STRUCUTRE, AFFINITY AND LIFE HISTORY OF ROTIFERA

INTRODUCTION

The word Rotifera is derived from latin meaning wheel barrier. They are commonly known as “wheel animalcules” as they are rotating wheel like structures. There size differ from 100 to 30 nanometres. They are visible microscope. It was discovered by Antivon leewenhook in 1703. Linnaeus and landmark put rotifers under protozoa. Their body is covered with cilia at anterior surface. They are aquatic sometimes perform planktonic motion while other time their movement changes to worm like.

They mainly consist of 3 classes. All three classes reproduce by three different mechanism

Seisonidea – only sexually

Bdelloidea – exclusively parthenogenesis

Monognonta – alternate between above two mechanism

Parthenogenesis is common in rotifer. They are pseudocoelomate metazoans. They are sexually dimorphic female are larger than male. They lack larval stages.

STRUCTURE OF ROTIFERA

• They are usually transparent in colour  their colour may vary from brown to orange and red. Body is divided into corona bearing head trunk and foot.
• Foot are extended by toe and consist of tail. Tail is covered with cuticles and at terminal of tail pedal gland are present in adhesive disc that secretes adhesives.
• Trunk is covered with lorica made up of cuticles. They are immovable and divided into rings. It provide the function of extension and contraction of body.

Nervous system

• Brain – located above mastax in head. It consist of dorsal single bilobed ganglion.
• Cephalic sense organ and dorsal antennae are innervated from brain.
• Geniculate ganglion passes nerve to lateral antennae.
• Pedal or caudalvesicular ganglion passes nerve to caudal sense organ.
• Sense organs –  present near the corona in the formed of bilobed brain there is presence of small eye spot. Ciliate pits are chemoreceptor and antenna are tactile organ.

Digestive system

• Mouth is marked in rotifer from here food entry takes place.
• Pharynx are modified by chitinous hard jaw called trophi
• Digestive and salivary gland are present in stomach.
• Anus is present mid dorsally at posterior end at union of trunk and tail.
• Before anus cloacal bladder is present for collection of excretory waste.

Excretory system

• It consist of pair of protonephridial tubules terminates in flame bulb.

AFFINITIES OF ROTIFERA

• Rotifera posses similar morphologial characters with many invertebrates.

Affinities with Platyhelminthes

Resemblance

• Primitive form of corona may come from complete or ventral ciliation in Turbellaran.
• Formation of trophy is quite common in Turbellaran worms.
• Flame cell containing protonephridial system is similar with Rhadocoel.
• Frontal organ of turbellaran may possibly identical with retrocerebral organs.

Differences

• Anus is absent in Turbellarian
• Absence of sub-epidermal continous muscles.
• Lack of epidermal nerve plexus.

Affinities with the Nematods

Similarities

• A syncytial epidermis
• Both consist of pseudocoelomic body cavity.
• Gut containing both mouth and anus.
• Absences of proper circulatory system
• No larval form in life cycle

Affinities with Arthropods

• Incomplete metamerism
• Presences of cuticles on body
• Bristle bearing arms similar to appendages of crustacean larva.

There are only morphological similarities.

Affinities with Annelids

There are structural similarities between trocophora larva and peculiar Rotifer trochospaera it was describe by Hatschek in 1878.

• Following parts of trochospaera is similar with trochophora that is the cilliary girdle, bent intestine and excretory organs.

According to him rotifers are simply annelids that remained in larval stage only.

It assumes that such similarities can be coincidence as there is no taxonomy relation and phylogenetic history.

LIFE HISTORY OF ROTIFERA

• The overall of lifespan of rotifera is about 3-4 days at an optimal temperature of 25 degree Celsius. Males are haploid and they are about of one fourth of female in size with no digestive tract and bladder only possess 1 proportionate testis which is filled with sperms. While female may be haploid or diploid.
• There are two modes of reproduction one is parthenogenesis and other is sexual reproduction. Parthenogenesis is usual mode of reproduction while sexual reproduction occur under unfavourable conditions.
• During parthenogenesis the diploid that is amictic female produces amictic that is haploid eggs.
• While sometime in unfavourable environmental condition female changes its mode to sexual form. Sexual reproduction is more complex it will allow to mictic and amicitc females.  Mictic and mictic females can be differentiated by the external appearance. Mictic female produces haploid eggs. Larvae hatching out of this unfertilised haploid eggs will produce haploid males.
• Haploid eggs of male are also significantly smaller in size. While the mictic fertilised egg are larger and have thick faintly granulated outer layer refer to as resting eggs. Although the mechanism of formation of resting egg is not well understood they are for survival strategy during unfavourable condition such cold or drought. Resting eggs will develop into amictic female.

CONCLUSION

• Rotifera are small microscopic animals they are filter feeder they mainly feed on dead material and other microscopic living material hence they are very important component of aquatic food web.
• About 2,200 species has been identified till date. They are cylindrical and worm shape and highly variable. They show various similarities with arthropods and annelids and other phyla but lack to show any phylogenic relation with them hence kept under minor phyla.
• It has been classified in 3 classes but it classification is still kept in consideration. They lack larval form. Body cavity is not true hence comes under category of pseudocoelomate.
• Although they are greater in species number but they lack participation in animal community hence considered in minor phyla. They only make fraction in animal community.