und ulus ▁heter oc lit us ) ▁is ▁a ▁small ▁kill if ish ▁found ▁along ▁the ▁Atlantic ▁coast ▁of ▁the ▁United ▁States ▁and ▁Canada . ▁Also ▁known ▁as ▁Atlantic ▁kill if ish , ▁m umm ies , ▁g ud ge ons , ▁and ▁mud ▁min n ows , ▁these ▁fish ▁inhab it ▁bra ck ish ▁and ▁coast al ▁waters ▁including ▁est u aries ▁and ▁salt ▁mar sh es . ▁The ▁species ▁is ▁noted ▁for ▁its ▁hard iness ▁and ▁ability ▁to ▁toler ate ▁highly ▁variable ▁sal inity , ▁temperature ▁fl uct u ations ▁from ▁, ▁very ▁low ▁o xygen ▁levels ▁( down ▁to ▁ 1   m g / L ), ▁and ▁heavily ▁poll uted ▁e cos ystem s . ▁As ▁a ▁result , ▁the ▁m umm ich og ▁is ▁a ▁popular ▁research ▁subject ▁in ▁emb ry ological , ▁phys i ological , ▁and ▁to xic ological ▁studies . ▁It ▁is ▁also ▁the ▁first ▁fish ▁ever ▁sent ▁to ▁space , ▁ab o ard ▁Sk yl ab ▁in ▁ 1 9 7 3 . ▁ ▁Tax onomy ▁The ▁genus ▁name ▁Fund ulus ▁comes ▁from ▁, ▁meaning ▁bottom , ▁from ▁the ▁fish ' s ▁habit ▁of ▁sw imming ▁near ▁mud dy ▁bottom s . ▁The ▁species ▁name ▁heter oc lit us ▁means ▁irregular ▁or ▁unusual . ▁The ▁type ▁spec imen ▁was ▁first ▁described ▁by ▁Carl ▁Lin na eus ▁in ▁ 1 7 6 6 , ▁from ▁near ▁Char l eston , ▁South ▁Carolina . ▁Other ▁scientific ▁names ▁now ▁considered ▁syn onym s ▁for ▁this ▁species ▁include