Abstract
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.
Highlights
Hermatypic corals have developed mutualistic relationships with unicellular, photosynthesizing, endosymbiotic dinoflagellate algae of the family Symbiodiniaceae [1]
These cells provide the coral host with a large portion of its daily energy needs [7]. This symbiosis is of fundamental importance to the ecological fitness of coral reefs, and it is clear from a multitude of studies that the coral-algal relationship is highly dynamic and flexible, primarily with respect to recruitment/infection, expulsion, cell division, and interhostdistribution of the algae [8]
This allowed us to closely track the sequence of events that are associated with algal acquisition. Using this Lateral Skeleton Preparative (LSP) assay on nubbins from two Red Sea branching coral species, Pocillopora damicornis and Stylophora pistillata, we studied zooxanthellae recruitment rates into substratum-spreading new tissues at the levels of colony and species
Summary
Hermatypic (reef-building) corals have developed mutualistic relationships with unicellular, photosynthesizing, endosymbiotic dinoflagellate algae of the family Symbiodiniaceae [1] (often referred to as zooxanthellae). To document and to elucidate routes of algal infections into newly formed coral tissues, we used the Lateral Skeleton Preparative (LSP) assay [12,24,25] (Figure 1a,b), in which observations were made on newly formed flat and transparent coral tissues spread on glass slides This allowed us to closely track the sequence of events that are associated with algal acquisition. Using this LSP assay on nubbins from two Red Sea branching coral species, Pocillopora damicornis and Stylophora pistillata, we studied zooxanthellae recruitment rates into substratum-spreading new tissues at the levels of colony and species. We analyzed the gradients of symbiotic cell abundance along axes from peripheral sites towards the centers of the spreading tissues, as well as the zooxanthella vertical distributions at marginal coral tissues
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