hoover” up diatoms and other algae from the leaf surface. The scale bar is 25 micrometres (1/40th of a millimetre).

The presence of so many rotifers tells me a little more about the story of the River Ehen. So many organisms adapted to capturing tiny food particles from the water flowing past must be a sign that there is a plentiful supply of such particles. Perhaps these are washed out of the sediment, perhaps from the surrounding catchment. I suspect, too, that the rotifers are not the only bugs feeding on these particles, and that concentrations will be highest just after spates (which will flush much more of these particles into the river). There was a spate just a few days before we visited. And, I suspect, these particles also act as micro “compost heaps” for the algae which are my main interest in the river. This might partly explain the conundrum of why so many algae are growing in a river that apparently contains so few nutrients.