The Rotifers by OtGO 2020 - 2023 Berlin
Studio, mixed media, acryl on canvas consists
of 40 equal-sized single
paintings, each measuring 20 by 15 cm
rotifers (/ˈroʊtɪfərz/, from the Latin rota, "wheel", and -fer,
"bearing"), commonly called wheel animals or wheel animalcules, make up
a phylum (Rotifera /roʊˈtɪfərə/) of microscopic and near-microscopic
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 0.1–0.5 mm (0.0039–0.0197 in) long (although their size can
range from 50 μm (0.0020 in) to over 2 mm (0.079 in)), and are common
in freshwater environments throughout the world with a few saltwater
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. 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.
Taxonomy and naming
Rev. 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
Die Infusionsthierchen als vollkommene Organismen in 1838 that the
rotifers were recognized as being multicellular animals.
species of rotifers have 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 three species of Seisonidea.
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.
strictly speaking, are confined to the Bdelloidea and the Monogononta.
Rotifera, Acanthocephala and Seisonida make up a clade called
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 (though the organ does not actually rotate).
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.
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.
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.
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.
Rotifers have a
small 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. Close
to the brain lies a retrocerebral organ, consisting of two glands
either side of a medial sac. The sac drains into a duct that divides
into two before opening through pores on the uppermost part of the
head. The function of the retrocerebral organ is unclear.
The nervous system comprises about 25% of the roughly 1,000 cells in a rotifer.
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.
The coronal cilia pull the animal, when unattached, through the water.
Like many other
microscopic animals, adult rotifers frequently exhibit eutely—they
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.
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
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
reproductive system consists of one or two ovaries, each with a
vitellarium gland that supplies the eggs with yolk. Together, each
ovary and vitellarium form a single syncitial structure in the anterior
part of the animal, opening through an oviduct into the cloaca.
Males do not
usually have a functional digestive system, and are therefore
short-lived, often being sexually fertile at birth. They have a single
testicle and sperm duct, associated with a pair of glandular structures
referred to as prostates (unrelated to the vertebrate prostate). The
sperm duct opens into a gonopore at the posterior end of the animal,
which is usually modified to form a penis. The gonopore is homologous
to the cloaca of females, but in most species has no connection to the
vestigial digestive system, which lacks an anus.
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"). 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. 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.
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 Rotaria, are ovoviviparous, retaining the eggs
inside their body until they hatch.
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
The life span of monogonont females varies from two days to about three weeks
Loss of sexual reproduction system
asexuals': Bdelloid rotifers are assumed to have reproduced without sex
for many millions of years. Males are absent within the species, and
females reproduce only by parthenogenesis.
However, a new
study provided evidence for interindividual genetic exchange and
recombination in Adineta vaga, a species previously thought to be
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
enclose an embryo encysted in a three layered shell that protects it
from external stressors. They are able to remain dormant for several
decades and can resist adverse periods (e.g., pond desiccation or
presence of antagonists). 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.
females cannot produce resting eggs, but many can survive prolonged
periods of adverse conditions after desiccation. This facility is
termed anhydrobiosis, and organisms with these capabilities are termed
anhydrobionts. Under drought conditions, bdelloid rotifers contract
into an inert form and lose almost all body water; when rehydrated they
resume activity within a few hours. Bdelloids can survive the dry state
for long periods, with the longest well-documented dormancy being nine
years. While in other anhydrobionts, such as the brine shrimp, this
desiccation tolerance is thought to be linked to the production of
trehalose, a non-reducing disaccharide (sugar), bdelloids apparently
cannot synthesise trehalose. In bdelloids, a major cause of the
resistance to desiccation, as well as resistance to ionizing radiation,
is a highly efficient mechanism for repairing the DNA double-strand
breaks induced by these agents. This repair mechanism likely involves
mitotic recombination between homologous DNA regions.
prey to many animals, such as copepods, fish (e.g. herring, salmon),
bryozoa, comb jellies, jellyfish, starfish, and tardigrades.
The genome size
of a bdelloid rotifer, Adineta vaga, was reported to be around 244 Mb.
The genomes of Monogononts seem to be significantly smaller than those
of Bdelloids. In Monogononta the nuclear DNA content (2C) in eight
different species of four different genera ranged almost fourfold, from
0.12 to 0.46 pg. Haploid "1C" genome sizes in Brachionus species range
at least from 0.056 to 0.416 pg