Symbiont spatial organisation is dynamically regulated within cnidarian host tissues

The symbiotic relationship between corals and photosynthetic dinoflagellate algae is crucial for the growth and maintenance of coral reef ecosystems. Coral polyps house these algal symbionts in their epithelial endoderm cells, but lose them under environmental stress, resulting in coral bleaching. The mechanism behind how the host integrates symbionts into its body plan remains unclear.

Utilizing advanced imaging techniques, quantitative analysis, and environmental stressors in sea anemone Exaiptasia diaphana and reef-building coral Pocillopora damicornis, researchers have discovered that symbionts are spatially organized along the aboral-oral axis of cnidarian polyps, emerging due to the long-range translocation of symbionts between host cells via a fluid-filled cavity.

This symbiont distribution is specifically enriched in the tentacle bud endoderm during Aiptasia polyp development, a pattern that can occur in darkness and with inert spheres of similar size to algae. This suggests an innate, host-intrinsic mechanism. The symbiont-occupied host cells are mechanically bound within the endoderm, preventing rearrangement, and instead undergo cycles of symbiont expulsion and re-uptake through the host's gastric cavity.

The rates of these behaviors are biased regionally, allowing for symbiont enrichment in the tentacles. Under increased light conditions, adult coral polyps remodel their symbiont organization, exhibiting a characteristic pattern of decreased tentacle enrichment, lateral clustering, and retention in the body column over a timescale of days.

The study reveals that the spatial organization of symbionts in cnidarian host tissues is dynamically regulated, a capacity that could influence both symbiosis establishment and resilience to environmental changes.