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3.3. Biology and life history

The life span of rotifers has been estimated to be between 3.4 to 4.4 days at 25°C. Generally, the larvae become adult after 0.5 to 1.5 days and females thereafter start to lay eggs approximately every four hours. It is believed that females can produce ten generations of offspring before they eventually die. The reproduction activity of Brachionus depends on the temperature of the environment as illustrated in Table 3.1.

The life cycle of Brachionus plicatilis can be closed by two modes of reproduction (Fig. 3.2.). During female parthenogenesis the amictic females produce amictic (diploid, 2n chromosomes) eggs which develop and hatch into amictic females. Under specific environmental conditions the females switch to a more complicated sexual reproduction resulting in mictic and amictic females. Although both are not distinguishable morphologically, the mictic females produce haploid (n chromosomes) eggs. Larvae hatching out of these unfertilized mictic eggs develop into haploid males. These males are about one quarter of the size of the female; they have no digestive tract and no bladder but have an over-proportionated single testis which is filled with sperm. Mictic eggs which will hatch into males are significantly smaller in size, while the mictic fertilized eggs are larger and have a thick, faintly granulated outer layer.

Figure 3.2. Parthenogenetical and sexual reproduction in Brachionus plicatilis (modified from Hoff and Snell, 1987).

These are the resting eggs that will only develop and hatch into amictic females after exposure to specific environmental conditions. These can be the result of changes in environmental conditions eventually creating alternations in temperature or salinity or changing food conditions. It should be emphasized that the rotifer density of the population also plays an important role in the determination of the mode of reproduction. Although the mechanism is not completely understood, it is generally believed that the production of resting eggs is a survival strategy of the population through unfavourable environmental conditions such as drought or cold.

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