3D Photogrammetry Reveals Dynamics of Stony Coral Tissue Loss Disease (SCTLD) Lesion Progression Across a Thermal Stress Event

Abstract

Stony coral tissue loss disease (SCTLD) was first observed in the United States Virgin Islands in January 2019 on a reef at Flat Cay off the island of St. Thomas. A year after its emergence, the disease had spread to several reefs around St. Thomas causing significant declines in overall coral cover. Rates of tissue loss are an important metric in the study of coral disease ecology, as they can inform many aspects of etiology such as disease susceptibility and resistance among species, and provide critical parameters for modeling the effects of disease among heterogenous reef communities. The present study quantified tissue loss rates attributed to SCTLD among six abundant reef building species (Colpophyllia natans, Montastraea cavernosa, Diploria labyrinthiformis, Pseudodiploria strigosa, Orbicella annularis, and Porites astreoides). Field-based 3D models of diseased corals, taken approximately weekly, indicated that the absolute rates of tissue loss from SCTLD slowed through time, corresponding with the accumulation of thermal stress that led to mass bleaching. Absolute tissue loss rates were comparable among species prior to the bleaching event but diverged during and remained different after the bleaching event. Proportional tissue loss rates did not vary among species or through time, but there was considerable variability among M. cavernosa colonies. SCTLD poses a significant threat to reefs across the Caribbean due to its persistence through time, wide range of susceptible coral species, and unprecedented tissue loss rates. Intervention and management efforts should be increased during and immediately following thermal stress events in order maximize resource distribution when disease prevalence is decreased.

Fine Scale Temporal and Spatial Dynamics of the Stony Coral Tissue Loss Disease Outbreak Within the Lower Florida Keys

“…Few new cases occurred during the summer months when thermal stress was highest and disease progression slowed considerably. A similar negative correlation was quantified between progression rates of SCTLD and DHW in St. Thomas, United States Virgin Islands (Meiling et al, 2020) and a decline in SCTLD activity coincided with increasing water temperatures in the Middle Keys in 2018, but not in 2019 (Sharp et al, 2020). Within the present study, there appeared to be a threshold at approximately 2-3 • C weeks (DHW) of accumulated thermal stress where the disease stopped progressing, and thus the diseased material in the coral tissue was no longer sloughing off into the water potentially transmitting to new corals as supported by the low incidence values.…”

“…Within the present study, there appeared to be a threshold at approximately 2-3 • C weeks (DHW) of accumulated thermal stress where the disease stopped progressing, and thus the diseased material in the coral tissue was no longer sloughing off into the water potentially transmitting to new corals as supported by the low incidence values. These findings could be the result of other seasonal environmental differences not studied here, however, our study and others (Meiling et al, 2020;Sharp et al, 2020) indicate that temperature may have an important influence on SCTLD dynamics. Furthermore, from our observations of physiological signs of stress (bleaching and/or paling) we did not find evidence of thermally stressed corals being more at risk to SCTLD.…”

“…However, Muller et al (2020) did not find a significant relationship between SCTLD occurrence and severity within reefs and sea surface temperatures in their study from May 2014 to December 2017. In fact, Meiling et al (2020) showed a decrease in tissue loss rate associated with SCTLD as thermal stress accumulated prior to a bleaching event. Sharp et al (2020) also saw a decrease in SCTLD incidence and death rates during the 2018 coral bleaching season in the Middle Keys, but not during the 2019 coral bleaching season.…”

Variable Species Responses to Experimental Stony Coral Tissue Loss Disease (SCTLD) Exposure

“…In the field, M. cavernosa has the highest prevalence of SCTLD compared to other species (Brandt et al, in preparation), likely because it is one of the first few species affected, yet it is one of the slowest to experience full mortality. Case fatalities of marked M. cavernosa colonies were of the lowest across species in the USVI (∼40% over 5 months; Meiling et al, 2020). This may be because M. cavernosa is better at fighting the SCTLD infection compared to other species.…”

“…The M. cavernosa fragments in the present experiment had the lowest lesion prevalence and one of the slowest lesion progression rates. Variable SCTLD lesion progression rates (Meiling et al, 2020) and survival (Aeby et al, 2019) among colonies of M. cavernosa suggest intraspecies genotypic differences in disease resistance. In the field, M. cavernosa has the highest prevalence of SCTLD compared to other species (Brandt et al, in preparation), likely because it is one of the first few species affected, yet it is one of the slowest to experience full mortality.…”