Bdelloid Rotifer

William Dembowski, Fellow of the Royal Astronomical Society (FRAS)

Although no larger than the largest protozoa, rotifers are multi-cellular animals composed of about a thousand cells. Often difficult to identify to a genus level, this creature is of the order Bdelloidea and the family Philodinidae. This rotifer is one of the more common of its kind and can be found everywhere from pond water to bird baths.

Rotifers are characterized by twin whirling crowns of cilia, which serve both to propel them through water and sweep up food. It is these features which prompted their discoverer, Anton von Leeuwenhoek, to give them the most descriptive name of "wheel animalcules."

When extended (Fig. 1) the head, body and foot are plainly visible. The foot is used to anchor the rotifer, usually to a piece of vegetation, while the cilia on the head create a vortex to capture its food. If the rotifer is disturbed (by tapping the microscope slide with a pen, for example) it will contract into its defensive mode (Fig. 2). In this instance the rostrum, which contains sensory organs, including rudimentary eyes, is left protruding from the "shell." Far from the top of the food chain, rotifers feed on bacteria and tiny protozoa. Then, in turn, they are eaten by small worms and crustaceans.

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