Advect, retain, sense: cilia-driven vortices deliver symbiont bacteria and flowborne cues to a mucociliary olfactory organ

Abstract

Animals that acquire a microbial symbiont from the environment must find it against an overwhelming background of other bacteria. The bobtail squid Euprymna recruits its luminous symbiont Vibrio fischeri from seawater in which the symbiont occurs at fewer than ten cells per millilitre, and the host is transcriptionally primed for colonisation as soon as the first handful of symbionts become detectable at the light organ, too few to signal directly and implying a detection event upstream of it. We hypothesised that the paired olfactory organs, which sit at the entrance to the mantle cavity and hence in the path of all inhaled water, are that site. Here we characterise cilia-driven flow, particle capture and neuronal activity at the olfactory organ in hatchlings of Euprymna scolopes and E. berryi and in three non-symbiotic cuttlefish and squid species, i.e. species with no known light-organ symbiosis. In Euprymna the organ is a densely ciliated, mucus-coated epithelium packed with putative chemosensory cells and expressing the peptidoglycan-recognition receptor EsPGRP4, and its motile cilia drive a vortical flow that converges head-on onto the sensory face and persists to at least 96 h after hatching, independent of colonization status. Suspended beads accumulate there more strongly than at the light organ, and swimming symbionts follow the same flow yet are retained an order of magnitude more strongly than inert beads of the same size. Cells within the organ show spontaneous calcium transients, and the subset of cells and the firing rate change in response to waterborne stimuli, including live V. fischeri. In the three non-symbiotic species the equivalent organ sweeps water past its face rather than onto it, confining capture to the margins. Taken together, these findings suggest that in Euprymna the olfactory organ may concentrate, hold and detect arriving bacteria at a chemosensory surface before they reach the light organ.