- like ▁chemical s ) ▁and ▁has ▁found ▁that ▁about ▁ 2 0 % ▁of ▁their ▁gen es ▁were ▁modified ▁as ▁compared ▁to ▁populations ▁living ▁in ▁clean ▁sites . ▁ ▁Be havior ▁ ▁M umm ich ogs ▁live ▁in ▁dense ▁sho als ▁that ▁can ▁include ▁several ▁hundred ▁individuals . ▁ ▁During ▁cold ▁winter ▁months ▁in ▁the ▁northern ▁parts ▁of ▁their ▁range , ▁m umm ich ogs ▁move ▁to ▁up stream ▁tid al ▁po ols , ▁where ▁they ▁bur row ▁into ▁the ▁mud ▁at ▁depth s ▁up ▁to ▁ ▁to ▁over w inter . ▁They ▁can ▁also ▁b ury ▁themselves ▁in ▁mud ▁if ▁they ▁are ▁caught ▁in ▁a ▁dry ing ▁tid al ▁pool ▁between ▁spring ▁t ides . ▁Alternatively , ▁they ▁can ▁travel ▁short ▁distances ▁on ▁land ▁to ▁get ▁back ▁to ▁the ▁sea . ▁ ▁In ▁the ▁labor atory , ▁m umm ich ogs ▁have ▁yield ed ▁clear ▁examples ▁of ▁free - running ▁circ ad ian ▁rh yth ms , ▁in ▁both ▁body ▁colour ▁and ▁sw imming ▁activity . ▁For ▁the ▁latter , ▁clear ▁rh yth ms ▁were ▁obtained ▁in ▁single ▁individuals ▁as ▁well ▁as ▁in ▁groups ▁of ▁ 5 ▁or ▁ 2 5 ▁individuals . ▁Ev idence ▁of ▁free - running ▁semi - l un ar ▁rh yth ms ▁have ▁also ▁been ▁obtained ▁in ▁m umm ich ogs : ▁in ▁constant ▁labor atory ▁conditions , ▁egg ▁production ▁pe aked ▁every ▁ 1 4 . 8 ▁days ▁for ▁up ▁to ▁ 5 ▁months . ▁ ▁Re production ▁ ▁Sp aw ning ▁takes ▁place ▁from