The image of this rotifer, Branchionus quadridentatus, shows internal organs, the elongated foot, and cilia. Image: Marco Spiller, eol.org
Rotifers are common freshwater zooplankton. They are often called ‘wheel animalcules’ because of the characteristic ring of cilia around their mouth. First observed and described in the late 1600s and early 1700s, they are a diverse group of organisms found in almost all freshwater environments. They can even live in the thin film of water found in moist soils. A few species live in salt water. Sadly, rotifers can’t be seen with the naked eye, but with a low magnification microscope you can easily follow their twists and turns through the water.
Wheel animalcules can live in several different kinds of habitats within a lake or pond. Some are full-time swimmers. Others prefer to remain near and creep along rocks, logs, and sediment. Some don’t move much at all and are attached to the substrate in tubes or by rootlike structures called holdfasts. Most live their lives as individuals, but there are several common colonial species. Sometimes you can see colonies without magnification.
Where Do Rotifers Fit Into the Food Web ?
Rotifers are filter feeders. The rotating cilia around their mouth help capture their food. They will eat just about anything, so long as it can fit in their mouth. That means organic debris, tiny phytoplankton, bacterial colonies may all be part of their diet. Some rotifers are carnivorous and known to eat members of their own species. Many species are transparent so some of what they have recently eaten may be visible in their guts.
Rotifers serve as tasty food for a lot of other critters, some that you wouldn’t expect to be interested in these microscopic swimmers. They are eaten by other zooplankton such as copepods, shrimps, tadpoles, and aquatic insects. Small fish will eat free-swimming rotifers. Dabbling waterfowl like mallards and shovellers will also eat rotifers that happen to be in the sediment or other material they are eating.
This rotifer (Keratella spp.) is carrying an egg. Image: Wolfgang Bettighofer, eol.org
Reproduction
Individuals are either male or female. Reproduction varies among the 3 major groups of rotifers. Seisonidea have both males and females, which mate to produce their eggs. Bdelloidea reproduce only by cloning (asexual parthenogenesis). Monogononta reproduce primarily by cloning, but will undergo sexual reproduction at times in their life cycle. Hatching eggs in most cases look like miniature adults.
Rotifers do produce resting eggs that can survive poor environmental conditions, sometimes for a very long time. Bdelloid rotifers don’t produce resting eggs, but instead can survive by desiccation. They lose almost all body water, but can revive within a few hours of rehydrating. Recently, scientists revived rotifers found in artic permafrost after being frozen for 24,000 years!
Learn More About Rotifers
There is a lot of information about rotifers on the web. You can enjoy some great microphotography using the links below:
- Journey to the Microcosm: Rotifers – fun video of live rotifers
- The Canadian Nature Photographer
- The Micropolitan Museum’s Rotifer Room
- The University of New Hampshire’s Rotifer key – browse image of common zooplankton
- download this video of feeding rotifers from the Nikon Small World Photography Competition. (Note this is a large file.)
