How do freshwater planarians produce eggs that survive winter freezing?

Freshwater planarians are capable of producing two distinct types of eggs to adapt to seasonal changes. While they produce thin-shelled summer eggs that hatch quickly, they also generate thick-shelled winter eggs. These specialized winter eggs are resistant to freezing temperatures, allowing them to survive the winter season and hatch in the spring.1

This reproductive strategy ensures the survival of the population through harsh environmental conditions. In many planarians, the development of these eggs involves a unique process where nutrition for the embryo is supplied by the addition of separate cells to the zygote before the entire mass is enclosed within the protective shell.1