Rotifera

Rotifera is a phylum of microscopic, pseudocoelomate animals common in freshwater, brackish water, and moist terrestrial habitats. Most species are less than 0.5 mm long, though some reach about 2 mm. They are among the most abundant metazoans in freshwater microfauna and occupy planktonic, benthic, and damp-soil niches. Their small size and rapid generation make them important in ecological and laboratory studies.

A distinctive feature is the corona, a wheel-like ring of ciliated lobes that creates water currents and helps draw food into the mouth. The muscular pharynx, or mastax, contains trophi, jaw-like elements that grind algae and detritus. The digestive tract is complete. Many rotifers possess a rigid external covering called a lorica. The body is often enveloped by a syncytial epidermis over a fluid-filled cavity, giving a hydrostatic skeleton, and a foot with toes in several species.

Reproduction is diverse. Monogononta typically reproduce by cyclical parthenogenesis, producing diploid female eggs that develop without fertilization and resting eggs under stress. Bdelloidea are exclusively parthenogenetic, with no observed males. Seisonidea are marine ectoparasites with sexual stages. Resting eggs and ephippia enable survival of unfavorable conditions and long-distance dispersal.

Ecologically, rotifers act as primary consumers of microalgae and detritus, contributing to nutrient cycling in freshwater and moist environments. They are widely used in ecotoxicology, developmental biology, and studies of cryptobiosis, and they provide live feed in aquaculture.