“…Approximately 75% of spawning and 10% of brooding coral species acquire their Symbiodiniaceae from the environment with each generation (Baird et al, 2009), and adult corals may take up environmental Symbiodiniaceae cells to replenish their microbiomes following abiotic stress (Boulotte et al, 2016;Lewis and Coffroth, 2004;Rouzé et al, 2019). However, the dispersal mechanisms that make Symbiodiniaceae cells available to prospective host corals have not been resolved (but see Coffroth et al, 2006;Nitschke et al, 2016), and Symbiodiniaceae cell densities in environmental reservoirs appear relatively low (sediments: 10 1 -10 3 cells ml -1 ; seawater: 10 0 -10 1 cells ml -1 ; macroalgae: 10 2 -10 3 cells ml -1 , Figure 1; Castro-Sanguino and Sánchez, 2012;Fujise et al, 2021;Littman et al, 2008) Cell densities reported in this study (all obligate corallivore and grazer/detritivore feces data and some data for facultative corallivore feces, sediments and seawater) represent live cell densities only, based on results from hemocytometry with the cell viability dye, trypan blue. Previously published cell densities for sediment and seawater (Fujise et al, 2021;Littman et al, 2008), macroalgae (Fujise et al, 2021), and facultative corallivore feces (Castro-Sanguino and Sánchez, 2012) may include both dead and live cells and were quanti ed using hemocytometry (Castro-Sanguino and Sánchez, 2012), a combination of ow-cytometry and hemocytometry (Littman et al, 2008), or quantitative PCR (Fujise et al, 2021).…”