Disease
THE IMPACT OF STONY CORAL TISSUE LOSS DISEASE ON THE NORTHERN USVI
Photo: Sonora Meiling
Photo: Nicole Krampitz
Stony coral tissue loss disease (SCTLD) is a rapidly spreading disease in the USVI that is causing dramatic loss of coral cover and declines in species diversity. SCTLD was first sighted in the US Virgin Islands in January 2019 at the Flat Cay TCRMP monitoring site. The disease was identified by its characteristic stony coral species-specific susceptibility characteristics. By March 2019 the disease had spread to the north side of St. Thomas and by January 2020 had spread eastward to the St. Thomas East End Reserves.
Numerous local teams at the University of the Virgin Islands, the Department Planning and Natural Resources, the National Park Service, the Caribbean Oceanic Restoration & Education (CORE) Foundation and The Nature Conservancy have been coordinating monitoring, research, and response with national collaborators, such as the Florida Department of Environmental Protection, Woods Hole Oceanographic Institution, University of Texas Arlington, Louisiana State University, Mote Marine Lab, Rice University, and the National Oceanic and Atmospheric Administration. Updated details concerning SCTLD can be found at vicoraldisease.org
Detailed early impacts at TCRMP sites can be found in the site descriptions of the 2020 TCRMP annual report and are covered in more detail by Brandt et al. 2021. Since the 2020 monitoring period, SCTLD has spread to eastern St. John and as far as St. Croix. As of the 2020 TCRMP annual report, 85% of TCRMP locations have become impacted and are experiencing losses in coral species and cover. Ginsburg Fringe, Coral Bay, Buck Island STX, Great Pond, and Jacks Bay had no recorded instances of SCTLD during the 2020 monitoring surveys. The locations most adversely affected by SCTLD include Kings Corner, Seahorse Cottage Shoal, and Flat Cay with relative coral cover losses of 64.6%, 62.6%, and 61.5%, respectively, since before SCTLD arrived.
More detailed impacts at TCRMP sites can be found in the site summaries of the TCRMP reports, and other accounts of SCTLD research in the territory can be found in Meiling et al. 2020, Costa et al. 2021, and Meiling et al. 2021.
Photo: Kathryn Cobleigh
Photo: Viktor Brandteneris
Photo: Sonora Meiling
Monitoring locations unaffected by SCTLD have experienced relatively little change in coral cover since Hurricanes Irma and Maria (2017) to present. During their initial infection with SCTLD (“Year 1”), monitoring locations lost on average 14.1 ± 7.6% and 17.3 ± 3.1% coral cover in 2019 and 2020, respectively, relative to pre-SCTLD monitoring in 2017. It is also noteworthy that longer affected locations (“Year 2”) continued to lose coral cover after the initial appearance of SCTLD, resulting in average losses of almost 50% relative to pre-SCTLD monitoring. Coral species richness followed a similar pattern with large losses in number of species following initial appearance in 2019 and 2020 relative to 2017. However, the relative loss in number of species decreased during the second year, as the most highly susceptible species were extirpated in the initial wave of SCTLD.
MASS MORTALITY OF DIADEMA ANTILLARUM IN 2022 DIE-OFF EVENT
In late January of 2022, extensive die-offs of the long-spined sea urchin, Diadema antillarum, were first noticed off Flat Cay. Within a month, other reefs surrounding St. Thomas and St. John also experienced mass mortality of D. antillarum in what appeared to be a highly lethal disease event. Once affected, seemingly healthy individuals would begin to experience rapid spine loss and tissue necrosis. Following the infrastructure created to track Stony Coral Tissue Loss Disease, a large effort was very quickly coordinated to investigate and track this disease (Diadema Response Network: https://www.agrra.org/sea-urchin-die-off/). This response network tracked the disease’s rapid spread to nearby Caribbean islands and noted that the disease frequently first appeared close to harbors, suggesting primary dispersal has been through anthropogenetic mechanisms (Hylkema 2023). Mortality associated with this disease has an incredibly high rate (up to 99%; Hylkema 2023) and occurs very quickly, often within one to two days (AGGRA).
The causative agent for this disease has been identified as the scuticociliate Philaster apondigitiformis, and, in a rare occurrence for marine organisms, has fulfilled Koch’s postulates (Hewson 2023). Scientists from the University of the Virgin Islands contributed to the identification of the disease agent. The severity of this die-off event mirrors that which occurred in 1983-1984 when D. antillarum population densities were reduced by 93-99% in the Caribbean (Lessios 1988). Despite decades passing since that mass mortality event, recovery of these sea urchins has been incredibly slow; it is estimated that after over thirty years, population densities have only reached 12% of their pre-die-off densities (Lessios 2016). Consequently, the current mass mortality event is expected to reduce population densities to even lower than those seen following the 83-84’ die-off and, furthermore, recovery is expected to be very slow (Levitan 2023).
The loss of D. antillarum has detrimental effects on coral reef health due to the species’ key role in controlling algal growth. As seen following the die-off in 1983 and 1984, the removal of these herbivores promotes turf and macroalgal overgrowth on coral reefs (Carpenter 1988). This growing macroalgal dominance limits settlement space for juvenile coral settlement and increases competition with adult colonies.
Following the mass mortality event, TCRMP sites experienced a significant reduction in D. antillarum densities (Kruskal-Wallis rank sum; p=0.03) dropping from an average of 1.7 ± 0.87 urchins 100m-2 in 2021 to 0.24 ± 0.15 in 2022 (Figure 1). Furthermore, the number of sites where the long spined sea urchin has typically been observed in the past decade was halved to only 6 of the 34 surveyed sites in 2022. Of those urchins that were observed, the majority were small juveniles, whose functionality as a key herbivore in controlling algal growth would be limited.
Figure 1. Mean density of D. antillarum per 100m2 across all TCRMP sites from 2012-2022.
