Editing Squid (section)

==Development==
The eggs of squid are large for a mollusc, containing a large amount of yolk to nourish the embryo as it [[Direct development|develops directly]], without an intervening [[veliger]] larval stage. The embryo grows as a [[germinal disc|disc of cells]] on [[animal pole|top of the yolk]]. During the [[gastrulation]] stage, the margins of the disc grow to surround the yolk, forming a yolk sac, which eventually forms part of the animal's gut. The dorsal side of the disc grows upwards and forms the embryo, with a shell gland on its dorsal surface, gills, mantle and eyes. The arms and funnel develop as part of the foot on the ventral side of the disc. The arms later migrate upwards, coming to form a ring around the funnel and mouth. The yolk is gradually absorbed as the embryo grows. Some juvenile squid live higher in the water column than do adults. Squids tend to be short-lived; ''[[Loligo]]'' for example lives from one to three years according to species, typically dying soon after spawning.<ref name=Ruppert/>

[[File:Bobtail Squid Light Organ.svg|thumb|upright=1.2|[[Sagittal section]] of the large eye-like light-producing organ of [[Euprymna scolopes|Hawaiian bobtail squid]], ''Euprymna scolopes''. The organ houses symbiotic ''[[Aliivibrio fischeri]]'' bacteria.]]

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.&nbsp;fischeri''.<ref name="Nyholm2004"/> Colonization occurs in a horizontal manner, such that the hosts acquires its bacterial partners from the environment. The symbiosis is [[Symbiosis#Obligate versus facultative|obligate]] for the squid, but facultative for the bacteria. Once the bacteria enter the squid, they colonize interior [[epithelium|epithelial cells]] in the light organ, living in [[Crypt (anatomy)|crypts]] with complex [[Microvillus|microvilli]] protrusions. The bacteria also interact with [[hemocyte]]s, 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.<ref name="McFall-Ngai 2014">{{cite journal|last=McFall-Ngai|first=M. J.|title=The importance of microbes in animal development: lessons from the squid-''Vibrio'' symbiosis|journal=Annual Review of Microbiology|date=2014|volume=68|pages=177–194|doi=10.1146/annurev-micro-091313-103654|pmid=24995875|pmc=6281398}}</ref> Approximately 95% of the bacteria are voided each morning before the bacterial population builds up again by nightfall.<ref name="Jones 2004"/>