Rotifer
Rotifera Temporal range:
Possible Devonian and Permian records | |
---|---|
Bdelloid rotifer (Bdelloidea) | |
Pulchritia dorsicornuta (Monogononta) | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Clade: | Bilateria |
Clade: | Nephrozoa |
(unranked): | Protostomia |
(unranked): | Spiralia |
Clade: | Gnathifera |
Phylum: | Rotifera Cuvier, 1798 |
Classes and other subgroups | |
|
The rotifers (/ˈroʊtɪfərz/, from the Latin rota, "wheel", and -fer, "bearing"), commonly called wheel animals or wheel animalcules,[1] make up a phylum (Rotifera /roʊˈtɪfərə/) of microscopic and near-microscopic pseudocoelomate animals.
They were first described by
Some rotifers are free swimming and truly
In June 2021, biologists reported the restoration of bdelloid rotifers after being frozen for 24,000 years in the Siberian permafrost.[7][8] Early purported fossils of rotifers have been suggested in Devonian[9] and Permian[10] fossil beds.
Taxonomy and naming
About 2,200
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.[14] 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.
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The Rotifera, strictly speaking, are confined to the Bdelloidea and the Monogononta. Rotifera, Acanthocephala and Seisonida make up a clade called Syndermata.[15]
Etymology
The word rotifer is derived from a Neo-Latin word meaning "wheel-bearer",[16] due to the corona around the mouth that in concerted sequential motion resembles a wheel (though the organ does not actually rotate).
Anatomy
Rotifers have
The two most distinctive features of rotifers (in females of all species) are the presence of corona on the head, a 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
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.[17]
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. [17]
The nervous system comprises about 25% of the roughly 1,000 cells in a rotifer.[18]
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.[17]
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. [19] 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.[17][18][20]
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.[20]
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. [20]
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. [20]
Digestive system
The coronal cilia create a current that sweeps food into the mouth. The mouth opens into a characteristic chewing
Behind the mastax lies an
A pair of
Biology
The coronal
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
Feeding
Rotifers eat particulate organic detritus, dead bacteria, algae, and protozoans. They eat particles up to 10 micrometres in size. Like
Reproduction and life cycle
Rotifers are
The female reproductive system consists of one or two
Males do not usually have a functional digestive system, and are therefore short-lived, often being
The phylum Rotifera encloses three classes that reproduce by three different mechanisms: Seisonidea only reproduce sexually; Bdelloidea reproduce exclusively by asexual parthenogenesis; Monogononta reproduce alternating these two mechanisms ("cyclical parthenogenesis" or "heterogony").[22] Parthenogenesis (amictic phase) dominates the monogonont life cycle, promoting fast population growth and colonization. In this phase males are absent and amictic females produce diploid eggs by mitosis which develop parthenogenetically into females that are clones of their mothers.[22] Some amictic females can generate mictic females that will produce haploid eggs by meiosis. Mixis (meiosis) is induced by different types of stimulus depending on species. Haploid eggs develop into haploid dwarf males if they are not fertilized and into diploid "resting eggs" (or "diapausing eggs") if they are fertilized by males.
Fertilization is internal. The male either inserts his penis into the female's cloaca or uses it to penetrate her skin, injecting the sperm into the body cavity. The egg secretes a shell, and is attached either to the substratum, nearby plants, or the female's own body. A few species, such as members of the
Most species hatch as miniature versions of the adult. Sessile species, however, are born as free-swimming larvae, which closely resemble the adults of related free-swimming species. Females grow rapidly, reaching their adult size within a few days, while males typically do not grow in size at all.[17]
The life span of monogonont females varies from two days to about three weeks.
Loss of sexual reproduction system
'Ancient asexuals':
However, a new study provided evidence for interindividual genetic exchange and recombination in Adineta vaga, a species previously thought to be anciently asexual.[23]
Recent transitions: Loss of sexual reproduction can be inherited in a simple Mendelian fashion in the monogonont rotifer Brachionus calyciflorus: This species can normally switch between sexual and asexual reproduction (cyclical parthenogenesis), but occasionally gives rise to purely asexual lineages (obligate parthenogens). These lineages are unable to reproduce sexually due to being homozygous for a recessive allele.[24]
Resting eggs
Resting eggs enclose an embryo encysted in a three-layered shell that protects it from external stressors.[25][26] They are able to remain dormant for several decades and can resist adverse periods (e.g., pond desiccation or presence of antagonists).[27][28] When favourable conditions return and after an obligatory period of diapause which varies among species, resting eggs hatch releasing diploid amictic females that enter into the asexual phase of the life cycle.[22][29]
Anhydrobiosis
Bdelloid rotifer females cannot produce resting eggs, but many can survive prolonged periods of adverse conditions after
Predators
Rotifers fall prey to many animals, such as
Genome size
The genome size of a bdelloid rotifer,
Gallery
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Pair of Lepadella rotifers from pond water
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Locula of the rotifer Keratella cochlearis
References
- ^ a b Howey, Richard L. (1999). "Welcome to the Wonderfully Weird World of Rotifers". Micscape Magazine. Retrieved 19 February 2010.
- ^ a b Harmer, Sidney Frederic & Shipley, Arthur Everett (1896). The Cambridge Natural History. The Macmillan company. pp. 197. Retrieved 25 July 2008.
john harris rotifer.
- ^ "Rotifers". Freshwater Life. Archived from the original on 1 August 2012. Retrieved 19 February 2010.
- ^ Hendrik Segers (2007). Annotated checklist of the rotifers (Phylum Rotifera), with notes on nomenclature, taxonomy
- PMID 12206243.
- ^ Dec 2011 4th Internat. Barcode of Life conference, University of Adelaide
- ^ Renault, Marion (7 June 2021). "This Tiny Creature Survived 24,000 Years Frozen in Siberian Permafrost - The microscopic animals were frozen when woolly mammoths still roamed the planet, but were restored as though no time had passed". the New York Times. Retrieved 8 June 2021.
- S2CID 235365588.
- ^ "Spoilt attack in the Lower Devonian".
- ^ "The Oldest Bdelloid Rotifera from Early Permian sediments of Chamba Valley: A New Discovery". International Journal of Geology, Earth and Environmental Science.
- ^ a b Bourne, A.G. (1907). Baynes, Spencer and W. Robertson Smith (ed.). Encyclopædia Britannica. Vol. XXI (Ninth ed.). Akron, Ohio: The Werner Company. p. 8.
- ISBN 978-0-632-04761-1, p. 98
- ^ Baqai, Aisha; Guruswamy, Vivek; Liu, Janie & Rizki, Gizem (1 May 2000). "Introduction to the Rotifera". University of California Museum of Paleontology. Retrieved 27 July 2008.
- ^ Shimek, Ronald (January 2006). "Nano-Animals, Part I: Rotifers". Reefkeeping.com. Retrieved 27 July 2008.
- ISBN 978-0-03-025982-1, p. 788ff. – see particularly p. 804
- ISBN 978-0-07-234899-6.
- ^ ISBN 978-0-03-056747-6.
- ^ a b Robert Lee Wallace. "Rotifers: Exquisite Metazoans". 2002. quote: "What is the function of the retrocerebral organ?"
- ^ Fontaneto, D., & De Smet, W. H. (2015). Rotifera, chapter 4 Handbook of Zoology, Gastrotricha, Cycloneuralia and Gnathifera, Volume 3, Gastrotricha and Gnathifera Schmidt-Rhaesa, Andreas.
- ^ a b c d 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.
- PMID 14747655.
- ^ a b c Nogrady, T., Wallace, R.L., Snell, T.W., 1993. Rotifera vol.1: biology, ecology and systematics. Guides to the identification of the microinvertebrates of the continental waters of the world 4. SPB Academic Publishing bv, The Hague.
- PMID 33339818.
- PMID 20862222.
- PMID 564567.
- ^ Clément, P.; Wurdak, E. (1991). "Rotifera". In Harrison, F.W.; Ruppert, E.E. (eds.). Microscopic Anatomy of Invertebrates. Aschelminthes, vol. IV. Wiley-Liss. pp. 219–97.
- .
- ISBN 978-94-010-3820-1.
- .
- S2CID 235365588.
- ^ PMID 25105197.
- ^ Wallace, R.L., T.W. Snell, C. Ricci & T. Nogrady (2006). Rotifera Vol. 1: Biology, ecology and systematics. Guides to the identification of the microinvertebrates of the continental waters of the world 23, 299 pp. Kenobi, Ghent/Backhuys, Leiden
- PMID 23873043.
- PMID 34764494.
- PMID 21473744.
External links
- Jersabek, C. D. & Leitner, M. F. (2013): The Rotifer World Catalog. World Wide Web electronic publication.
- Introduction to the Rotifera
- Rotifers of Germany and Neighbouring Countries (Website with high-quality photos)
- Rotifers
- Tree of Life Web Project
- Rotifer Videos
- Detailed description of Rotifers
- The Rotifers, by Robert Abernathy, on Project Gutenberg
- Encyclopædia Britannica (11th ed.). 1911. .