Adaptations of Daphnia to Environmental Stress
In a good environment, most Daphnia are female and will reproduce without breeding, through a natural form of cloning. When reproducing this way, the eggs do not get fertilized, so the young are exact copies of their mothers. The unfertilized eggs develop into live embryos inside the female's body, and the young are released into the environment within two to three days.

When the environment becomes stressful, Daphnia adapt by producing male as well as female embryos. Once they become mature, these individuals breed and produce fertilized eggs that are encased in tough protective shells. These are released from the female's body and will have to go through several cycles of freezing and thawing before they hatch.

It is difficult to distinguish male and female Daphnia. They are basically identical except that males are generally smaller in size, have larger antenules, and the first legs have a stout hook used in clasping the female during mating. Rather than trying to identify males to detect stressful environments, it is much simpler to inspect the brood chambers of females (click here to see photos). If you see eggs encased in protective shells, that is a sign of unfavorable environmental conditions. If the brood chambers are full of embryos or eggs with no protective coating, you can conclude that the Daphnia are not feeling stressed.

Why do Daphnia respond to stress by reproducing sexually rather than through cloning? In this way, they produce young that are not exact copies of their mothers, and some of the babies may be better adapted than others to the stressful environment in which they must live. In addition, the fertilized eggs are enclosed in tough shells which help to protect them until the environment once again becomes favorable. This is a useful adaptation for organisms that live in ponds or other water bodies that may dry up for part of the year: although the adults will die, their eggs are adapted to surviving until the environment once again becomes favorable.