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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. [ABSTRACT FROM AUTHOR]