Threespine Stickleback

The threespine stickleback is a small fish, family Gasterosteidae, named for the three sharp spines along its back that it raises to deter predators. It lives in both marine and freshwater habitats across much of the Northern Hemisphere, and populations that colonized freshwater lakes after the last ice age have repeatedly evolved similar changes in armor and body shape, making it one of the most studied examples of rapid evolution in a wild vertebrate. Males build a nest from plant material and guard the eggs and young until they can swim independently.