▁environmental ▁model ▁organ ism ▁because ▁of ▁their ▁ability ▁to ▁toler ate ▁various ▁extrem es ▁of ▁chemical ▁( p oll ution , ▁etc .) ▁and ▁physical ▁( temper ature , ▁sal inity , ▁o xygen , ▁etc .) ▁conditions . ▁They ▁are ▁relatively ▁abund ant ▁in ▁nature ▁and ▁can ▁be ▁easily ▁captured , ▁transport ed ▁and ▁re ared ▁in ▁labor atory ▁facilities . ▁They ▁are ▁commonly ▁used ▁in ▁scientific ▁studies ▁of ▁stress ▁bi ology , ▁thermal ▁phys i ology ▁and ▁to xic ology , ▁and ▁have ▁also ▁been ▁studied ▁in ▁the ▁context s ▁of ▁evolution ary ▁bi ology , ▁development al ▁bi ology , ▁end ocr in ology , ▁cancer ▁bi ology , ▁and ▁chron obi ology ▁( stud y ▁of ▁circ ad ian ▁rh yth ms ). ▁With ▁the ▁successful ▁sequ encing ▁and ▁assembly ▁of ▁the ▁full ▁kill if ish ▁gen ome , ▁they ▁serve ▁as ▁a ▁premier ▁scientific ▁model ▁for ▁studying ▁bio chem ical ▁and ▁phys i ological ▁responses ▁to ▁varying ▁environmental ▁conditions . ▁ ▁Their ▁remarkable ▁ability ▁to ▁toler ate ▁various ▁extrem es ▁of ▁temperature ▁and ▁sal inity ▁has ▁made ▁them ▁popular ▁subjects ▁in ▁scientific ▁studies ▁of ▁to xic ology . ▁For ▁dec ades ▁the ▁kill if ish ▁has ▁been ▁a ▁useful ▁labor atory ▁model ▁for ▁to xic ological ▁studies ▁that ▁include ▁expos ures ▁to ▁single ▁chemical s , ▁chemical ▁mi xt ures , ▁and ▁complex ▁cont amin ated ▁media . ▁It ▁is ▁sometimes ▁the ▁only ▁fish ▁species ▁found ▁in ▁sever ely ▁poll uted ▁and ▁o xygen - dep riv ed