An amazing number of aquatic plants and animals form resting stages, or have parts of their life cycles that are capable of leaving water to move to new locations. Resting stages or emerging stages can allow plants and animals that normally live in water to "migrate." The protection of a hard outer shell can allow aquatic organisms to be blown by the wind or carried by birds for great distances. Adults that emerge from water into air can go to other water bodies to lay eggs. But why would it be useful to be able to "migrate" from one body of water to another?
Even the largest of water bodies is transitory in nature, given enough time. At times, rivers, streams, lakes, mud puddles, and even oceans can go dry. The current drought in Colorado reminds us of the transitory nature of even human-managed water bodies. Many aquatic plants and animals, especially the small ones, have developed coping mechanisms. Most nonvascular (lower) plants such as algae have one form or another of resting or spore stage, while vascular plants produce seeds- the ultimate in rugged, portable packaging of the next generation.