Rapid recruitment of symbiotic algae into developing scleractinian coral tissues

While the early acquisition of Symbiodiniaceae algae into coral host tissues has been extensively studied, the dynamics of the migration of algal cells into rapidly expanding coral tissues still lacks a systematic study. This work examined two Red Sea branching coral species, Pocillopora damicornis and Stylophora pistillata, as they were growing and expanding their tissue laterally on glass slides (January-June, 2014; 450 assays; five colonies/species). We measured lateral tissue expansion rates and intratissue dinoflagellate migration rates. Tissue growth rates significantly differed between the two species (with Stylophora faster than Pocillopora), but not between genotypes within a species. Using a "flow-through coral chamber" under the microscope, the migration of dinoflagellates towards the peripheral edges of the expanding coral tissue was quantified. On a five-day timescale, the density of the endosymbiotic dinoflagellate cells, presenting within a 90 μm region of expanding coral tissue (outer edge), increased by a factor of 23.6 for Pocillopora (from 1.2 × 10⁴ cells cm^-2 to 2.4 × 10⁵ cells cm^-2) and by a factor of 6.8 for Stylophora (from 3.6 × 10⁴ cells cm^-2 to 2.4 × 10⁵ cells cm^-2). The infection rates were fast (5.2 × 10⁴ and 4.1 × 10⁴ algal cells day-1 cm^-2, respectively), further providing evidence of an as yet unknown pathway of algal movement within coral host tissues.