i) A group of Daphnia under 200x magnification
ii) A female D. pulex clearly showing 2 developing embryos within her brood
chamber.
In a normal growth and breeding cycle, the Daphnia female will produce eggs that
will develop directly and be released as live young without a resting phase. When
adverse conditions become apparent the female will produce a different type of egg
in her brood pouch, this egg is designed for resting only hatching when favourable
conditions resume.
These resting eggs are protected and encased in a saddle like structure known as an
ephippium.
The ephippium will usually contain 2 large eggs and will be cast off during the next
moulting stage. The ephippia can either sink to the bottom or float via means of minute gas chambers, this means that the eggs can either be dispersed by the wind or
inadvertently hosted by other animals (i.e. attached to the feathers of waterfowl) for
transportation and to populate other water bodies they will only hatch as and when
conditions are right, wether this be due to photo-period, temperature, water quality,
availability of food or other factors. But from these resting eggs, only females hatch.
I suspect that this is purely Mother Nature?s way of preventing the species of becoming extinct.