first few entries which have some stunning pictures of various species. Ancient rotifers
are hard to find in the fossil record, as only some mouth parts arc made of sufficiently
tough material to resist decay for a while, and these are so minute that it is very hard to
see them in rock, even if you know what you are looking for.
Rotifers are in a Phylum all their own. In evolutionary history, they are usually dismissed
as a line that leads nowhere, an evolutionary 'dead end', a minor Phylum, but they are
fascinating. They usually have two circles of beating cilia (tiny hairs) on the 'head' (I use
the term guardedly, as any idea of brain is not really appropriate, but at least it is the bit
that usually leads the way when they move), and these beating cilia give the appearance
of two small wheels whirling very fast. This is what gave rotifers their name, literally
'wheel bearers'. The movement of the cilia washes food particles into the mouth, which
is nearby, and can help the animal to move about in water. Also nearby is a light-sensitive
spot or two, not developed enough to be called eyes. The hinder parts of the animals
vary, but one common group is called the bdelloid rotifers, because their back end is
leech-like, and they move like a leech (from the Greek for leech, if you did not know!).
We used to bring a tray of muddy water for children in Year 7 (age 11 - 12) to look at as
an introduction to using a microscope at school, and the children thought these Rotifers
were wonderful monsters from outer space: totally alien life-forms. Because their bodies
are transparent, all the innards can be seen, which increases the excitement. I was often
asked how they lived, which 1 could more or less answer, but all I knew about their
reproduction was that they lay eggs. Now I know a bit more.
Biologists have found out that most rotifers have two sexes, as many animals do, but
about 80 million years ago, when the dinosaurs roamed the world, the bdelloid rotifers
gave up on males and have been female only ever since. It has been known for some time
(though I did not) that they have been female only for 40 million years, but recent research
has doubled this time span. Many small animals can be female-only for a few generations,
such as green-fly or other aphids, but not for longer than a season or so. Not only do
bdelloid rotifers beat all animal records for longevity of lines of female-only animals,
but, in addition, they have radiated out to produce about 350 species world-wide, since
abandoning males. I suppose there must have been some spontaneous mutations that
enabled some differentiation without total elimination. Naturally, each of the species is
more or less a clone, though many small, harmless changes in their DNA have accumulated
down the generations. It is these fascinating creatures that biologists are using to try to
unravel why so many organisms use sexual reproduction; clearly there is more genetic
variation, but the downside is that making males is biologically so expensive. The
arguments are not as straight-forward as most people suppose. For more information
about why sex might be so useful, see the latest book by Richard Dawkins, called The
Ancestor's Tale (published Phoenix, 2005, £9.99). I readily acknowledge what a debt I
owe to this wonderful populariser of biology and evolution, and it is an excellent, albeit
atheistic, read. And not only about rotifers!
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Essex Field Club Newsletter No. 50, May 2006