Upwelling
The major upwellings in the ocean are associated with the divergence of currents that bring deeper, colder, nutrient rich waters to the surface. There are at least five types of upwelling: coastal upwelling, large-scale wind-driven upwelling in the ocean interior, upwelling associated with eddies, topographically-associated upwelling, and broad-diffusive upwelling in the ocean interior.
Polypodium (animal)
Although the fresh water habitat of "Polypodium" is unusual for cnidarians, it is not unheard of, especially within hydrozoans. For instance, the model organism "Hydra" and the jellyfish "Craspedacusta sowerbii" are both exclusively fresh-water hydrozoans. "Hydra" and "Craspedacusta sowerbii" are distantly related and they are not closely related to "Polypodium". Thus, it appears that in the evolution of cnidarians, invasion to fresh-water habitats has happened at least three separate times.