Organisms living in salty conditions generally need some form of active transport to get rid of the salt they take in with their food and drink.
Halophytes are a specialist group of plants, able to handle extremely salty conditions because they have special salt-resistant adaptations to help them.
An estuary provides special problems for animals and plants, in large part because of the wide variations in salinity and what this does to water balance.
Extreme environments which carry extremophiles include both the Arctic and the Antarctic, hot springs, and mid-ocean vents which are both hot and toxic.
Unusual life forms can be found in some very improbable places: high up in the atmosphere, deep in rocks and ice, in hot, cold, wet and dry places.
There are limits to what animals can survive: as a bare minimum, they need certain levels of moisture or fluid, enough oxygen and sufficient food.
Biologists hope that the oceans beneath the icy surface of Jupiter's moon, Europa, may also be extreme environments that carry some form of life.
Even in extremophiles, there are limits to the extremes of climate, starvation, drought, heat and general environment animals can survive in the longer term.
This file is http://members.ozemail.com.au/~macinnis/scifun/splatsextremo.htm, first created on February 16, 2008. Last recorded revision (well I get lazy and forget sometimes!) was on February 16, 2008.