Here's a semi-topical contribution...

One of the more amazing species of fish are the desert pupfish. They have amazing tolerances for tempurature and salinity. Here's a brief bit one one of the species, the Desert Pupfish, from Extreme Evolution, Desert-dwelling fish live on the edge of survival. from the California Academy Sciences website:

"Of the five species native to California, the biggest record holder is the desert pupfish, . The largest pupfish, this two-inch-long leviathan can live in waters from a frigid 40° F up to a scalding 112° F. It can endure the lowest levels of oxygen of any fish species, and a range of salinity from no-salt to twice the salt concentration of seawater."

What the article doesn't explain well is that the habitat will go through these extremes one or more times in a singe season. So the fish have adapted to being able to live in water that conditions that change from fresh water to saline. (another species can withstand 3x concentration of sea water!). Tough little buggers!

On a sad note, of the 13 sepecies, several are exinct or exist only in captivity. I know of a hobbyist breader who has a federal permit to bread them with the hopes of reintroducing them into the wild, although that may be a very remote possibility. Habitat destruction becasue of a falling water table is the main culprit followed by the introduction of alien species, such as game fish. Who knows what scientific discoveries will be lost forever if these unique fish disappear.