The Hawaiian bobtail squid is an established model system for the study of the colonization of epithelia by bacterial symbionts.
The image shows the squid Euprymna scolopes swimming in the water column. This species, which is a night-active predator in the shallow sand flats of the Hawaiian archipelago, uses the light produced by its luminous bacterial symbiont, Vibrio fischeri, as a camouflage. Much like the microbiota of the human gut, the squid's bioluminescent partner is acquired anew each generation and resides extracellularly along the surface of epithelial tissues. The relationship between E. scolopes and V. fischeri has been studied for over 25 years as a model for the establishment and maintenance of animal-bacterial symbioses. See McFall-Ngai.
Image Credit: Chris Frazee and Margaret McFall-Ngai