Invertebrates
Rotifer
Rotifera · also called rotifers, wheel animals
A rotifer is smaller than most animals you could name, often invisible without a microscope, yet it has a working gut, a brain, muscles, and a crown of beating hairs that spins like a tiny wheel to sweep food into its mouth. Some can dry out to almost nothing and sit lifeless for years, then spring back into full activity within minutes of touching water again.
Overview
Rotifers, phylum Rotifera, are microscopic animals found in fresh water and damp habitats worldwide. Their name means “wheel-bearer,” after the corona, a ring of cilia around the mouth whose coordinated beating looks like a spinning wheel and drives both feeding and swimming.
Tiny, tough, and often all-female
Despite their minute size, rotifers have complete organ systems. Bdelloid rotifers can survive total desiccation through anhydrobiosis, reviving within minutes of rehydration after months or years dormant, and reproduce entirely by parthenogenesis — no male has ever been documented in the group, a genuine evolutionary puzzle.
A hidden pillar of aquatic life
Enormously abundant, rotifers are vital food for small aquatic animals and fish larvae, and their fast, easy culture has made them a cornerstone live food in fish and shellfish aquaculture worldwide.
Status
Least Concern — abundant, resilient, and found on every continent including Antarctica, thriving on extreme tolerance and rapid reproduction.
Rotifer: common questions
Why are rotifers called wheel animals?
The name refers to the corona, a ring or pair of rings of hair-like cilia surrounding the mouth that beat in a coordinated, wave-like pattern to draw in food and, in free-swimming species, to help propel the animal through the water. Under a microscope, this rhythmic beating can create an optical illusion that looks remarkably like a pair of small spinning wheels, which is where both the common name wheel animal and the scientific name Rotifera, meaning wheel-bearer, come from.
How can a rotifer survive being completely dried out?
Certain rotifers, especially the bdelloid group, can enter a state called anhydrobiosis, in which the animal loses almost all of its internal water, its metabolism effectively stops, and its body takes on a stable, glass-like form that resists the physical damage drying would normally cause to living cells. In this dormant state, the rotifer can withstand not just drying but also extreme cold, high radiation, and other stresses for months or years, and remarkably it can rehydrate, resume active swimming and feeding, and go on to reproduce within minutes to hours of water becoming available again.
Do rotifers really reproduce without any males at all?
In the bdelloid group specifically, yes. No male bdelloid rotifer has ever been reliably documented, and females reproduce entirely through parthenogenesis, producing daughters that are essentially clones of themselves without any need for fertilisation. This has persisted for an estimated tens of millions of years, which is unusual because most lineages that abandon sexual reproduction entirely are thought to accumulate harmful mutations and go extinct relatively quickly, making bdelloid rotifers a long-studied puzzle in evolutionary biology, with their frequent drying-and-repair cycles and unusual capacity to absorb foreign genes proposed as part of the explanation.
Are rotifers important to aquariums and fish farming?
Yes, rotifers are one of the most widely used live foods in aquaculture, particularly for feeding the newly hatched larvae of many farmed fish and shellfish species, which are often too small to eat larger prey items when they first begin feeding. Because rotifers are tiny, reproduce very quickly, and can be cultured in large numbers relatively easily and cheaply, hatcheries commonly grow dense rotifer cultures specifically timed to be available as first food when a batch of fish larvae hatches, making rotifers a quietly essential part of the modern seafood and ornamental fish supply chain.