Killifish are small, brightly-colored tropical fish which number well over a thousand different species. What makes them of interest to renal physiologists is their ability to adapt to both freshwater AND saltwater.
It's not a trivial accomplishment: when swimming in fresh water, killifish are severely hypertonic relative to their environment. As a result they make use of apical placement of sodium transporters (such as the loop-sensitive Na-K-2Cl transporter and the thiazide-sensitive Na-Cl transporter) in their gut and their gills, so as to absorb what little sodium that comes into their diet.
In marked contrast, when killifish make their way into salt water, they rapidly switch molecular gears, dramatically increasing expression of the Na-K-2Cl transporter on the BASOLATERAL surface of their gills--thereby turning these organs into a potent salt secreting mechanism.
Killifish successfully make use of their osmotic flexibility by residing in any number of habitats, including river estuaries where fresh water rivers meet the ocean--an area where there can be variable mixing of salt and fresh water.