▁zo oid s ▁forms ▁around ▁the ▁central ▁st alk . ▁In ▁contrast , ▁several ▁species ▁reproduce ▁using ▁poly ps . ▁Poly ps ▁can ▁hold ▁eggs ▁and / or ▁s perm ▁and ▁can ▁be ▁released ▁into ▁the ▁water ▁from ▁the ▁posterior ▁end ▁of ▁the ▁si phon oph ore . ▁The ▁poly ps ▁may ▁then ▁be ▁fert il ized ▁outside ▁of ▁the ▁organ ism . ▁▁ ▁Si phon op hor es ▁use ▁g on op hor es ▁to ▁make ▁the ▁re product ive ▁gam etes . ▁Gon op hor es ▁are ▁either ▁male ▁or ▁female ; ▁however , ▁the ▁types ▁of ▁g on op hor es ▁in ▁a ▁col ony ▁can ▁vary ▁among ▁species . ▁Species ▁are ▁character ized ▁as ▁mon oe cious ▁or ▁dio e cious ▁based ▁on ▁their ▁g on op hor es . ▁Mon oe cious ▁species ▁contain ▁male ▁and ▁female ▁g on op hor es ▁in ▁a ▁single ▁zo oid ▁col ony , ▁whereas ▁dio e cious ▁species ▁har bor ▁male ▁and ▁female ▁g on op hor es ▁separately ▁in ▁different ▁colon ies ▁of ▁zo oid s . ▁ ▁Bi ol um ines c ence ▁ ▁Near ly ▁all ▁si phon op hor es ▁have ▁bi ol um ines cent ▁capabilities . ▁Since ▁these ▁organ isms ▁are ▁extremely ▁frag ile , ▁they ▁are ▁rarely ▁observed ▁alive . ▁Bi ol um ines c ence ▁in ▁si phon op hor es ▁has ▁been ▁thought ▁to ▁have ▁evol ved ▁as ▁a ▁defense ▁mechanism . ▁Si phon op hor es ▁in ▁the ▁genus ▁E ren