Horizontal transmission of Symbiodinium cells between adult and juvenile corals is aided by benthic sediment
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Of all reef-building coral species, 80–85 % initially draw their intracellular symbionts (dinoflagellates of the genus Symbiodinium) from the environment. Although Symbiodinium cells are crucial for the growth of corals and the formation of coral reefs, little is known about how corals first encounter free-living Symbiodinium cells. We report how the supply of free-living Symbiodinium cells to the benthos by adult corals can increase the rate of horizontal symbiont acquisition for conspecific recruits. Three species of newly settled aposymbiotic (i.e., symbiont-free) corals were maintained in an open aquarium system containing: sterilized sediment and adult coral fragments combined; adult coral fragments alone; sterilized sediment alone; or seawater at Heron Island, Great Barrier Reef, Australia. In all instances, the combination of an adult coral and sediment resulted in the highest symbiont acquisition rates by juvenile corals (up to five-fold greater than seawater alone). Juvenile corals exposed to individual treatments of adult coral or sediment produced an intermediate acquisition response (<52 % of recruits), and symbiont acquisition from unfiltered seawater was comparatively low (<20 % of recruits). Additionally, benthic free-living Symbiodinium cells reached their highest densities in the adult coral + sediment treatment (up to 1.2 × 104 cells mL−1). Our results suggest that corals seed microhabitats with free-living Symbiodinium cells suitable for many coral species during the process of coral recruitment.
KeywordsHorizontal transmission Free-living Symbiodinium Scleractinian corals Sediment Spawning
S.W. funded the project, and M.R.N. was supported through an Australian Coral Reef Society bench fee award. The authors thank Clémence Barral, Oliver McIntosh and Simon Lamping for assistance in the field, and the staff at HIRS for their aid during the coral spawning events. Sediment and coral collection were conducted under the conditions described in GBRMPA permits, Numbered G12/35077.1 and G12/35434.1 awarded to M.R.N. and S.W. We thank two anonymous reviewers for their contributions to this manuscript.
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