Daphnia ecology and life style
Daphnia are filter feeders that direct small suspended
particles into their mouth by a water current produced by
their leaf-like legs (Figure 1). Daphnia’s common name of
‘water flea’ comes from its jump-like movement, which
results from the beat of the large antennae used for
swimming (Figure 1). In a normal growth season Daphnia
generates diploid eggs by asexual reproduction (parthenogenesis).
These eggs develop directly into larvae in the
female brood chamber and are released into the water
after about 3 days. In most species the larvae go through
four to six larval stages before developing into sexually
mature adults. However, the Daphnia life cycle is adapted
to extreme environmental conditions such as cold winters
or summer droughts. If triggered by external stimuli such
as high population density and a scarcity of food, Daphnia
can produce haploid resting eggs by meiosis; these require
fertilization and a period of extended dormancy in order
to develop [3]. Resting eggs are distributed by wind or
animals and development is resumed in response to
external stimuli (for example, rising temperature). Cyclic
parthenogenesis, in which parthenogenesis and sexual
reproduction alternate, is common in most Daphnia
species, but lineages have been described that exclusively
reproduce asexually (obligate parthenogenesis). Cyclic
parthenogenetic Daphnia must contain the molecular
tools for the production of both haploid gametes (by
meiosis) and diploid eggs (by mitosis), the latter developing
parthenogenetically into diploid zygotes. This makes