|Rotifers (Wheel Animalicules)
Rotifera (Rotifers, Wheel Animalcules)
The rotifers are aquatic, microscopic Aschelminthes with an anterior ciliary organ (corona) or funnel. They have a
pharynx equipped with internal jaws and 1 pair of flame-bulb protonephridia. There are about 1500 known species.
Rotifers are 0.04 to 2 mm long with most below 0.5 mm. The anterior end is broad or narrowed and is usually equipped
with a ciliary apparatus. The trunk is elongated and often enlarged. There is a slender terminal posterior tail or foot.
Some rotifers are sacciform (sac-shaped) (e.g. Asplanchna) or spherical (Trochosphaera), while some are wide and
flattened (e.g. Brachionus, Euchlaris) and others are long and slender with a lengthened foot (e.g. Rotaria) or a
lengthened neck (e.g. Seisonacea).
The trunk is cylindrical and either circular in cross-section or with flattened ventral and lateral surfaces, which may be
concave. Rotifers exhibit more-or-less bilateral symmetry and may or may not be ventrally curved. There may be
asymmetries in the toes or the body. In the family Trichoceridae, the body is spirally twisted to the left.
The cuticle is yellowish and often ringed (pseudosegmentation) and may be thickened, especially on the trunk, to form
a lorica case made-up of one or more plates. The anterior end (or ‘head’) bears the mouth, the corona and various
projections. It is either, broad and truncate or slightly convex. The anterior end has a central apical field encircled by
the lobulated corona. The apical field may bear projections such as outlets for the retrocerebral organ ducts and
sensory bristles or hairs, etc.
In bdelloid rotifers the corona is partially subdivided into retractile trochal discs. One pair of auricles may be present
on the anterior end of rotifers, as in Notommata. In Synchaeta these auricles are retractable. The rostrum is a
middorsal projection of the head, found in bdelloids and many notommatids. The tip of the rostrum possesses cilia and
sensory bristles and may be protected by 2 thin cuticular plates called rostral lamellae. In bdelloids the rostrum is used
as the anterior end in looping leech-like locomotion (with the trochal discs retracted).
The eyes are pigment-spot ocelli – single or paired red flecks, situated in the brain or as a lateral pair in or near to the
corona or as a frontal pair on the apical field or rostrum. The eyes may be elevated on papillae or sunk into
The mouth is situated in the corona, in the midventral line of the head. Often a coronal protrusion serves as a lower
lip. In the Collothecacea the mouth is located at the bottom of the funnel-shaped anterior end (this funnel is an
expansion of the corona).
The trunk may bear non-movable lorical spines, as in Brachionus, Keratella and Notholca. These spines are longest in
Notholca longispina, which is a pelagic species. Long movable spines are used in skipping locomotion in the Filiniidae
and Polyarthra (a synchaetid). There are 6 skipping blades on each side of the trunk. Pedalia has body extensions
tipped with setose bristles. Some bdelloids, especially those subject to desiccation and freezing, such as moss-
dwellers and those in Polar Regions, possess cuticular tubercles and spines.
A single or paired dorsal antenna is situated on the middorsal line of the anterior end of the trunk either as a finger-
like projection tipped with sensory hairs or as a bristly tuft or single bristle. Lateral antennae, lacking in the Bdelloidea
and Seisonacea, are situated on either side of the posterior part of the trunk. The lateral antennae are situated more
anteriorly in many sessile forms.
The anus is located on the middorsal line at or near the trunk/foot boundary. It is elevated on a papilla in the
Flosculariacea. In the Notommatidae the anus is often overhung by a caudal projection.
The cuticle of the foot is commonly ringed with few to many joints. It is used to cling to objects in creeping types, in
which it is positioned axially. Alternatively, the foot is used as a rudder in swimming types, in which case it is offset
ventrally. In sessile forms the foot forms a long stalk. In some rotifers the foot is reduced or absent, especially in
The foot may terminate in an adhesive disc, as in the Seisonacea, or a flat attachment surface in sessile forms or else
there are commonly 1-4 movable projections or toes at or near the foot’s end. The toes are either short/conical as in
bdelloids or slender/spine-like. In the Trichoceridae, which have spiral asymmetry, the right toe is reduced or lost.
Pedal glands commonly open at the tips of the toes. The foot may also bear styles or spurs, etc.
Rotifers are dioecious and in most the males are greatly reduced. In the Seisonacea and some Ploima, however, the
males are similar to the females. There are no males in the Bdelloidea, only parthenogenetic females. Male rotifers
only appear at certain seasons and for brief intervals.
The cuticle is transparent and colourless or yellowish. The digestive tract is usually coloured brown, orange or red,
especially in alpine and Polar Regions.
The metachronous beating of cilia on the corona gives the illusion that the corona is a rotating wheel and the corona
is sometimes called “wheel organ”. Rotifer literally means ‘wheel bearer’ (L. rota: a wheel, ferre: to bear). The corona
possibly evolved from a ventral ciliated region near the mouth, which was used for creeping. In Dicranophorus, the
corona is used in creeping locomotion.
The corona consists of a large oval ventral buccal field around the mouth and a circumapical band extending from the
buccal field and encircling the margin of the head. The circumapical band is absent in Dicranophorus. The buccal field
is reduced in swimming forms, but some of the cilia may remain as stiff sensory cirri.
The circumapical band may have enlarged marginal cilia and may give out a pair of lateral auricles. These lateral
auricles are used in swimming and retract during creeping locomotion. In some rotifers the margins of the apical band
are modified into an inner anterior circlet or membranelle, the trochus, and an outer posterior circlet, the cingulum.
These two circlets incorporate the buccal field so that the whole corona consists of two circlets or membranelles with a
ventral mouth between them.
The corona of Pedalia and Testudinella consists of a band encircling the head. In many other rotifers the corona is
expanded into circular, oval or lobed discs for food gathering, and then there is a groove between the trochus and the
cingulum, which carries microscopic food to the mouth.
In the Bdelloidea the apical field, enclosed by the trochus, subdivides into two trochal discs raised on pedicels. The
cingulum encircles the base of both pedicels and passes below the mouth. In this corona type the mouth is outside the
In the sessile Flosculariacea, the mouth is also outside the disc. In these rotifers the apical field comprises a round,
oval or lobed disc edged by the two circlets of the trochus and cingulum, of which the trochus is the largest. This type
of corona is used in feeding and is non-locomotory.
The trochal discs are employed in swimming and feeding and are folded up and retracted during creeping locomotion.
In the Collothecacea the corona is extended into 1,2,3,4,5, or 7 lobes that form an anterior funnel with the mouth at
the centre and bottom of the funnel. This functions as a trap to catch small organisms.
In exclusive swimmers, like Asplanchna and Synchaeta, the circumapical band is reduced to a single girdle of
membranelles often broken into arcs and tufts. The buccal field is also greatly reduced.
Corona cilia are often compounded into cirri, membranelles or styles, etc. The corona of male rotifers is usually less
modified than that of females. However, adult males of Atrochus, Cupelopagis and Acyclus have no corona.
The body wall consists of cuticle, epidermis and subepidermal muscles. The cuticle is non-chitinous (and is possibly
made of scleroproteins (?)). The cuticle is often annular, especially in forms that move by creeping or looping. These
annulations provide flexibility and accompany a telescopic head joint and a neck (cervical) joint, etc. The cuticle is
usually non-annulated in exclusive swimmers.
The cuticle may have polygonal patterns, longitudinal or transverse striations, grooves, or ridges and may bear
spines or tubercles, etc. The cuticle may be hardened into a lorica, which is slightly flexible. Such a lorica consists of
several pieces or single dorsal and ventral plates or else is a single piece with or without a longitudinal suture.
Loricate forms are often laterally or dorsoventrally flattened and the margins may project as teeth or spines. A head
shield may be demarcated from the trunk lorica by a groove. The head shield may form a hood covering the entire
dorsal surface of the head, as in Lepadella.
The epidermis is a syncytium containing scattered nuclei. The nuclei are bilaterally arranged and have constant
positions and numbers within each species. The epidermal cushions are projections into the pseudocoel containing
one or more nuclei and basal bodies and roots of coronal cilia (this epidermal part of the corona is called the coronal
Some pelagic forms are enclosed in a loose gelatinous capsule. In the sessile Flosculariacea and Collothecacea, this
forms a tube into which the animal can withdraw. This gelatinous material is possibly a secretion of the foot epidermis
or the pedal glands (?). The tubes harden and incorporate foreign bodies or definite pellets of particles formed in a
special ciliated pit under the pouch-like lower lip. These pellets are arranged in a regular array. Other
flosculariaceans use faecal balls or sand grains in a similar way.
In the Seisonacea some epidermal glands occur in the neck region. In the Flosculariacea the pits that form pellets for
the tube produce a sticky secretion. Other epidermal glands are rare.
The retrocerebral organ is a gland situated above and behind the brain. The organ consists of either one or both of a
retrocerebral sac and a pair of lateral subcerebral glands. Both the sac and the glands consist of syncytium. The duct
of the sac (accompanied by the gland outlets) forks and opens on the apical field via pores borne on a single papilla
or a pair of papillae. The function of the retrocerebral organ is unknown.
The pedal glands are unicellular or multinucleate syncytia in the foot. The foot may contain up to 30 (1-15 pairs) of
these glands and several types may be present. The pedal glands are reduced or absent in the adults of sessile
forms. The pedal glands open by ducts on the tips of the toes or at the sides or bases of the toes or on spurs or on
the end of the foot. One or more reservoirs may be present. These glands produce an adhesive secretion used in
permanent attachment, creeping, and construction of tubes and cases.
The muscles of rotifers may be smooth and/or striated. There are no definite subepidermal layers, but rather an array
of single muscles. Cutaneovisceral muscles extend from the body wall to the viscera, especially the digestive tract.
Visceral muscles occur in the walls of viscera.
In the Order Ploima, the body wall circular musculature is best developed and causes body extension. In this order
this musculature usually comprises 3-7 widely spaced bands beneath the epidermis. These form complete or
incomplete rings. Incomplete rings are ventral with short lateral arcs.
The circular fibres are anucleate and are cytoplasmic extensions of the epidermal syncytium. The circular muscle in
the neck and trunk is reduced in loricate forms. In flattened loricate forms, however, circular fibres form dorsoventral
bands (by becoming free from the epidermis except at their ends) that serve to bring the lorica plates together. The
coronal sphincter is made of 1-7 circular muscle bands (which may be cross-connected) that close the neck over the
retracted corona. Some forms also possess a pedal sphincter of circular muscle at the junction of trunk and foot.
Circular muscle also gives rise to muscles operating the external movable parts, such as the arms of Pedalia.
The longitudinal body wall muscles in the order Ploima comprise several muscle groups:
- Bands, beneath the circular bands, running the length of the body and attached to the epidermis at intervals.
- Head and foot retractors. The head retractors consist of central, dorsal and ventral pairs. The lateral is often
subdivided into superior, median and inferior bands. The central retractors extend from the dorsal trunk wall to
the apical or buccal field. The dorsal retractors extend from the corona to the end of the foot or else are divided
into separate head and foot retractors. In the latter case, the dorsal head retractors extend from the dorsal
trunk wall to dorsally either side of the circumapical band, whilst the dorsal foot retractors extend from the
posterior dorsal trunk wall to the distal end of the foot. The lateral retractors extend from the sides of the trunk
to the lateral regions of the head and corona or sometimes extend into the posterior trunk or foot. The ventral
retractors extend from the distal end of the foot to the buccal field and the ventral region of the head.
- Smaller retractors, such as the auricle retractors, dorsal antenna retractors, etc.
These longitudinal retractors have one or more nuclei. They cause the head, corona and foot to be retracted
accompanied by telescoping of the cuticular rings into each other or else accompanied by a spreading apart of the
lorica plates. The muscle systems of other orders, besides the Ploima, are less well known. Sessile forms have long
retractors that retract the anterior end into the tube. Swimming forms have modified ring muscles that form
dorsoventral bands that operate the appendages.
The rotifer pseudocoel is a spacious cavity with no epithelial lining and no mesenteries. It is of blastocoel origin and is
fluid-filled. The pseudocoel contains a loose network of branched amoeboid cells (of ectodermal origin) that
anastomose to form a syncytium.
Some rotifers are sessile, attached by their foot to the substrate or living in a secreted tube. Most rotifers, however,
are motile. Some are exclusive swimmers, swimming constantly by means of their corona cilia that force water
posteriorly along the body and driving the rotifer forward, often in a spiral path. Others are semi-sessile, temporarily
attaching via their foot and intermittently swimming and/or creeping. Some rotifers crawl in an inch-worm manner,
attaching their posterior end to the substratum by pedal gland secretion, stretching forward, then attaching the
anterior end to the substratum and then releasing the foot and drawing the rear end forward. In Dicranophorus, the
corona cilia are used in creeping locomotion
The CNS consists of a bilobed brain (cerebral ganglion) dorsal to the mastax. The brain gives out sensory and
motor nerves and two main ventral nerve cords. Paired sensory nerves innervate the eyes, sensory bristles,
apical pits, rostrum and dorsal antennae. Motor nerves from the brain innervate the dorsal, lateral and central
retractors and the salivary glands. Pharyngeal nerves innervate the mastax where they form a plexus on each side
of the mastax wall. This plexus connects to the unpaired mastax ganglion in the mid-ventral wall of the mastax. A
visceral nerve extends from the mastax ganglion to the digestive tract on each side.
The brain or pharyngeal nerves give out one pair of epipharyngeal nerves that connect to a small epipharyngeal
ganglion on each side of the epipharynx. The brain also gives out the pair of lateroventral ganglionated nerve cords
that extend to the foot. Near the brain these cords bear an anterior ganglion and further posterior a geniculate
The geniculate ganglion (or the ventral nerve cords) gives out one pair of lateral sensory nerves to the lateral
antennae and a scalar nerve that innervates the trunk muscle rings. The scalar nerve has one ganglion cell for each
muscle band. The anterior parts of the ventral cords or the geniculate ganglion also send nerves to the longitudinal
retractors. The geniculate ganglion, or anterior regions of the cords, also sends out a nerve to the coronal sphincter.
The cords terminate posteriorly in the vesicular ganglion (supplying the urinary bladder) and the caudal or pedal
ganglion (supplying the foot). These ganglia may fuse to form a caudovesicular ganglion. In Asplanchna two dorsal
ganglionated cords take the place of the scalar nerves.
The anterior end is equipped with sensory membranelles or styles, ciliated pits and papillae, etc. The styles
(membranelles) are stiff tactile bristles along the edge of the circumapical band and apical field. They may also occur
near the mouth. The paired ciliary pits are thought to be chemoreceptors (?) and may also occur on the apical field.
Conical or finger-like palps, sometimes with hairs on their tips, may also occur on the apical field. The papillae
carrying the exits of the retrocerebral organ may also bear sensory structures.
Red pigment spot ocelli are common in the Ploima, Bdelloidea and the free stages of sessile orders. There is usually
a single ocellus, the cerebral eye in the dorsal or ventral surface of the brain. In some rotifers 1-2 pairs of ocelli are
present. Some have 1-2 lateral eyes on each side of the corona and some have one pair of apical eyes on the apical
field, often on the rostrum in Bdelloidea. Cerebral and apical or cerebral and lateral eyes may both be present. The
cerebral eye is a single cell, resembling a brain cell. The lateral and apical eyes are epidermal cushions with one or
more nuclei with red pigment in the optic cell or epidermal cushion and sometimes have a lens-like body.
The dorsal antenna or tentacle is a movable papilla or finger-like projection with one or more tufts of sensory hairs on
its tip. Alternatively, the dorsal antenna may be a tuft of sensory hairs or 1-2 styles, or styles encircled by sensory
hairs projecting through an opening in the cuticle or lorica. The dorsal antenna may be reduced to an unciliated pit.
The paired lateral antennae, absent in the Seisonacea and Bdelloidea, are situated from near the mouth to the end of
the trunk. They may be shifted dorsally or ventrally and exhibit the same range of structure as the dorsal antenna, but
are equipped with only one sensory cell each.
The caudal antenna is found in the Ploima, above the base of the toes as a hairless papilla or a tuft of bristles or a pit.
A supra-anal sense organ may also be present. A pharyngeal sense organ is found in the Collothecacea as a tuft of
sensory hairs on the diaphragm.
Four forms of feeding are employed by the different species of rotifer.
- Many benthic rotifers are current producers and feed off protozoans etc.
- Some creeping or pelagic rotifers are grasping carnivores with protrusible forceps-type trophi.
- The Collothecacea are trapping carnivores equipped with a coronal funnel lacking motile cilia but equipped
with long immotile bristles. The prey animal enters the outspread funnel and then the lobes/arms close over it
and the bristles prevent its escape.
- Atrochus creeps about ingesting algae gathered in by the coronal tentacles.
The mouth is ventral and situated on the head. It is rounded, slit-like or triangular. The cingulum below the mouth
may form a definite lower lip. In the Collothecacea the coronal lobes form a funnel divided into the outer
infundibulum and the inner vestibule, by a ridge (diaphragm) bearing pharyngeal sensory tufts. Contraction of the
diaphragm traps the prey in the vestibule.
The mouth may be closable by a spincter muscle. Two pairs of dilator muscles may open the mouth and are
especially characteristic of raptorial feeders. The mouth opens into the buccal tube (a buccal field invagination?),
especially in ciliary feeders. The buccal tube leads into the pharynx. The buccal tube may be absent, in which case
the mouth opens directly into the pharynx.
The pharynx or mastax is highly muscular and rounded, trilobed or elongated. The inner pharyngeal wall forms the
masticatory apparatus. This masticatory apparatus is formed from a set of hard cuticularised pieces called the
trophi. The rotifer mastax is a remarkable, efficient micro-machine made up of some 50 or so mechanical parts
(skeletal parts and muscles) and yet it is microscopic. There are 7 main pieces: the unpaired fulcrum, paired rami,
paired unci and paired manubria. The fulcrum and rami together form the incus, whilst the unci and manubria form
the malleus. The rami extend from the fulcrum plate. Additional subunci may be present as small pieces between
the unci and rami. The right and left pieces may differ in size and shape and interlock when used. The caudae are
the pointed rear ends of the manubria. There are eight principle types of trophi / mastax:
Malleate Type. In the malleate type of trophi the unci bear teeth for chewing and grasping prey, as in Brachionus and
Epiphanes, for example. The submalleate type is a modified malleate and is found in Euchlaris.
Virgate Type. The fulcrum and manubria are elongated and rod-like. The rami are broad, triangular plates that
support the piston and the unci bear 1-2 teeth. The prey is grasped by the unci and its contents sucked in by the
action of the piston or hypopharynx, which is a muscular mass in the centre of the mastax and is operated by muscles
attached to the posterior of the fulcrum. The epipharynx consists of one or more cuticular plates that stiffen the
anterior dorsal wall of the virgate trophi. This type of trophi occurs in pelagic raptorial rotifers, e.g. many
notommatids, Trichocercidae, Gastropodidae and Synchaetidae.
Cardate Type. This trophi is used in sucking and is found in Lindia, for example. Suction results from movements of
the unci and there is no piston. The epipharynx is present and the manubria are forked.
Forcipate Type. This type of trophi is protruded from the mouth to grasp prey, as in Dicranophorus, for example. The
curved rami and the fulcrum form a forceps-like structure with sharp tips. The pieces are slender and elongated.
Incudate Type. This type of trophi is forceps-like with rudimentary manubria. It is stouter than the forcipate type and
can be protruded from the mouth and rotates 90-180 degrees as it does so. It is used to grasp prey and is found in
the asplanchnids, for example.
Ramate Type. This type of trophi occurs in the Bdelloidea. It is stout with reduced fulcrum and reduced manubria.
The unci are large and plate-like and bear several parallel ridges that grind against each other in a masticatory
function. A variation called the malleoramate type occurs in the Flosculariacea.
Uncinate Type. The fulcrum and manubria are reduced. The unci bear a few teeth. The rami are stout and the
subunci are large. This type occurs in the Collothecacea that ingest large prey. The mastax is sacciform and
voluminous and the trophi play a minor role.
Fulcrate Type. This type of trophi occurs in the Seisonacea and consists of a median elongated piece (the fulcrum?)
and one pair of leaf-like manubria attached to the anterior end, and several additional small pieces
The mastax lumen is lined by cuticle and is ciliated only in current-feeding forms (that possess a ramate mastax). The
epithelium is wholly or partly syncytial and secretes the trophi. The mastax muscles appear to be cytoplasmic
extensions of the epithelium.
Muscles extend between the pieces of the trophi and from the trophi to soft tissues. The various muscles and their
functions are listed below:
- The adductor rami, situated between the rami, close the rami.
- The abductor rami (fulcroscapalis) open the rami.
- The fulcro-manubricus muscles connect the fulcrum to each manubrium.
- The ramo-manubricus extends from the ramus to the cauda of the manubrium.
- The flexor mallei extend from the cauda to the unci and cause the unci to strike against the rami.
- The uncicus connects the uncus to both ends of the manubrium and rubs the uncal teeth on the ramus.
- The adductor mallei are continuous bands in the pharynx wall between the caudae of the two manubria and
cause the chewing action of the unci.
- The lateralis manubrii assist the adductor mallei.
- The abductors / adductors of the cauda.
- The fulcro-oralis extends from the fulcrum to the buccal-field and adjacent areas.
- The fulcro-mucosus connects the fulcrum to the pharynx wall (between the rami) and acts as the piston
muscle in the virgate-type mastax.
- The fulcro-oesophagus connects the fulcrum to the oesophagus.
- One or more bands of the scapalis connect the fulcrum to the pharynx walls.
All the muscles 1-13 are paired, cross-striated and mostly anucleate. Additionally, cutaneo-pharyngeal muscles
extend between the mastax and the body wall. These include the dorsopharyngeus, which connects the mastax to the
dorsal body wall, and the retractor pharyngeus that extends from the pharynx to the ventral body wall (and also has
an extension to the corona in Euchlaris).
One pair of protonephridia with flame bulbs opens into the urinary bladder (‘cloacal bladder’). The flame bulbs are
anucleate and fused cilia comprise the ‘flames’. The tubules extend the length of the animal, one on each side.
There are usually 2-8 flame bulbs on each side, but there are as many as 50 in the asplanchnids. Each flame bulb
opens into a ciliated capillary that opens into the main tubule.
Some rotifers possess a transverse capillary between the anterior parts of the main tubules, called Huxley’s
anastomosis, which may receive additional flame bulbs.
The bulb cap gives out protoplasmic filaments that anchor it to the body wall. Instead, in the asplanchnids, external
flagella beat in the pseudocoel. Additional driving cilia may be present in the tubules.
The bladder contracts 1-4 times per minute. The protonephridia are also involved in osmoregulation.
Sexual dimorphism. In the Ploima, the male may be reduced to about an eighth to one-half the size of the female and
may occur seasonally and may lack an anus, no urinary bladder and no cloaca. The male gut is reduced. The males
are often a different shape with differing lorica and corona.
In the Flosculariacea and the Collothecacea, the free-swimming male is 10% or less the size of the female and
possesses a simple ciliated anterior end plate. In pelagic and sessile rotifers the male is reduced. In the Seisonacea,
the male is smaller but has the same level of organisation as the female.
The male may come from smaller eggs and may not grow after hatching and often resembles a juvenile female.
There are no males at all in the Bdelloidea and the females are parthenogenetic.
Female system. The female usually possesses a single syncytial ovary and a syncytial vitellarium in a common
membrane that is connected to the cloaca via an oviduct. In the Bdelloidea the germovitellaria and oviducts are
paired. The Seisonacea have paired ovaries but no vitellaria.
Male system. The male rotifer usually has one large sacciform testis that is connected to the genital pore via a
ciliated sperm duct. One or more pairs of prostatic glands supply the sperm duct. The posterior genital-pore end of
the sperm duct is a protrusible cirrus that may have a cuticular lining. Alternatively the sperm duct may end in a
cuticular protrusible tube or penis or the body wall may form a copulatory tube. The Seisonacea have one pair of
testes and no copulatory apparatus and produce spermatophores.
In some rotifers the copulatory organ is inserted into the female cloaca. However, in most rotifers impregnation is
hypodermic and the sperm enter the pseudocoel. There are two types of sperm present together. One type has a
large rounded or oval head and an undulating membrane for a tail. The second type are rod-shaped bodies that
possibly aid cuticle penetration during insemination.
A feeding tube connects each egg to the vitellarium. The eggs are laid on the substratum or stuck to the female body
or to other animals or else they float. Some species are viviparous and have no eggshells. The maximum number of
eggs that can be produced is determined by the number of nuclei in the ovary at birth, and is typically from 10 to 50.
The egg has a shell (and may also have egg membranes) and may have a membranous or gelatinous float.
In the Monogononta three types of eggs are produced: amictic eggs that cannot be fertilised and develop
parthenogenetically into females; mictic eggs, which if not fertilised develop into males, and dormant eggs, which are
fertilised mictic eggs and develop into amictic females. A given female is either mictic or amictic.
The eggs may have a furrow along which the shell springs open upon hatching, within about a day. Dormant eggs
take weeks or months to hatch and are desiccation and cold resistant.
Determinate spiral cleavage of the egg produces a stereoblastula. The hatchling females are free-swimming with
adult form and reach sexual maturity within a few days. Males are often mature at birth. In sessile rotifers, the free-
swimming juvenile hatchlings or larvae attach to the substrate and the foot elongates into a stalk and the coronal
lobes develop, or are lost in some genera.
Rotifers often have no regenerative capacity. Some young rotifers exhibit wound healing, especially in anucleate
regions such as the coronal lobes. This healing often takes place at the expense of growth and egg production. The
nuclear number is constant at about 900-1000 nuclei per rotifer.
O. Seisonacea: marine, epizoic rotifers.
O. Bdelloidea: the fresh-water bdellid rotifers, typically benthic.
SO. Ploima: includes the majority of rotifers, benthic fresh-water or pelagic fresh-water.
SO. Flosculariacea: sessile and fresh-water free-swimming rotifers.
SO. Collothecacea: mostly sessile.
Most rotifers are fresh-water, but some occur in brackish water and some are oceanic (mostly littoral, but some down
to 400 m depth) and some live in damp terrestrial habitats. Rotifers are either carnivorous or bacteria feeders. There
are creeping, swimming, pelagic and sessile types.
The sessile, and some pelagic, rotifers are commonly enclosed in gelatinous or cuticular tubes or envelopes, which
may incorporate foreign bodies. Some rotifers are epizoic and some are parasitic. Most rotifers are solitary, but
some sessile types form spherical swimming colonies with no organic connections between the individuals.
About 5000 free-swimming rotifers per liter have been estimated to live in weedy ponds. Large numbers also occur
in the water-film covering sand grains just above the water’s edge of lakes (the psammolittoral habitat). It is
estimated that 10 cm3 of beach sand (with 2-3 cm3 water) contains about 4x106 bacteria, 104 protozoans and 400
rotifers (in addition to tardigrages, nematodes, copepods and oligochaetes).
Some 25 000 semisessile rotifers have been estimated on one sprig of Utricularia (bladderwort) in one liter of water.
The motile young of rotifers select plants for attachment and the young of different species tend to prefer different
plant species. Some also prefer certain locations on the plant body. Populations of sessile rotifers peak in summer
and overwinter as dormant eggs.
Pelagic and limnetic rotifers are estimated at ~1000/liter in lakes and large ponds. They swim continuously, by
means of their locomotory corona, and capture prey with the corona and/or with mastax protrusion. These rotifers
are transparent and sacciform or stout with the foot reduced, lost or displaced ventrally. Several mechanisms
provide buoyancy: oil droplets, long projecting spines or a jelly hull. The eggs are carried by the mother or fastened
to other plankton, including other rotifers, or the eggs float with the aid of spines, air cells or jelly hulls.
Some planktonic Ploima exhibit form variations depending on season (cyclomorphoses). They enlarge their surface
area during summer, by body spine elongation, to increase bouyancy (since the specific weight of water decreases
as temperature increases). In Keratella cochlearies, Keratella aculeata and Asplanchna, long-spined forms hatch
from dormant eggs and the spines shorten with each parthenogenetic generation and may disappear altogether.
Terrestrial rotifers (mostly bdelloids) inhabit moss/lichen cushions and are active when moist. These rotifers are
often spiny or warty and often coloured red (especially in alpine regions) due to the presence of carotenoids in the
stomach and germovitellarium. They may survive for up to 3-4 years in a desiccated state. Some encyst, whilst
others contract and shrivel. They survive desiccation better if they are well fed and dried slowly. When water is
added they swell, unfold and activate within minutes to hours.
Epizoic and parasitic rotifers have reduced heads, corona, sense organs and mastax and show reduced activity. The
foot functions as an adhesive organ.
Many species of rotifer are cosmopolitan. Their dormant eggs and desiccated states are blown about or carried by
Bibliography / References
- L. H. Hyman, 1951. The Invertebrates vol. III: Acanthocephala, Aschelminthes and Entoprocta. McGraw-Hill
A 3D computer model of a rotifer. The two ciliated semicircular lobes around the mouth
look as if they are spinning round under the microscope - an illusion created by the
beating cilia which drive currents of food-laden water into the mouth. This gave them their
name (rotifer = 'wheel-bearer'). This model is of a thresh water bedelloid rotifer of the
genus Philodina. The organs have been coloured for clariety. Rotifers are generally
transparent and colourless, though some, like Philodina roseola have orange/red
carotenoids to colour their stomach and germovitellaria and the gut may also be coloured
by algae the animal has ingested. The toe 'toes' on the tail are adhesive organs that can
anchor the animal to the substrate. Rotifers come in a bewildering diversity of forms and
make beautiful specimens for microscopy. Being transparent, their internal moving parts
can also be directly observed.
There may be 2-7 ‘salivary’ glands in the mastax wall. These are uninucleate or syncytial masses with ducts, and
sometimes a reservoir, which open anterior to the trophi or in the buccal tube or near to the mouth. These possibly
function in ingestion and/or digestion.
The mastax leads into a short or long aglandular oesophagus, which may be cuticle lined or ciliated. The
oesophagus has a syncytial wall supplied with muscle fibres and there may be a sphincter at the entrance to the
stomach, to which the oesophagus connects.
The stomach and intestine may not be distinctly demarcated and then form a stomach-intestine. Otherwise, the
stomach is an enlarged thick-walled sac or tube (U-shaped in the Synchaetidae). The wall of the stomach is either
composed of a definite number (30-45) of large cells that may be ciliated or else is a syncytial lining.
One pair (occasionally more) of gastric glands empty into the stomach at the oesophageal-stomach junction. These
glands are composed of a syncytium with a constant number of nuclei and open into the stomach via a simple pore
on each side and sometimes via a pair of intervening tubular ducts. These possibly secrete enzymes (?).
The intestine may not be distinctly delineated from the stomach, or else a pyloric sphincter demarcates the two. The
intestine is tubular or bladder-like and composed of a thin syncytium, which is sometimes ciliated, and is supplied by
muscle fibres continuous with those of the stomach wall.
The protonephridial tubules and oviducts open into the end of the intestine into the cloaca chamber, which functions
as a urinary bladder in some rotifers. The cutaneo-visceral muscles attach the stomach and intestine to the body
wall. One or more pairs of cutaneo-gastric muscles connect the stomach to the lateral body wall, whilst cutaneo-
intestinal muscles connect the intestine to the body wall. The cloaca is equipped with dilator muscles.
The anus and cloaca are absent in some pelagic forms, for example Asplanchna, Ascomorpha and Chromogaster,
which suck fluid from their prey and ingest no food waste for egestion. Digestion in rotifers occurs in the stomach and
is both intra- and extracellular.