Non-invasive investigation of the morphology and optical properties of the upside-down jellyfish with optical coherence tomography.

The jellyfish largely cover their carbon demand via photosynthates produced by microalgal endosymbionts, but how holobiont morphology and tissue optical properties affect the light microclimate and symbiont photosynthesis in remain unexplored. Here, we use optical coherence tomography (OCT) to study the morphology of medusae at high spatial resolution. We include detailed 3D reconstructions of external micromorphology, and show the spatial distribution of endosymbionts and white granules in the bell tissue. Furthermore, we use OCT data to extract inherent optical properties from light-scattering white granules in and show that granules enhance local light-availability for symbionts in close proximity. Individual granules had a scattering coefficient of = 200-300 cm, and scattering anisotropy factor of = 0.7, while large tissue-regions filled with white granules had a lower = 40-100 cm, and = 0.8-0.9. We combined OCT information with isotopic labelling experiments to investigate the effect of enhanced light-availability in whitish tissue regions. Endosymbionts located in whitish tissue exhibited significantly higher carbon fixation compared to symbionts in anastomosing tissue (i.e. tissue without light-scattering white granules). Our findings support previous suggestions that white granules in play an important role in the host modulation of the light-microenvironment.