▁from ▁. ▁They ▁are ▁among ▁fish ▁species ▁most ▁toler ant ▁of ▁sal inity ▁changes ▁( e ury hal ine ). ▁M umm ich og ▁lar va e ▁can ▁grow ▁in ▁sal in ities ▁r anging ▁from ▁ 0 . 4 ▁to ▁ 1 0 0 ▁parts ▁per ▁thousand , ▁the ▁latter ▁being ▁about ▁three ▁times ▁the ▁normal ▁sal inity ▁of ▁se aw ater . ▁Ad ult ▁m umm ich ogs ▁toler ate ▁low ▁o xygen ▁levels ▁down ▁to ▁ 1   m g / L , ▁at ▁which ▁they ▁resort ▁to ▁aqu atic ▁surface ▁resp iration ▁( bre ath ing ▁in ▁the ▁surface ▁layer ▁of ▁water , ▁rich er ▁in ▁o xygen ▁because ▁of ▁contact ▁with ▁air ) ▁to ▁surv ive . ▁They ▁can ▁even ▁surv ive ▁for ▁a ▁few ▁hours ▁in ▁mo ist ▁air ▁outside ▁of ▁water , ▁breath ing ▁air ▁directly . ▁ ▁Pop ulations ▁have ▁developed ▁resistance ▁to ▁m eth yl mer cur y , ▁k ep one , ▁dio x ins , ▁pol ych lor in ated ▁bi phen yl , ▁and ▁pol yar om atic ▁hydro car b ons . ▁One ▁study ▁has ▁looked ▁at ▁the ▁genom ic ▁variation ▁exhib ited ▁by ▁m umm ich ogs ▁populations ▁living ▁in ▁New ark ▁Bay , ▁New ▁Bed ford ▁Har bor , ▁and ▁the ▁Elizabeth ▁River ▁( V ir gin ia ) ▁( in ▁some ▁areas ▁heavily ▁poll uted ▁with ▁pol ych lor in ated ▁bi phen yl s ▁and ▁cre os ote , ▁a ▁complex ▁mixture ▁containing ▁dio x in