Cilia-driven fluid vortices deliver symbiotic bacteria onto the mucociliary olfactory organ of the bobtail squid

The bobtail squid Euprymna acquires its luminous symbiont Vibrio fischeri from seawater in which the symbiont occurs at fewer than ten cells per millilitre, a minute fraction of the ambient bacterioplankton. The host is transcriptionally primed for colonisation as soon as the first handful of symbionts are detectable at the light organ, insufficient for direct biochemical signaling. How the arriving symbiont is sensed so early remains unknown. We hypothesized that the paired olfactory organs, which sit at the entrance to the mantle cavity and hence in the path of all inhaled water, could be a site of early symbiont detection. Here we characterise cilia-driven flow and capture of bacteria and bacteria-sized beads at the olfactory organ in hatchlings of Euprymna scolopes and E. berryi and in three non-symbiotic cuttlefish and squid species. In Euprymna, suspended beads accumulate more strongly at the olfactory organs than at the light organ over the same exposure time. The olfactory organ is a densely ciliated, mucus-coated epithelium packed with putative chemosensitive cells and expressing the peptidoglycan-recognition receptor EsPGRP4. Its motile cilia drive a vortical flow that converges on the organ and persists to at least 96 h post hatching. Swimming symbiont bacteria follows the same flow and capture pattern as inert beads yet are retained an order of magnitude more strongly. Hatchlings of the three non-symbiont cephalopod species also exhibit motile cilia at the olfactory organ, but the resulting flow pattern sweeps past the surface, not onto it, which limits capture of flow-borne matter to the organ's edges. Taken together, these findings suggest that in Euprymna, the olfactory organ may be able to capture and detect symbiont bacteria before they reach the light organ by effectively sorting and trapping bacterial candidates near the chemosensory surface.