Immune priming using poly I:C as a prophylactic treatment against stony coral tissue loss disease (SCTLD) and Investigating changes in algal symbiont load and community structure in response to in situ shading

Stony coral tissue loss disease (SCTLD), which first appeared in South Florida in 2014, has spread throughout Florida and the wider Caribbean effectively altering coral reef community structure and diminishing stony coral populations which are vital for promoting biodiverse ecosystems, protecting coastlines, and contributing to local economies. Here we tested the use of polyinosinic-polycytidylic acid (poly I:C) as a prophylactic treatment against SCTLD in a laboratory-based disease assay. Coral colonies with active SCTLD lesions were sourced from the Cayman Islands, as sourcing multiple colonies of various species with SCTLD proved to be difficult. Experimental fragments of Pseudodiploria clivosa (sourced from Miami-Dade County in 2021 and kept in running seawater facilities on Virginia Key since collection) were exposed to poly I:C via injections and water bath methodologies and then exposed to SCTLD. We found no transmission to either the poly I:C treated, or the control fragments during a 60-day exposure. Despite long exposure times and significant disease lesions in the source colonies, we found extremely low rates of SCTLD transmission to the experimental colonies, even after resorting to direct contact of 3 C40BCE June 2025 healthy colonies to SCTLD lesions. We hypothesize the low experimental transmission is the result of reduced susceptibility of the remaining corals on reefs in Miami-Dade County Florida corals after a decade of chronic exposure, which we elaborate on in the first portion of this report. 

Coral bleaching is widely recognized as a major factor contributing to the global declines of scleractinian corals. While often attributed primarily to elevated ocean temperatures, light also plays a critical role in influencing the severity of coral bleaching. Both thermal and light stress can damage the photosynthetic machinery of the algal symbionts (Family Symbiodiniaceae), initiating the bleaching response and leading to a positive feedback loop. Shading, which reduces light exposure during heat stress events, presents a practical management tool that may help lessen the impacts during summer bleaching. In collaboration with Dr. Karen Neely, we found the temporary deployment of in situ shade structures reduced bleaching severity in colonies of Colpophyllia natans and Pseudodiploria clivosa at Newfound Harbor, FL. This was evident both visually, through bleaching index scores, and at the molecular level, based on symbiont to host cell ratios – an indicator of algal symbiont “load” on the coral host – compared to nearby unshaded coral colonies. This effect is likely due to reduced light-driven stress on the Symbiodiniaceae, leading to the lower production of reactive oxygen species and reducing the need for the coral to expel associated algal symbionts as a protective response. These findings highlight shading as a promising short-term strategy to alleviate stress on vulnerable coral populations during marine heatwaves