▁one ▁is ▁almost ▁flat ▁and ▁fits ▁inside ▁the ▁left . ▁The ▁inner ▁surface ▁is ▁smooth , ▁wh it ish ▁or ▁bl u ish - gre y . ▁ ▁Bi ology ▁Ost rea ▁ed ul is ▁are ▁gre g ari ous ▁m oll us cs ▁that ▁start ▁their ▁lives ▁as ▁males . ▁They ▁m ature ▁sex ually ▁after ▁eight – ten ▁months ▁and ▁may ▁change ▁sex ▁depending ▁on ▁the ▁water ▁temperature . ▁Usually ▁the ▁lif es pan ▁can ▁reach ▁about ▁six ▁years , ▁with ▁a ▁maximum ▁of ▁ 1 5 ▁years . ▁Ad ult ▁o yst ers ▁feed ▁by ▁fil tr ation . ▁ ▁Distribution ▁The ▁species ▁naturally ▁ranges ▁along ▁the ▁western ▁and ▁southern ▁coast s ▁of ▁Europe ▁from ▁Norway ▁to ▁Mor oc co ▁and ▁including ▁most ▁of ▁the ▁British ▁Is les ▁and ▁the ▁Mediter rane an ▁coast . ▁ ▁Natur ally ▁vi able ▁populations ▁have ▁appeared ▁in ▁eastern ▁North ▁America ▁from ▁Maine ▁to ▁Rh ode ▁Island ▁subsequent ▁to ▁artificial ▁introduction ▁in ▁the ▁ 1 9 4 0 s ▁and ▁ 1 9 5 0 s . ▁ ▁Hab itat ▁Ost rea ▁ed ul is ▁can ▁be ▁found ▁in ▁est u ar ine ▁and ▁shall ow ▁coast al ▁water ▁with ▁hard ▁substr ata ▁of ▁mud ▁and ▁rocks . ▁ ▁Human ▁use ▁Ost rea ▁ed ul is ▁has ▁been ▁har v ested ▁throughout ▁Europe ▁as ▁an ▁important ▁food ▁source ▁since ▁pre history . ▁During ▁Roman ▁occupation ▁of ▁Britain ▁O . ▁ed ul is ▁o yst ers ▁were ▁export ed ▁in ▁large ▁quantities ▁back ▁to ▁Italy