Characteristic of euryhaline and freshwater habitats, extremes of temperatures, variations in dissolved oxygen and high degree of habitat disturbance are the main parameters shaping the lives of killifishes. The ability to move from fresh to saltwater requires tremendous osmoregulation adjustments. The sheepshead minnow (Cyprinodon variegatus) is found in marsh ponds from Maine to the West Indies, and can tolerate a wider range of salinity than any other fish. Killifish kidneys have specially adapted renal tubules to process sodium chloride salts as concentration increases from freshwater to saltwater. Special chloride cells located in the opercular epithelium of the gills help facilitate this effort for the mummichog (F. heteroclitus). Metabolic regulation of intercellular activity provides for greater hemoglobin-oxygen binding efficiency in mummichogs, allowing them to respond quickly to changes in dissolved oxygen levels, regulate pH, and thus effectively deliver oxygen to muscle tissues. This model may be the same for all killifishes, although a genetic component is probably involved, explaining the variety of tolerance levels limiting distribution of certain species.
The pupfishes of the California deserts show the most amazing temperature tolerance, commonly withstanding months of summer water temperatures between 35–40°C (95–104°F), with extremes of 47°C (116.6°F) not uncommon. Very few vertebrates can survive at this thermal level. Their small size (60-75 mm, 2-3 in), while providing a distinct advantage for survival in shallow, densely populated areas with limited food resources, would seem to present a major thermoregulatory challenge. Desert nights can experience temperatures below freezing as well, at which times the pupfish burrows into detritus on the bottom and remains dormant until temperatures rise again. Since all pupfish species are considered rare or endangered, further study of this remarkable ability awaits better captive rearing techniques.
Another important adaptation necessary for survival in salt marsh habitats is the ability to tolerate high levels of dissolved hydrogen sulfide. This gas is released as a by-product of anaerobic marsh decomposition and can achieve potentially toxic levels. Experiments with the California killifish (F. parvipinnis) indicate that metabolic tolerance is achieved by response of cellular mitochondrial oxidation, allowing a tolerance two to three times greater than that of most fishes.