I know it took a while to get through hatching a resting egg, but a new a new baby Daphnia has finally been hatched. The growing up part of the Daphnia life cycle is almost simple. The newborn will go through 4 molts (5 instars) over the next week or so and by the fifth instar it is a mature female, ready to produce young of her own. A couple of minor details along the way.
The time to maturation depends on water temperature. Remember, the number of instars is fixed and it takes growth and cell divisions to get to each molt. Daphnia are poikilotherms, that is, they are the temperature of the environment. Their development is fast in warm water and slow in cold water. At room temperature, around 70F, the process takes a week. In colder water it can take 10 days or more, even for species adapted to cold water. In warm water there is a limit to the speed of the process and it is unlikely for Daphnia to reach maturity in less than 5 days.
Another detail returns to my theme of the medium is the message. A common predator of juvenile Daphnia is the larval form of the phantom midge, Chaoborus. The larvae float horizontally in the water. 
When a Daphnia swims by the predator grabs hold and forces the prey into the mouth. The grabbing and forcing is accomplished by the use of the predator’s antennae 
that have evolved into grasping appendages, a nifty adaptation. But while the antennae have changed, the mouthparts are pretty typical and they are hard and so the mouth can only open so far. And antennae modified for grasping have limitations.
What this digression has to do with juvenile Daphnia growing up is it is not in the best interest of prey to be eaten. There are only so many ways to avoid being dinner. They can’t leave and go to a new pond. They can’t swim to some other part of the pond, as there is nowhere to go. They can’t have a thicker exoskeleton because that would make them heavier and they couldn’t swim. There is no point to camouflage as the predator hunts by sensing vibrations in the water and not by sight.
The options available to Daphnia are related to what I described about their predator. The mouth size is fixed meaning that sufficiently large Daphnia can’t be forced in readily. In general, a first instar Daphnia is about 0.6 mm and is mature at about 2.0 mm. Being large is a way not to get eaten; size is a refuge. But the juveniles have to go through 4 molts during most of which they are of a vulnerable size. That’s where the Daphnia have evolved an incredibly cool trick based on their ability to taste Chaoborus in the water. If Chaoborus is present in the pond, the juvenile Daphnia are able to change shape with a trick called phenotypic plasticity. More on this in my next post.
