rical ▁curr ents ▁or ▁food ▁delivery . ▁At ▁night , ▁Spanish ▁h og fish , ▁ ▁blue head ▁wr asse , ▁the ▁wr asse ▁Hal ich o eres ▁b iv itt atus , ▁the ▁c un ner ▁T aut og ol ab rus ▁ad sp ers us , ▁and ▁even ▁requ iem ▁sh arks , ▁can ▁be ▁picked ▁up ▁by ▁hand ▁without ▁el ic iting ▁a ▁response . ▁A ▁ 1 9 6 1 ▁observ ational ▁study ▁of ▁approximately ▁ 2 0 0 ▁species ▁in ▁European ▁public ▁aqu aria ▁reported ▁many ▁cases ▁of ▁apparent ▁sleep . ▁ ▁D ivers ▁can ▁easily ▁see ▁fish es ▁sett ling ▁down ▁for ▁the ▁night ▁in ▁typical ▁sh el ters , ▁such ▁as ▁holes ▁and ▁cre v ices , ▁under ne ath ▁led ges , ▁amid st ▁veget ation , ▁inside ▁sp ong es , ▁or ▁buried ▁in ▁sand . ▁ ▁Some ▁extra ▁protection ▁can ▁be ▁derived ▁from ▁special ▁secret ions , ▁such ▁as ▁the ▁m uc ous ▁en velope ▁produced ▁by ▁several ▁species ▁of ▁wr asse ▁and ▁par rot fish , ▁either ▁around ▁the ▁fish ▁themselves ▁or ▁at ▁the ▁opening ▁of ▁their ▁shelter . ▁These ▁en velop es ▁screen ▁the ▁sleep ing ▁fish ▁from ▁pred ators ▁ ▁and ▁e ct op ar as ites . ▁ ▁In ▁the ▁labor atory , ▁periods ▁of ▁in activity ▁often ▁alternate ▁with ▁periods ▁of ▁activity ▁on ▁a ▁ 2 4 - h ▁basis , ▁or ▁a ▁near ▁ 2 4 - h ▁basis ▁when ▁the ▁light ing ▁conditions ▁are ▁constant .