Rotifers, commonly known as wheel animals or wheel animalcules, are microscopic aquatic organisms belonging to the phylum Rotifera. The name ‘rotifer’ derives from a Neo-Latin word meaning ‘wheel-bearer’ due to their characteristic ciliated crowns that resemble rotating wheels. These cilia are used for both locomotion and feeding. Despite their small size, often no larger than a grain of sand, rotifers possess complex bodies with specialized organ systems, including a brain, muscles, and complete digestive, nervous, and reproductive systems.
There are over 2,000 known species of rotifers, classified into three classes: Monogononta, Bdelloidea, and Seisonidea. While most are free-swimming and planktonic, some are sessile, living in tubes or gelatinous holdfasts.
Despite their mostly soft bodies, which make fossilization rare, rotifer fossils have been found, including species from Pleistocene peat deposits and Dominican amber dating to the Eocene. They play a crucial role in aquatic ecosystems by consuming organisms at the base of the food chain and serving as prey for many invertebrates.
Rotifers generally measure between 0.1 and 0.5 mm in length, though some can grow as large as 2 millimeters. The tiniest rotifers are about 6 to 8 micrometers, which is roughly six times the size of a human red blood cell.
Rotifers exhibit bilateral symmetry and diverse shapes, typically divided into three segments: the head, trunk, and foot, covered throughout by a well-developed, transparent cuticle. The cuticle is a body covering that varies in rigidity. A thick, rigid cuticle gives the rotifer a box-like shape and is often adorned with spines, ridges, or other ornamentations, known as loricate rotifers. Conversely, flexible cuticles result in a worm-like appearance called illoricate rotifers.
All rotifers possess an organ called the corona in their head, which is typically used for feeding and locomotion. In primitive species, the corona is a simple ring of cilia around the mouth, while in more evolved species, it has a complex structure with modifications such as bristles or tufts. In bdelloids, the corona consists of two rotating wheels on a pedestal.
The trunk is the body’s main part, housing all the internal organs. The foot, projecting from the trunk’s rear, often appears segmented due to ring-like cuticle formations. In sessile and crawling species, the foot ends in one to four toes with adhesive glands, enabling attachment to substrates like plants, rocks, and other surfaces. In many free-swimming species, the foot is reduced or absent. The toes, when present, may number up to four and aid in temporary attachment.
They lack a specialized circulatory or respiratory system, and thus gaseous exchange occurs across the body surface.
The ciliated corona, which sweeps food into the mouth, makes way for the muscular pharynx or mastax, which helps chew the food. The pharyngeal wall bears jaw-like structures called trophi, the shape of which varies depending on the nature of the diet. While the trophi in suspension feeders have grinding ridges, those in carnivorous species are modified like forceps for biting.
The mastax leads to the tubular esophagus, which, in turn, makes way for the stomach, where most of the digestion and absorption occurs. About seven salivary glands in front of the esophagus release digestive enzymes into the mouth.
The stomach is linked with two gastric glands and connects to a short intestine that opens into a cloaca on the posterior end of the body.
Rotifers possess a pair of protonephridia or flame cells that help expel excess water and nitrogenous waste from the body, maintaining the internal osmotic balance.
Their nervous system comprises about 25% of the total body cells (around 1,000) and is characterized by a small cerebral ganglion (considered the brain) above the mastax. A network of nerves originates from the ganglion and spreads throughout the body.
These invertebrates also bear one or two pairs of short, sensory antennae accompanied by simple eyes or ocelli (up to five) containing a single photoreceptor cell.
Females possess one or two ovaries, each with a vitellarium gland that provides yolk to the eggs. The ovary and vitellarium form a syncytial structure that opens through an oviduct into the cloaca. In the genus Asplanchna, females lack an anus but retain the cloacal opening for excretion and egg release.
Males possess a single testicle and sperm duct connected to a pair of glands called prostrates. The sperm duct opens into the penis through a gonopore, which is homologous to the cloaca of females.
Rotifers were first described in 1696 by Rev. John Harris, who noted their ability to contract into a spherical shape and stretch out again, likening them to large maggots. In 1702, Antonie van Leeuwenhoek detailed Rotifer vulgaris and later described Melicerta ringens and other species. He was also the first to document the revivification of certain species after drying. However, it wasn’t until 1838, with the publication of Christian Gottfried Ehrenberg’s Die Infusionsthierchen als vollkommene Organismen, that rotifers were recognized as multicellular animals.
About 2,200 rotifer species have been described, and their taxonomy remains in flux. They are generally classified under the phylum Rotifera, which includes three classes: Seisonidea, Bdelloidea, and Monogononta. The largest class, Monogononta, comprises about 1,500 species. Bdelloidea includes approximately 350 species, while Seisonidea is the smallest class, with only three known species.
Acanthocephala, previously considered a separate phylum, has been shown to be modified rotifers. Their relationship with other rotifers has led to their inclusion in a clade called Syndermata, which reflects their close evolutionary connection.
Although most rotifers are cosmopolitan and mostly found in freshwater habitats, like ponds and lakes, a few, like Testudinella clypeata, are found in marine environments. They are also found attached to mosses and lichens growing on tree trunks or even in permafrost or sewage treatment tanks.
Some gastropods are also endemic to certain regions of the world. For instance, Cephalodella vittata is endemic to Lake Baikal, Russia. These mollusks, however, are absent in Antarctica.
Rotifers primarily filter-feed on algae, bacteria, protozoans, phytoplankton, and organic detritus, none exceeding 10 µm in size. They modify their feeding behavior depending on environmental conditions and prey abundance.
Filter-feeding rotifers use well-developed coronal cilia to create feeding currents that draw particles into a ciliated feeding groove, feeding them to the buccal field and then to the mouth. The mastax, equipped with trophi, then grinds the food.
Raptorial rotifers employ a different feeding strategy, using their pincer-like mastax to grasp or pierce food items, which are then ground up. These rotifers often target larger prey items, showing a more predatory feeding behavior. Some rotifers have evolved to trap prey using a funnel-shaped corona lined with immotile bristles or spines. When prey enters the funnel, these structures prevent escape, guiding the prey to the mouth at the funnel’s center.
Other rotifers gather food using coronal tentacles, while some have developed symbiotic relationships with crustaceans or survive as endoparasites within invertebrates, living off their body fluids.
While most rotifers are planktonic and free-swimming, others, like those in the families Atrochidae, Collothecidae, and Flosculariidae, are sessile and attach themselves to a substrate. Some motile rotifers also creep along the bottom of water bodies.
Rotifers have short lifespans, typically ranging from 6 to 45 days.
Rotifers exhibit diverse reproductive strategies, including sexual reproduction, asexual reproduction through parthenogenesis, and cyclical parthenogenesis. They are dioecious, meaning they have distinct male and female individuals, but males are often rare or absent in many species.
In sexually reproducing species, females produce mictic eggs, which are haploid and require fertilization by male sperm to develop into diploid resting eggs. These resting eggs can endure harsh environmental conditions by entering a dormant state. If not fertilized, mictic eggs develop parthenogenetically into haploid males. Males are typically smaller, have a single testis, lack a functional digestive system, and are short-lived, often being sexually mature at birth. Fertilization involves the male inserting his copulatory organ into the female’s cloaca or directly injecting sperm through her body wall into the blastocoel.
Bdelloid rotifers reproduce exclusively by asexual parthenogenesis, a form of reproduction where females produce diploid eggs through mitosis. These eggs develop directly into female clones of their mothers without the need for fertilization.
Bdelloids are one of the few animal groups that have evolved without males for approximately 40 million years. Their populations consist entirely of females, enabling rapid and consistent reproduction without the genetic contribution of males.
Monogonont rotifers alternate between asexual and sexual reproduction, a process known as cyclical parthenogenesis or heterogony. Most of their life cycle is dominated by the amictic phase, during which females produce diploid eggs that develop parthenogenetically into more females. Under certain environmental conditions, amictic females can produce mictic females, which generate haploid eggs through meiosis. These haploid eggs develop into males if unfertilized or into resting eggs if fertilized.
Most rotifer species hatch as miniature versions of the adult, growing rapidly to reach their adult size within a few days. Sessile species, however, hatch as free-swimming larvae that later attach to a substrate. The lifespan of monogonont females varies from two days to about three weeks, and they reach sexual maturity within 18 hours of hatching.
Rotifers are preyed upon by other invertebrates, such as bryozoans, comb jellies, copepods, jellyfish, starfish, tardigrades, and insects (including their larvae). Additionally, certain birds and fish, like herring and salmon, also eat them.
Rotifers, like crustaceans, play a crucial role in nutrient cycling. They are often used in tanks to keep the water free from toxic waste expelled by fish. This ability helps prevent excess clouding of the water and allows light to reach the bottom of the tanks.
Rotifers help decompose organic wastes in soil, replenishing its mineral content and enhancing fertility. Some of them are ectoparasitic or endoparasitic on other animals. For example, members of the genera Seison and Paraseison attach to the legs and gills of certain crustaceans, such as Nebalia. Additionally, some rotifers are endoparasitic on nematodes, crustaceans, brachiopods, bryozoans, sponges, and even other rotifers.
As an important component of freshwater plankton, they are integral to the food chain and are a preferred diet for many other invertebrates.
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