Abstract
Reef-building scleractinian corals are threatened by many disturbances, prompting a surge in coral reef research, conservation, and scientific interventions in support of restoration. Coral science and management often try to build upon corals’ natural adaptive processes to help them survive environmental changes, and the symbiotic relationship between scleractinian corals and single-celled dinoflagellates in the family Symbiodiniaceae is of specific interest when considering this adaptive potential. Corals regulate the density of algal symbionts in their tissues by expelling excess cells or digesting them. This regulatory mechanism presents an avenue for non-invasive sampling to provide valuable information on symbiont community composition without harming the coral host. Here, we designed a novel, noninvasive methodology to identify Symbiodiniaceae cells expelled from juvenile corals under ambient conditions by filtering water collected from small closed-system incubations and extracting, purifying, and analyzing symbiont DNA from the filtrate. We found that symbiont DNA in the water was a reliable indicator of Symbiodiniaceae community composition in the coral. While the non-invasive sampling was not as effective as taking a tissue sample (which typically requires sacrificing the coral recruit), efficiency can be improved by selecting coral recruits with specific qualities or placing multiple recruits in the same incubation chamber. Generally, age was the most significant indicator of sampling efficiency for spawning species. At four months of age, individual Diploria labyrinthiformis showed a 71% chance of expelling sufficient symbionts over a 16 h incubation to be detected with quantitative polymerase chain reaction (qPCR). Placing 6-13 recruits in an incubation chamber is recommended to increase the likelihood of symbiont detection to >95%. Non-fatal sampling is vital as it allows the preservation of genetically unique individuals and contributes to coral conservation efforts. Researching algal symbiont community or expulsion dynamics in juvenile corals has applications for coral holobiont growth, thermotolerance, disease, and conditioning studies, and non-invasive sampling techniques allow this information to be gathered without impacting the success of coral conservation efforts.