In a well-studied bioluminescent species, the Hawaiian bobtail squid, a special light organ in the squid's mantle is rapidly colonized with Aliivibrio fischeri bacteria within hours of hatching. This light-organ colonization requires this particular bacterial species for a symbiotic relationship; no colonization occurs in the absence of A. fischeri. Colonization occurs in a horizontal manner, such that the hosts acquires its bacterial partners from the environment. The symbiosis is obligate for the squid, but facultative for the bacteria. Once the bacteria enter the squid, they colonize interior epithelial cells in the light organ, living in crypts with complex microvilli protrusions. The bacteria also interact with hemocytes, macrophage-like blood cells that migrate between epithelial cells, but the mechanism and function of this process is not well understood. Bioluminescence reaches its highest levels during the early evening hours and bottoms out before dawn; this occurs because at the end of each day, the contents of the squid's crypts are expelled into the surrounding environment. Approximately 95% of the bacteria are voided each morning before the bacterial population builds up again by nightfall.
in A Well-sTUDIED BIOLuMInesceNt Species, thE hawaIIan bOBtAil SQUid, A sPeCIal ligHt orgAN IN The squid's maNtle iS rA.piDlY coLonized With AliivibRIo fiSChEri bactERIa WitHin hoURs of haTcHIng. ThiS lIGht-oRgan ColoNizatIon requiRes this ParTicular baCterial SpecIes FoR a symBIotIc relationShip; no CoLoNizaTioN oCcurS in the absENCe of a. fisCherI. coloNization oCcURS in A hoRIZONtal ManNer, suCh that THE HOsts aCQUirEs iTs BActeriAl pArtners FroM the enVIRoNment. ThE SymBiosiS iS obliGAte fOr the sqUid, buT facUlTaTIve FOR thE bActERia. oNce the bacteria EnTeR the squid, tHeY coLoniZe intErior ePITheliAL cELlS iN the lighT oRgaN, liViNG In cRypts wItH complEx microVilli PRoTrusions the bACtEria aLso InteraCT witH heMoCYtES, macropHagE-liKe BLOOd cElLs That miGRate betweEN ePiTHeliaL Cells, buT tHe MeChAnIsM and FUNction oF tHIs ProCess IS noT well unDerStood. biolUminEscEnce rEaches iTs hIghEst levels DuRing The early eVeNINg hourS And bOTtOMs out befOrE dAwn; thiS occurS BEcauSe aT The enD of eAch DAY, tHE CONtENTs of THe squId's crypTS aRE EXPeLled iNtO The sURroundiNg eNvIronmenT. appRoXImatELy 95% oF the bactEriA arE VOidED EACH moRNinG beFOrE the baCTerIAl PoPuLaTiON bUiLds uP AgAin by nigHtfall.