Identification and Ecology of Australian Freshwater Invertebrates

Rotifera (wheel animalcules)

Wheel animalcules derive their common name from movements of the cilia on the corona that create the appearance of revolving wheels. They are often the most numerically abundant and diverse metazoans in freshwaters.

Descriptive Features:

morphology variable

body slightly elongated, rounded in cross-section, unsegmented, divided into 3 regions, head, trunk and foot

cuticle covers body surface, may be delicate and flexible, or thickened and rigid, forming a strong case (lorica) enclosing most of the body, lorica varies from a few cuticular plates to a box-like structure bearing spines

head bears a mouth, various protuberances (sometimes retractile) and a corona (zone of cilia encircling a central apical field)

corona may be simple, lobulated, scalloped or subdivided into retractile trochal discs

apical field bare but may have several projections

trunk with dorsal and lateral antennae; anus occurs middorsally at or about junction of foot and trunk

foot absent in most planktonic rotifers, reduced in others, toes may be present terminally, (1, 3 or 4 toes pairs), or absent

Total length: minute to microscopic animals, less than 0.5 mm

Brachionidae

Distribution: Australia wide

Sensitivity Rating: none. High densities of rotifers are found in highly eutrophic waters.

Functional Feeding Group: gathering collector, predator

Barramundi Creek, Kakadu NP NT

Ecology: Rotifers occur in littoral or benthic areas of billabongs, lakes, rivers and puddles amongst floating or submerged vegetation. Most rotifers are free living, sessile or free floating, a few are parasitic, or ectocommensal on other freshwater animals. Planktonic forms float with the aid of an inflated, balloon-like body. Semiplanktonic species use an adhesive secretion from the toe tips to attach to flocculent detritus, algal filaments, crustaceans or even other rotifers. Rotifers can reproduce rapidly to fill available niches. High population densities can be easily achieved when food is not limiting, hence they are in turn a significant part of the diet of macroinvertebrates, tadpoles or juvenile fish. Rotifers are omnivorous eating microscopic particulate organic matter, dead or alive. Food is gathered by the cilia on the corona. Some species are carnivorous Bdelloidea reproduce parthenogenetically, Collothecacea and Flosculariacea have alternate generations (heterogony), that is, after parthenogenetic phases, male generations may occur especially at the onset of unfavourable environmental conditions. Eggs give rise to sexual females which produce eggs that give rise to males. Copulation involves injection of sperm into the body cavity of the female and this results in resting eggs. Resting eggs are thick shelled, resistant and hatch only after a dormant period when conditions become favourable. They always hatch as females. Most eggs hatch outside the body of their parent, although a few species are known to be viviparous. If the rotifer arising from the egg is free-swimming, then the egg hatches directly into a more or less complete and fully formed adult. If it is destined to be a sessile rotifer, the egg hatches into a larval form that swims freely briefly then attaches to a submerged object and assumes adult morphology.

Information Sources: Shiel 1995, Williams 1980

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