Rotifers are more complex than protozoan (classified as metazoan) and exist in a wide range of shapes and sizes. The taxonomic classification of rotifers is currently in flux, however it is generally accepted that over 2,000 species of rotifers exist. 

The majority or rotifers are motile and may also attach to flocs. Rotifers feed on small particulate matter such as dead bacteria, protozoan, and other particulate organic material. In practical experience, we have cited the presence of rotifers in SRT values of three to four days, so therefore it is not considered good practice to attempt to correlate rotifers with SRT values. More accurately, rotifers appear to predominate when there is an abundance of “rotifer food.” Rotifer’s preferred substrate in wastewater appear generally be pin floc or broken-up floc material. In lagoon wastewater treatment processes, many rotifers are algal predators. 

Female rotifers are generally larger than male rotifers (up to 10 times larger), and in most species, males lack digestive systems, causing them to have very short lifespans with an almost sole existing purpose of mating. Chances are that if you are viewing a rotifer under the microscope, it is a female. In nature, the lifespan of a female rotifer typically ranges from two days to three weeks. Reproduction methods vary depending on rotifer species, with the egg secreting a shell which may attach to other material. Or, often, the rotifer may carry the eggs inside their body until they hatch.

The body of a rotifer is divided into a head, trunk and foot. The most recognized and distinguished feature of rotifers (female rotifers) is the corona on the head. Depending on species there is some configuration of cilia around the mouth which then enter the mastax (where the food is chewed). Rotifers also have stomachs, a nervous system, two pairs of antennae, and up to five eyes. Mucus secreted by rotifers at the mouth or the foot is believed to help aid in floc formation. Rotifers, while generally much hardier than ciliates, are still strict aerobes and can die when threshold levels of stress occur. 

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About the author: Ryan Hennessy is the principal scientist at Ryan Hennessy Wastewater Microbiology. He was trained and mentored by Dr. Michael Richard for over 10 years in wastewater microbiology, and serves as a microbiology services consultant. Hennessy is a licensed wastewater treatment and municipal waterworks operator in the state of Wisconsin and fills in as needed for operations at several facilities. He can be reached at ryan@rhwastewatermicrobiology.com.