all about Rotifers

1. ZOO 211 Lower Invertebrates
Phylum Rotifera: General
characteristics, biology and
phylogeny
Mr Ibrahim A. G
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2. Phylum Rotifera
• Rotifera – Latin (rota-wheel, fera-
those that bear ) i.e“to bear a wheel”.
• Rotifers have an anterior corona.
• Corona –
– a ciliated organ
– appears to rotate like a wheel
– For locomotion & feeding
• There are about 2000 species
of Rotifers
They range from 40µm to 3mm, but most are between 100 and 500µm long
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Rotifer body
• Head
– Corona with cilia
– Mouth
– Sensory organs
– brain
• Trunk
– Largest part, varies from long to spherical or sac-like
– Contains organs: mastax, salivary glands, pseudocoelom,
protonephridia, stomach, intestine, anus
• Foot
– Locomotion, attachment
– Pedal glands, one to two toes
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5. Characteristics
• Bilaterally symmetrical.
• Body has more than two cell layers, tissues and organs.
• Body cavity is a pseudocoelom.
• Body possesses a through gut with an anus.
• Body covered in an external layer of chitin.
• Although some rotifers have a true, secreted cuticle, all have a fibrous layer
within their epidermis.
• The fibrous layer in some is quite thick and forms a caselike lorica, which is often
arranged in plates or rings.
• Has a nervous system with a brain and paired nerves.
• Has no circulatory or respiratory organs.
• Reproduction mostly parthenogenetic (an asexual form of reproduction found in
females where growth and development of embryos or seeds occurs without
fertilization by a male), otherwise sexual and gonochoristic (reproduction with
two distinct sexes).
• Feed on bacteria, and protists, or are parasitic.
• All live in aquatic environments either free swimming or attached.
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6. Forms
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Sessile Floaters/Swimmers
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Habitat
• Rotifers live in still water and flowing water
environments such as lake bottoms, rivers, or
streams.
• Rotifers are also commonly found on mosses
and lichens growing on rocks and aquatic
insect larvae.
• Some Rotifers are epizoic (live on the body of
another animal) or parasitic
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Rotifer Anatomy
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Feeding
• Cilia on corona beat
• Water currents bring food, they feed on bacteria,
protists, and other small animals
• Food enters mouth
• Food enters pharynx (mastax)
• Trophi (jaws) grind food
• Food passes into esophagus
• Salivary and gastric glands secrete enzymes that
break down food
• Nutrients are absorbed in the stomach
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Excretory system
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•Consist of a pair of protonephridial tubules with several
flame cells
•Flame cells empty contents into a common bladder
•Bladder by pulsating, empties into a cloaca
•Protonephridial also serves as osmoregulatory organ

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Nervous system
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•It contains a bilobed brain which sends paired nerves to
the sense organs
• sensory organs include paired eyespots, sensory
bristles, papillae, ciliated pits and dorsal antennae

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15. Reproduction
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•The reproduction of rotifers can be sexual (dioecious) and by
parthenogenesis. In dioecious organisms, males are usually smaller
than females.
•However, despite having separate sexes, males are entirely unknown
in the class Bdelloidea, and in the Monogononta they seem to occur
only for a few weeks of the year.
•The female reproductive system in the Bdelloidea and Monogononta
consists of combined ovaries and yolk glands (germovitellaria) and
oviducts that open into the cloaca.
•The mictic female produces eggs that undergo the usual double
meiotic division and are haploid while amictic eggs are diploid and
develop parthenogenetically
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16. • In Bdelloidea (Philodina), all females are parthenogenetic and produce diploid
eggs that hatch into diploid females. These females reach maturity in a few days.
• In class Seisonidea, females produce haploid eggs that must be fertilized and that
develop into either males or females
• Sexual reproduction occurs in the rotifers of Class Monogononta where the
females produce two kinds of eggs (amitic and mitic).
• During most of the year female produces amictic eggs which develop
parthenogenetically into diploid (amictic) females
• Several environmental factors—crowding, diet, or photo-period etc—may induce
amictic eggs to develop into diploid mictic females that produce thin-shelled
haploid eggs.
• If these eggs are not fertilized, they develop into haploid males. But if fertilized,
the eggs, called mictic eggs, develop a thick, resistant shell and become dormant.
• They survive over winter (“winter eggs”) or until environmental conditions are
again suitable, at which time they hatch into amictic females.
• Dormant eggs are often dispersed by winds or birds, which may explain the
peculiar distribution patterns of rotifers.
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17. • The male reproductive system includes a single testis and a
ciliated sperm duct that runs to a genital pore (males usually
lack a cloaca).
• The end of the sperm duct is specialized as a copulatory organ.
• Copulation is usually by hypodermic impregnation; the penis can
penetrate any part of a female’s body wall and inject sperm
directly into her pseudocoel.
• The zygote undergoes modified spiral cleavage.
• Females hatch with adult features, needing only a few days’
growth to reach maturity.
• Males often do not grow and are sexually mature at hatching.
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Rotifer Life Cycle
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20. Classification of Phylum Rotifera
• Class Seisonidea (seison-earthen vessels, eidos-form)
 Marine
 Enlongated form
 Corona vestigial
 Sexes similar in size and form
 Females with pair of ovaries and no vitellaria
 Epizoic on gills of a crustacean
Example: Seison
• Class Bdelloidea (bdella-leech, eiodos-form)
 Swimming or creeping forms
 Anterior end retractile
 Corona usually with pair of trochal discs
 Males unknown
 Parthenogenetic
 Two germovitellaria
Examples: Philodina, Rotaria
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21. • Class Monogononta (monos-one, gonos-primary sex gland)
about 90% of known species
 Swimming or sessile forms
 Single germovitellarium
 Males reduced in size
 Eggs of three types (amictic, mitic, dormant)
Examples: Asplanchna, Epiphanes
Some authorities classified Phylum Rotifera into:
a. Class Digonata (Seisonidea and Bdelloidea) and
b. Class Monogononta
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22. Assignment
• Phylogeny of rotifers
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