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ROTIFERA

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(Cuvier, 1798 )

Sections
  • General Description
  • Development
  • General Description

    Rotifers are an unusual group of animals, in that growth is not by an increase in cell number, but by the cells enlarging, so the number of body cells remain constant. They are bilaterally symmetrical and occur in a great variety of different shapes (Figure. 1). Despite their small size, they have a complex anatomy. The body can be divided into three main sections: a head, a trunk, and a foot. The head bears the corona, which is ciliated (Figure 1A); the cilia arranged in a circle beating one after the other, giving the appearance of a rotating wheel. The trunk is the main part of the body, and is protected by stiffening armour known as the lorica. This 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. The lorica may be divided into plates or rings, with each plate or ring containing spines, ridges, etc (Figure 1E, F). The last section, the foot, can end in contractile toes (Figure 1B) used for attachment while feeding, although the toes may be missing and this section bear spines or be rounded (Figure 1 G, H).

     

    Figure 1. Phylum Rotifera.

    Rotifers are generally transparent, but can appear green, brown, red, or orange, depending on the colour of the food in the digestive tract. Even though they are small, they do have organs, including a modified pharynx (the mastax) that grinds the food, an oesophagus, stomach, reproductive organs etc. They also have a nervous system consisting of nerves that extend to the sensory organs and other areas of the body, sending messages to and from the "brain", a mass of cells known as a ganglia. The sensory organs consist of bristles on the corona, antennae, and one or two red or black “eyes” on the brain that contain a few photo receptors. Most spend solitary lives, but some live in colonies (Figure 1I). They may be free swimming and truly planktonic, others move by inchworming along the substrate, while some are sessile, living inside tubes or gelatinous holdfasts.

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    Development

    Reproduction is varied. For most species, the males have never been discovered, and may not even exist. This is because many species reproduce asexually by parthogenesis, when eggs develop without ever being fertilized by a male. The female produces one or two diploid eggs (eggs with the full number of chromosomes), which then develop into females. In other species, in which both sexes are present, sexual reproduction may occur. In this case, the female produces haploid eggs (eggs with half the number of chromosomes. These haploid eggs can either remain unfertilized and undergo parthogenesis to develop into haploid males, or they can be fertilized by the male and develop into diploid females.

    Individuals of some species produce two distinct types of parthenogenetic eggs. One type develops into a normal parthenogenetic female, while the other occurs in response to a changed environment and develops into a degenerate male. This male lacks a digestive system, but does have a complete male reproductive system, used to inseminate females thereby producing fertilized 'resting eggs'. Resting eggs are able to survive extreme environmental conditions, such as may occur during winter or dessication. These eggs resume development and produce a new female generation when conditions improve.

    Distribution: Particularly in shallow coastal and brackish waters.

    Size: ~ 0.05 to 2 mm.
     
    Further information: Larink & Westheide, 2006; Berzins 1960a-f.

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