Symbiont spatial organisation is dynamically regulated within cnidarian host tissues

The symbiosis with photosynthetic dinoflagellate algae enables corals to build and sustain reef ecosystems. Individual coral polyps hold algal symbionts in their epithelial endoderm cells and lose them under environmental stress, leading to coral bleaching. How the host integrates symbionts into its body plan is not well understood. Here, using a combination of high-resolution imaging, quantitative analysis, and environmental perturbations in the sea anemone Exaiptasia diaphana (Aiptasia) and reef-building coral Pocillopora damicornis, we uncover a spatial organisation of symbionts along the aboral-oral axis of cnidarian polyps that emerges under the long-range translocation of symbionts between host cells through a fluid-filled cavity. The symbiont distribution becomes specifically enriched in the tentacle bud endoderm during Aiptasia polyp morphogenesis. This pattern can form in darkness and with algae-sized inert spheres, suggesting an innate host-intrinsic mechanism. Symbiont-occupied host cells are mechanically constrained within the endoderm and thus unable to rearrange; instead, they go through cycles of symbiont expulsion and re-uptake via the host gastric cavity, with regionally biased rates of these behaviours providing a route to enrich symbionts in the tentacles. Symbiont organisation is remodelled under increased light in adult coral polyps, with a characteristic pattern of reduced tentacle enrichment, lateral clustering and retention in the body column emerging over a timescale of days. Together, our findings reveal that the spatial organisation of symbionts is dynamically regulated in cnidarian host tissues, a capacity that may shape both the establishment of symbiosis and its resilience under environmental change.