3. Biology and Life History
At 25°C the life span of rotifers has been estimated to be 3.4 to
4.4 days. Generally, the larvae become adult after 0.5 to 1.5 days whereafter
females 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.
Figure 3.2. Parthenogenetical and sexual reproduction in Brachionus plicatilis (modified from Hoff and Snell, 1987).
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 eggs
(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. 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 weather 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 determining of the mode of reproduction.
Although the mechanism is not completely understood it is generally
accepted that the production of resting eggs is a survival of the population
through unfavourable environmental conditions as drought or cold.
It is obvious that for culture purposes the sexual reproduction should be
avoided since it is slow and results in resting eggs that need to be hatched
while the mictic females generally die resulting in a sudden collapse of the
culture.
1.
Introduction
2. Morphology
3. Biology and Life History
4. Strain Differences
5. General Culture Procedures
