Rotifers try "abstinence only"--for 30 million years

Rotifers live in temporary pools of water, like your bird bath or rain garden.

Rotifers are tiny multicellular animals that live in pools of water. For one kind of rotifer, no males have ever been found.

Photos with permission by Aydın Örstan.

Talk about a dry spell!

One kind of rotifer, the Bdelloid rotifers, haven't had sex for 30 million years. For three centuries since their discovery, no one has seen any eggs or male rotifers. Just females. Ouch!

This had scientists scratching their heads, since theory argues that sex has many advantages. It's supposed to help rabbits keep one step ahead of the foxes, in the evolutionary arms race. Giving up sex is thought to be an evolutionary dead end--less than 1% of animal species reproduce without sex.

So what gives with the rotifers? How have they turned "no sex" into a good deal?

Rotifers do have one mortal enemy--it's a tiny fungus. If rotifers ingest fungal spores, the spores catch in their throats, sprout, and digest the rotifer from the inside out. If spores of the fungus are present in your bird bath, it won't be long before all the rotifers are dead.

And rotifers can't use sex to jazz up their biological defenses against the fungus. Now scientists at Cornell have discovered that the rotifers escape from the deadly fungus--by a kind of "hide and seek" strategy.

Dry up and blow away

It turns out that rotifers are one of the few kinds of animal that can survive completely drying out--and they can do this at any stage in their life cycle. When your rain garden dries up, the rotifers turn to dust, and are blown about from place to place. They can survive for as long as 9 years as dust. Then, add a drop of water, or a film of moisture on some moss, and they come back to life within an hour!

The fungus can also survive drying, but not for so long. And they don't blow about so readily. So when the rotifer lands in another damp spot, the chances are--there won't be any fungus there. The rotifers take a long drink, plump up, and go about their business, filtering tiny particles of food out of the water. But whatever their business is, it isn't... sex.

Reported in Science, 29 January 2010, p. 574-6

More on rotifers--Aydın Örstan's wonderful blog on invertebrate animals.

Wikipedia article.