Friday, March 19, 2010
Last weekend when it was rainy and I could have been doing my taxes, I decided instead to track down a rainy day animal, the rotifer.
I encountered rotifers for the first time a few years ago when I was looking for another tiny animal, the water bear, and hadn’t thought much about them again until the end of January when they were featured in a Science Friday program on NPR. The program was about a class of rotifer, called bdelloid, in which all members are female. Without sex, the genome is more or less fixed so favorable genes won’t spread fast enough for the species to remain competitive or adapt to attacks from enemies and it will become extinct. The Science Friday program described the bdelloid method for eluding a certain fungal parasite as an example of how these animals have not only survived but are very successful despite millions of years of reproducing without sex. There are around 450 species and they are so plentiful that you can find them not only in ponds and slow-flowing streams but in your roof gutters, bird bath, leaf litter, clumps of moss, etc. Like motorcycles, they are everywhere. And, like motorcycles, you may not be aware of them unless you look carefully.
Classification: The name, rotifer, refers to the pair of “wheel” organs on the head and is the phylum name. Paul Sherman, professor of animal behavior at Cornell University, likened the wheel organs to a Norelco shaver. The first time I saw one I thought of an upside down street sweeper. Bdelloid is the class name and comes from the Greek for “like a leech” and refers to the way they move.
How to see them: The easiest way to see a bdelloid rotifer in action is to ask Google to find you a video (there are some very good ones). If you want to see a Carlisle rotifer, you will need access to a microscope with at least 40x magnification. The rotifers I have been finding are just less than half a millimeter long when fully extended. With 100x magnification you will see detail of the organs.
Where to find them: The method that always works for me is to take a small clump of moss and submerge it in a jar of water for a few hours or overnight. Then take the moss out and squeeze the water it holds into another container. This squeezed out water will almost certainly contain dozens of bdelloid rotifers – and perhaps a water bear. Use an eye dropper to take a drop of water and make a slide. The film of water under the cover slip is deep enough for the rotifer to move around and feed. I also collected a water sample from one of the streams in the Towle woods and in the first slide I made, I found a different bdelloid species from the ones I find in moss.
How to spot them: Scan your slide for anything that moves, paying attention to the slower moving critters. I’m not familiar enough with how leeches move to understand the “bdelloid” name. I find an inchworm analogy useful. The bdelloid rotifer secretes a sticky substance from a pair of pointy toes at the tip of its tail section. It then attaches itself by its toes, stretches its body forward, releases the toes, telescopes the hind part of the body into the front, attaches the toes again and on and on. At the head end you see the “wheels” which don’t actually turn. They have a ring of cilia which beat in a circular motion and look like wheels. (There are two – like a motorcycle.) The cilia create a current which can be used for movement but is also used to sweep food into the mouth.
Survival of the species: Bdelloid rotifers have the ability to exist in a desiccated form called a tun. Their active lifespan is just a few days but they can revive from a tun after many years. Christopher Wilson and Paul Sherman of Cornell University showed that a deadly fungal parasite that infects bdelloids, survives in the tun for only 3 to 4 weeks. Rotifers that are rehydrated after a month or more will be fungal-free. But they also need to relocate. Wilson and Sherman also demonstrated that rotifer tuns are more aerodynamic than the fungal spores and will “fly” on the wind to new sites some of which will be free of the dreaded fungus.
Bdelloid rotifers have a talent for repairing their DNA which breaks up in the tun state. Some scientists believe that rotifers incorporate genes from other sources, like the food in their stomachs, when they are rebuilding after desiccation. Genetic variations introduced this way would undoubtedly have contributed to their success and diversification into hundreds of species.
Question: Are the bdelloid rotifers sitting in the bird bath yearning for males, or are they too busy wondering why we find DNA repair so difficult?
Sources: www.sciencefriday.com/program/archives/201001293; Guide to Microlife by Kenneth G. Rainis and Bruce J. Russell; Living Invertebrates by Pearse/Buchsbaum.
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