Rotifera (Genusː Philodina) - 40X View

The rotifers (, from Latin 'wheel' and 'bearing'), sometimes called wheel animals or wheel animalcules, make up a phylum (Rotifera ) 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 (although their size can range from to over ), 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 (e.g., ''Sinantherina semibullata''), 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

John Harris first described the rotifers (in particular a bdelloid rotifer) 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 in 1838 that the rotifers were recognized as being multicellular animals.

In the landmark monograph on ''The Rotifera'' (1886-9) 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.

Etymology

The word ''rotifer'' is derived from a Neo-Latin word meaning 'wheel-bearer' due to the corona around the mouth that in concerted sequential motion resembles a wheel (although the organ does not actually rotate).

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. There is a well-developed cuticle, which 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''. Rigid cuticles are often composed of multiple plates, and may bear spines, ridges, or other ornamentation. Their cuticle is nonchitinous and is formed from sclerotized proteins.

The two most distinctive features of rotifers (in females of all species) are the presence of ''corona'' on the head, a structure ciliated in all genera except ''Cupelopagis'' and 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.

Nervous system

Rotifers have a small cerebral ganglion, effectively its brain, located just above the mastax, from which a number of nerves extend throughout the body. 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 antennae and up to five eyes. 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.

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" (RCO), 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.
Robert Lee Wallace
"Rotifers: Exquisite Metazoans"

2002.

quote:
"What is the function of the retrocerebral organ?"

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. Hochberg, R., Araújo, T. Q., Walsh, E. J., Mohl, J. E., & Wallace, R. L. (2023). Fine structure of the retrocerebral organ in the rotifer Trichocerca similis (Monogononta). Invertebrate Biology, 142(1), e12396. https://doi.org/10.1111/ivb.12396.

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 (i.e., attachment of eggs to a substrate), although more research is needed to explore function and evaluate the homology between species.

Digestive system

The coronal cilia create a current that sweeps food into the mouth. The mouth opens into a characteristic chewing pharynx (called the ''mastax''), sometimes via a ciliated tube, and sometimes directly. The pharynx has a powerful muscular wall and contains tiny, calcified, jaw-like structures called ''trophi'', which are the only fossilizable parts of a rotifer. The shape of the trophi varies between different species, depending partly on the nature of their diet. In suspension feeders, the trophi are covered in grinding ridges, while in more actively carnivorous species, they may be shaped like forceps to help bite into prey. In some ectoparasitic rotifers, the mastax is adapted to grip onto the host, although, in others, the foot performs this function instead.

Behind the mastax lies an oesophagus, which opens into a stomach where most of the digestion and absorption occurs. The stomach opens into a short intestine that terminates in a cloaca on the posterior dorsal surface of the animal. Up to seven salivary glands are present in some species, emptying to the mouth in front of the oesophagus, while the stomach is associated with two gastric glands that produce digestive enzymes.

A pair of protonephridia open into a bladder that drains into the cloaca. These organs expel water from the body, helping to maintain osmotic balance.

Biology

The coronal cilia pull the animal, when unattached, through the water.

Like many other microscopic animals, adult rotifers frequently exhibit eutely—they have a fixed number of cells within a species, usually on the order of 1,000.

Bdelloid rotifer genomes contain two or more divergent copies of each gene, suggesting a long-term asexual evolutionary history. For example, four copies of hsp82 are found. Each is different and found on a different chromosome excluding the possibility of homozygous sexual reproduction.

Feeding

Rotifers eat particulate organic detritus, dead bacteria, algae, and protozoans. They eat particles up to 10 micrometres in size. Like crustaceans, rotifers contribute to nutrient recycling. For this reason, they are used in fish tanks to help clean the water, to prevent clouds of waste matter. Rotifers affect the species composition of algae in ecosystems through their choice in grazing. Rotifers may compete with cladocera and copepods for planktonic food sources.

Reproduction and life cycle

Rotifers are dioecious and reproduce sexually or parthenogenetically. They are sexually dimorphic, with the females always being larger than the males. In some species, this is relatively mild, but in others the female may be up to ten times the size of the male. In parthenogenetic species, males may be present only at certain times of the year, or absent altogether.

The female reproductive system consists of one or two ovaries, each with a ''vitellarium

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