Rotifer
The rotifers, sometimes called wheel animals or wheel animalcules, make up a phylum of microscopic and near-microscopic pseudocoelomate animals.
They were first described by Rev. John Harris in 1696, and other forms were described by Antonie van Leeuwenhoek in 1703. Most rotifers are around long, and are common in freshwater environments throughout the world with a few saltwater species.
Some rotifers are free swimming and truly planktonic, others move by inchworming along a substrate, and some are sessile, living inside tubes or gelatinous holdfasts that are attached to a substrate. About 25 species are colonial, either sessile or planktonic. Rotifers are an important part of the freshwater zooplankton, being a major foodsource and with many species also contributing to the decomposition of soil organic matter. Genetic evidence indicates that the parasitic acanthocephalans are a highly specialised group of rotifers.
Most species of the rotifers are cosmopolitan, but there are also some endemic species, like Cephalodella vittata to Lake Baikal. Recent barcoding evidence, however, suggests that some 'cosmopolitan' species, such as Brachionus plicatilis, B. calyciflorus, Lecane bulla, among others, are actually species complexes. In some recent treatments, rotifers are placed with acanthocephalans in a larger clade called Syndermata.
In June 2021, biologists reported the restoration of bdelloid rotifers after being frozen for 24,000 years in the Siberian permafrost. The earliest record of the rotifer clade is of an acanthocephalan from the Middle Jurassic of China. Earlier purported fossils of rotifers have been suggested in Devonian and Permian fossil beds.
Taxonomy and naming
first described the rotifers in 1696 as "an animal like a large maggot which could contract itself into a spherical figure and then stretch itself out again; the end of its tail appeared with a forceps like that of an earwig". In 1702, Antonie van Leeuwenhoek gave a detailed description of Rotifer vulgaris and subsequently described Melicerta ringens and other species. He was also the first to publish observations of the revivification of certain species after drying. Other forms were described by other observers, but it was not until the publication of Christian Gottfried Ehrenberg's Die Infusionsthierchen als vollkommene Organismen in 1838 that the rotifers were recognized as being multicellular animals.In the landmark monograph on The Rotifera by C.T. Hudson, assisted by P.H. Gosse, 400 British and foreign species were included; by 1912, the total reached 607 species. About 2,200 species of rotifers have now been described. Their taxonomy is currently in a state of flux. One treatment places them in the phylum Rotifera, with three classes: Seisonidea, Bdelloidea and Monogononta. The largest group is the Monogononta, with about 1,500 species, followed by the Bdelloidea, with about 350 species. There are only two known genera with four species of Seisonidea.
The Acanthocephala, previously considered to be a separate phylum, have been demonstrated to be modified rotifers. The exact relationship to other members of the phylum has not yet been resolved. One possibility is that the Acanthocephala are closer to the Bdelloidea and Monogononta than to the Seisonidea; the corresponding names and relationships are shown in the cladogram below.
The Rotifera, strictly speaking, are confined to the Bdelloidea and the Monogononta. Rotifera, Acanthocephala and Seisonida make up a clade called Syndermata. The findings of a fossil called Juracanthocephalus shares features with both Seisonidea and Acanthocephala, suggesting that they are sister groups.
Giribet & Edgecombe and Brusca et al. accept the following classification:
- class Hemirotatoria/Hemirotifera
- * subclass Bdelloidea
- * subclass Acanthocephala
- * subclass Seisonidea/Seisonacea
- class Eurotifera
- * subclass Monogononta
Etymology
Anatomy
Rotifers have bilateral symmetry and a variety of different shapes. The body of a rotifer is divided into a head, trunk, and foot, and is typically somewhat cylindrical. The trunk contains visceral organs, and often, sensory antennae. There is a well-developed cuticle, found everywhere except in the corona, which is secreted by a fibrous layer in the syncytial epidermis. This fibrous layer may be thick and rigid, giving the animal a box-like shape, or flexible, giving the animal a worm-like shape; such rotifers are respectively called loricate and illoricate. Loricate fibrous layers are often composed of multiple plates or rings, and may bear spines, ridges, or other ornamentation. Certain species have superficial rings in the body wall imitating segments. Also, sub-epidermal muscles, which may be circular, longitudinal, or traversing the pseudocoel to the visceral organs. This large fluid-filled pseudocoel contains certain muscles and mesenchymal ameboid cells. Their cuticle is nonchitinous and is formed from sclerotized proteins.The two most distinctive features of rotifers are the presence of corona on the head, a structure ciliated in all genera except Cupelopagis, and the presence of mastax. In the more primitive species, the corona forms a simple ring of cilia around the mouth from which an additional band of cilia stretches over the back of the head. In the great majority of rotifers, however, this has evolved into a more complex structure.
Modifications to the basic plan of the corona include alteration of the cilia into bristles or large tufts, and either expansion or loss of the ciliated band around the head. In genera such as Collotheca, the corona is modified to form a funnel surrounding the mouth. In many species, such as those in the genus Testudinella, the cilia around the mouth have disappeared, leaving just two small circular bands on the head. In the bdelloids, this plan is further modified, with the upper band splitting into two rotating wheels, raised up on a pedestal projecting from the upper surface of the head.
The trunk forms the major part of the body, and encloses most of the internal organs. The foot projects from the rear of the trunk, and is usually much narrower, giving the appearance of a tail. The cuticle over the foot often forms rings, making it appear segmented, although the internal structure is uniform. Many rotifers can retract the foot partially or wholly into the trunk. The foot ends in from one to four toes, which, in sessile and crawling species, contain adhesive glands to attach the animal to the substratum. In many free-swimming species, the foot as a whole is reduced in size, and may even be absent. Rotifers move by swimming with the coronal cilia and/or foot-assisted leech-like creeping.
Nervous system
Rotifers have a small bilobed cerebral ganglion, effectively its brain, located just above the mastax, from which a number of paired nerves extend throughout the body, namely, sense organs, mastax, muscles, and viscera. The number of nerves varies among species, although the nervous system usually has a simple layout.The nervous system comprises about 25% of the roughly 1,000 cells in a rotifer.
Rotifers typically possess one or two pairs of short dorsal antennae, and with usually paired eyespots. The eyes are simple in structure, sometimes with just a single photoreceptor cell. In addition, the bristles of the corona are sensitive to touch, and there are also a pair of tiny sensory pits lined by cilia in the head region, as well as bristles and papillae.
Retrocerebral organ
Despite over 100 years of research, rotifer anatomy still has many poorly understood components. One of the more mysterious organs in rotifers is the "retrocerebral organ", which still remains very enigmatic in its morphology, function, development, and evolution. Lying close to the brain, this organ usually consists of one or more glands and a sac or reservoir. The sac drains into a duct before opening through pores on the uppermost part of the head. Current data shows a wide diversity in structure and potential function. In some species it is reduced or may even be absent completely. Benthic species have larger RCO's than planktonic species. Despite this diversity, positional correspondence of RCOs strongly suggests homology.A 2023 study using transmission electron microscopy and confocal laser scanning microscopy has illuminated the fine structure of this organ further. The study, the first of its kind, investigated the RCO in one species, Trichocerca similis. It was determined to be a syncytial organ, composed of a posterior glandular region, an expansive reservoir, and an anterior duct. The glandular portion has an active cytoplasm with paired nuclei, abundant rough ER, ribosomes, Golgi, and mitochondria. Secretion granules accumulate at the anterior end of the gland where they undergo homotypic fusion to create larger granules with numerous "mesh-like" contents. These contents gradually fuse into tubular secretions that accumulate in the reservoir, awaiting secretion. Cross-striated longitudinal muscles form a partial sleeve around the reservoir and may function to squeeze the secretions through the gland's duct that often penetrates through the cerebral ganglion.
Retrocerebral organ secretions
Much like the organ itself, the precise function and biochemical makeup of the secretions is still unknown. The small size of rotifers and small volume of the secretions makes isolation immensely difficult. The secretions have some similarities to the hydrogel secretions that form gelatinous housings in some rotifer species.Ultrastructure analysis of T. similis secretions showed them to be a series of tube-like secretions with a highly filamentous framework. This is highly suggestive of a glycosaminoglycan structure- proteins with negatively charged polysaccharide chains forming proteoglycan molecules. These molecules are standard in vertebrate and invertebrate gelatins such as mucus.
Despite recent advancements in understanding RCO organ and secretion ultrastructure, the exact function of the organ is still ultimately unclear. The leading hypotheses are that the RCO secretes a mucus-like substance that aids in benthic locomotion, adhesion, and/or reproduction, although more research is needed to explore function and evaluate the homology between species.