Rotifera (Rotifers)

(invertebrate zoology) A class of the phylum Aschelminthes distinguished by the corona, a retractile trochal disk provided with several groups of cilia and located on the head.

(Rotifers)

Phylum: Rotifera

Number of families: 34

Thumbnail description
Group of microscopic animals characterized by the presence of a complex jaw apparatus and a ciliary wheel organ used for locomotion and feeding

Evolution and systematics

The Rotifera traditionally have been considered part of a group called Aschelminthes or Pseudocoelomata that comprised most of the microscopic animal groups without a true body cavity. Modern phylogenetic analyses have rejected this group, however, and today most studies support a close relationship between Rotifera, Acanthocephala (thorny-headed worms), Gnathostomulida (jaw worms), and the recently described Micrognathozoa (jaw animals) and unite them in a superphylum named Gnathifera (meaning, "those that possess jaws"). The acanthocephalans usually are considered a sister group to Rotifera, but this has been questioned by molecular data, which imply that acanthocephalans are highly advanced rotifers. This hypothesis still needs support from morphological data. The phylogenetic position of Gnathifera remains uncertain, but the most recent phylogenetic analyses suggest that they either are a basal group in Spiralia or form a monophyletic group with Gastrotricha and Platyhelminthes. In 2003 Rotifera included about 1,817 species, distributed among five orders and 34 families and divided into three classes: Seisonidea, Bdelloidea, and Monogononta. The latter class contains approximately 80% of the known species and displays the greatest morphological diversity.

Physical characteristics

Rotifers may range in size from less than 0.00394 in (100 µm) to 0.098 in (2,500 µm), but most species measure between 0.00591 and 0.0197 in (150–500 µm). The body generally is divided into a head, a trunk, and a foot region, but this basic pattern may vary greatly. The most conspicuous organ in the head is the wheel organ, also called the corona, which is composed of metachronously beating cilia that are arranged in different, distinct bands. Generally, the corona comprises a large buccal field that surrounds the mouth and a circumapical band that encircles the aciliate apical head region. Numerous modifications from this basic plan have made the corona morphological highly variable. In the class Bdelloidea, for example, the circumapical band is divided medially so that it forms two trochal discs. In live swimming or feeding animals, the ciliary beat of the disc gives the illusion that the animal carries two small, rotating wheels, and the whole phylum is named after this feature (rota, meaning "wheel," and fero, meaning "to bear"). The corona is used both for locomotion and feeding. The ciliary beat leads food particles toward the mouth opening, which always is located more or less ventrally.

The trunk may vary in shape from a very elongated form, sometimes divided into telescopic, retractable pseudo-segments, to a much more globular or sacciform form. The foot is composed of one to several pseudo-segments, and it often has two terminal toes with adhesive gland openings at their tips. Both the foot and the toes may be reduced in several genera and species. The rotifer integument is syncytial, which means that the epidermal cells are not separated by cell membranes. An outer cuticle, which is present in most other invertebrates, is lacking in rotifers, and instead they have an intracellular filamentous lamina. The thickness of the lamina varies, and in some taxa parts of it are so thick that it forms a heavy body armor. Such taxa are called loricate, whereas those with a thinner lamina are referred to as illoricate.

The mouth is located ventrally or apically in the corona. It leads to the pharynx, which contains a complex masticatory apparatus referred to as the mastax. The mastax is made up of hard jaw parts, called trophi, and minute muscles that connect the jaw elements. The trophi comprise four principal elements: paired rami, paired unci, paired manubria and an unpaired fulcrum. There also may be different associated elements that together are referred to as epipharyngeal elements. The rami are the central elements in the trophi and often are equipped with teeth or denticles. They are joined caudally with a flexible ligament such that they are able to open and close and in that way crush food items. The only unpaired principal element, the fulcrum, extends caudally from the articulation point of the rami. It primarily serves as a muscle attachment point for different muscle fibers, for example, the large abductor muscles that run to the rami. The unci are located rostral or ventral to the rami. They may be rod-shaped with a single sharp tooth or plate-shaped with several strong teeth, and they may be used to grab and manipulate food particles or, in some predatory rotifers, to penetrate the integument of the prey. Proximally, the unci join the manubria. The manubria often are rod-shaped with a well-developed head and are responsible mainly for the movement of the unci. The morphological features of the four principal elements differ greatly, and the rotifer trophi have been divided into nine different types, depending on the size and shape of the principal sclerites. These types are highly significant for rotiferan taxonomy; several families and genera can be recognized solely on the basis of jaw type.

From the pharynx a short esophagus leads to the stomach, where the food is digested. Digestive enzymes are produced in syncytial gastric glands that empty into the stomach. After the stomach comes the gut, which terminates in a dorsal cloaca. Pairs of protonephridia control excretion and maintenance of osmotic balance. Each protonephridium is composed of one or more multiciliated terminal cells and multiciliated canal cells that lead to the collecting tubules, which guide the wastes on to the urinary bladder. The female reproductive system comprises one or two syncytial germovitellaria that each consists of a germinal region and a yolk-producing vitellarium, surrounded by a follicular layer. The classes Seisonidea and Bdelloidea have paired gonads, whereas the Monogononta have only a single germovitellarium. An oviduct formed by the follicular layer may connect the germovitellarium with the cloaca. The male organs comprise an unpaired testis and a penis. (See Reproductive biology for a more detailed description of the complex rotifer reproduction.)

Distribution

Rotifers have been recorded from all parts of the world, and the distribution pattern for the different species may vary from truly cosmopolitan to endemic. In particular, the bdelloid genera Philodina and Rotaria and various monogonont genera, such as Brachionus, Keratella, Lecane, and Lepadella, contain some extremely abundant species that have been recorded from most places in the world. Other species also are rather common but have a more limited distribution, being restricted, for example, to the Eurasian continent or the Holarctic or pantropical regions. Furthermore, some species appear to be endemic. For instance, no less than 11 species from the genus Notholca are endemic to Lake Baikal.

Among the key factors of rotifer success in terms of distribution are their cryptobiotic capabilities and their ability to produce resting eggs (see Habitat and Reproductive biology). Both resting eggs and dormant bdelloids may disperse over large distances with the aid of wind or water. Furthermore, resting eggs from many species have sculptured shells with tiny spines and hooks that enable them to attach to other animals (for example, birds) and spread by epizoic dispersal.

Habitat

Rotifers are found in all aquatic and semiaquatic habitats, but they reach the greatest diversity and largest population sizes in freshwater. They may inhabit the sediment, live in association with submerged plants (live as well as dead and partly decayed), or be restricted to plankton. They also may be adapted for more special habitats, such as the ice of the Arctic Ocean, terrestrial mosses and lichens, or meltwater ponds on glaciers. Furthermore, some species are specialized parasites and live in the intestines or gills of various invertebrates. The optimal environment for many species is warm, nutrient-rich, slightly alkaline freshwater, but several species also are capable of surviving in more demanding habitats, such as temporarily dry or frozen ponds. Many bdelloid rotifers have cryptobiotic capabilities, which means that they are able to stop their metabolism, dehydrate their cells, and enter a state of dormancy. When entering cryptobiosis, the animal is capable of surviving under conditions that normally would be hazardous, such as complete dehydration, freezing, or oxygen deficiency. Many monogonont rotifers may survive under similar conditions by producing thick-shelled resting eggs. When the circumstances become more optimal, the populations may grow rapidly, because of their ability to reproduce asexually. The population and species richness is generally lower in marine habitats. The greatest diversity is found in the periphyton, but many species also may inhabit the interstices of sand grains on beaches. Marine rotifers are found mostly in the plankton or in the littoral zone and are extremely rare in deeper waters.

Behavior

Studies on rotifer behavior are extremely limited. With their minute size and simple nervous system, there is no basis for a complex type of behavioral biology. Various observations can be made concerning their locomotive patterns, escape behavior, and communication in relation to reproduction. When rotifers move, they typically swim or, alternatively, crawl on the substratum. Swimming specimens often move gently through the water in a characteristic helical motion that makes them easy to distinguish from most other microinvertebrates. Their sensory structures enable them to avoid obstacles, which distinguishes them from rotifer-like protozoa that often bump into obstacles and subsequently perform a rapid jump backward. Bdelloid rotifers either crawl or swim with their two ciliary discs. When crawling, they use the adhesive glands in the foot and in the rostrum. First, they adhere to the substratum with their pedal glands, and then they extend the body and attach to the substratum with the rostrum. Subsequently, the foot is detached, moved forward, and attached again. This inchworm-like way of crawling is very characteristic, and it makes it easy to distinguish crawling bdelloids from other animals.

When a rotifer is touched by another animal or by a thin dissecting needle, it often reacts by retracting the corona or else changes its swimming direction. Some species have special features that are used to "escape." For example, species in the genus Polyarthra have bundles of leaflike fins; when the animals are disturbed, they can flick with the fins and rapidly move as far as twelve body lengths. In the genus Scaridium the foot is equipped with strong muscles that allow it to act as a spring, which enables the animal to make a quick jump away from the source of disturbance.

Feeding ecology and diet

Rotifers typically feed on suspended organic particles, microalgae, ciliates, or bacteria. Most rotifers are filter feeders or suspension feeders and collect food with a water current created by the ciliary beat of the corona. There also are several forms of specialization. Instead of filtering, some rotifers creep along plants or sediment particles and graze on the bacterial layer. Others are predatory and may feed on algae, flagellates, or even other rotifers. In the latter case, it often is possible to identify the ingested prey by analyzing its indigestible trophi in the predator's stomach contents. Species that feed on algae often employ their unci to penetrate the filament, consequently using the large hypopharyngeal muscles to create a vacuum and in this way suck out the cytoplasm of the prey.

Reproductive biology

Information on rotifer mating behavior is scarce. It has been shown, however, that females of Brachionus plicatilis carry a glycoprotein on the surfaces of the body that acts as a sex pheromone. This protein can bind to chemoreceptors in the corona of conspecific males. Hence, the pheromone probably serves as a mating recognition signal that helps rotifers avoid mating with nonspecific specimens.

The rotifer reproductive cycle differs between the three classes. Whereas seisonids reproduce sexually and bdelloids are solely asexual, the monogonont rotifers have a complex cycle that includes both a sexual and an asexual phase. In seisonids males and females are the same size, and both probably are diploid (contrary to the haploid monogonont dwarf males, discussed later). The male stores sperm in a spermatophore that is transferred to the female. Fertilization and initial cell divisions occur inside the female's germarium; later, the female attaches the eggs to the host, the crustacean Nebalia, where they stay until the juveniles hatch. In bdelloids there are only females, and they reproduce exclusively by asexual parthenogenesis. This means that the maternal individual produces diploid eggs via mitosis and that these eggs can develop into new embryos without initial fertilization. As a consequence, the daughters are cloned individuals that always are genetically identical to the mother.

The monogonont reproductive cycle is divided into an asexual (the amictic) and a sexual (the mictic) phase. The amictic phase resembles the bdelloid cycle, with parthenogenetically reproducing amictic females and complete absence of males; during this phase the population is capable of growing very quickly. Certain physical stimuli may induce the production of another kind of female, named a mictic female. Mictic females are morphologically similar to amictic females, but they produce eggs by meiotic cell division, which means that the eggs become haploid. The haploid mictic egg either waits to be fertilized by a male or, if it is not fertilized, starts to develop into a haploid male. Males are much smaller than females, and internal organs, such as the alimentary canal, often are reduced. The short-lived males seek mictic females immediately after hatching and fertilize eggs by hypodermic impregnation. Fertilization results in a thick-shelled resting egg that can survive extreme conditions, such as freezing and dehydration; after a period of dormancy an amictic female hatches from the egg and enters the amictic phase again. Most rotifers have direct development, and mitosis never occurs after hatching. Larvalike stages are found among permanently attached rotifers, so that the newly hatched animals can move to a suitable place before they settle permanently.

Conservation status

No species is listed by the IUCN.

Significance to humans

Rotifers in the wild have little significance to humans. They may have some economic significance, however, because many species are cultured as a food source for aquariums and cultured filter-feeding invertebrates and fish fry. They also may be used as biological pollution indicators.

Species accounts

Resources

Donner, J. Ordnung Bdelloidea. Berlin: Akademie Verlag, 1965.

Koste, Walter. Rotatoria: Die Rädertiere Mitteleuropas. Stuttgart: Gebrüder Borntraeger, 1978.

Nogrady, T., R. L. Wallace, and T. Snell. Rotifera. Vol. 1, Biology, Ecology, and Systematics. Guides to the Identification of the Microinvertebrates of the Continental Waters 4, edited by H. J. F. Dumont. Amsterdam: SPB Academic Publishing, 1993.

Wallace, R. L., and C. Ricci. "Rotifera." In Freshwater Meiofauna: Biology and Ecology, edited by S. D. Rundle, A. L. Robertson, and J. M. Schmid-Araya. Leiden, The Netherlands: Backhuys Publishers, 2002.

Ricci, C., and G. Melone. "Key to the Identification of the Genera of Bdelloid Rotifers." Hydrobiologia 418 (2000): 73–80.

[Article by: Martin Vinther Sørensen, PhD]

A phylum of pseudocoelomate, microscopic, mainly free-living aquatic animals, characterized by an anterior ciliary apparatus, the corona, whose cilia when in motion have the appearance of a pair of rapidly rotating wheels. This structure is implicit in the phyletic name (literally “wheel bearers”) and the older popular name wheel animalcules.

The Rotifera show considerable diversity in form and structure, but all are bilaterally symmetrical, pseudocoelomate animals possessing complete digestive, excretory, nervous, and reproductive systems, but lacking separate respiratory and circulatory systems. They possess two features unique to their phylum: the corona, which is a retractile trochal disk, and the mastax, which is a gizzardlike structure derived from the modified pharynx.

Rotifers are dioecious and sexually dimorphic; females are commoner than males, some of which are degenerate organisms lacking a mouth and digestive organs. Males, when produced in the life cycle, are short-lived and survive for only hours or at the most a few days.

The three major subdivisions of the Rotifera, now given class status, are the Seisonacea, Bdelloidea, and Monogononta. See also Bdelloidea; Monogononta; Seisonacea.