Besides coral disease, natural disasters, and other anthropogenic sources of coral degradation, many of the threats to coral reefs are outbreaks of another kind: changes in the number of biotic threats, such as predators and competitors. One of the largest direct threats to coral was the exponential population growth of one of its major predators, Acanthaster planci, or the Crown-of thorns Starfish. An equally harmful, but indirect threat to coral, was the die-off of one of coral's major allies, Diadema antillarum, or Black Sea Urchins, which keep coral competitors, such as fleshy algae, in check.
Black Sea Urchin Mass Mortality
D. antillarum tend to remain in more sheltered aquatic habitats, such as depressions in coral, during the day and migrates out to more open seagrass areas to feed during the night. The sea urchin has established itself as a major herbivore in reef and seagrass habitats. Its preferred food appears to be benthic algal turf and macroalgae, but it's diet is wide and unrestricted.
Starting in 1983, D. antillarum experienced a two-year period of mass-mortality with some areas losing up to 97% of their black sea urchin populations. Presence of a disease among the sea urchins was first noticed along the Caribbean coast of Panama. . Affected organisms initially developed an accumulation of sediment, lost most of their dark pigment, and often had their spines break off. As the mysterious disease progressed, the urchins did not seek shelter during the day, were unable to remain attached to their substrates, and eventually became easy targets for predacious fish looking for an accessible meal. The source of the disease has been linked to a waterborne, host-specific pathogen that impacted Caribbean sea urchins as it traveled with ocean currents and in the ballast water of ships crossing the Panama Canal. In one 1985 study, healthy urchins kept in an aquarium were exposed to seawater, developed diseased symptoms, and eventually died. Two types of Gram-positive Clostridium bacteria were cultured from these urchins, suggesting the bacterial basis for the disease was from the highly aggressive and resistant Clostridiaceae family. 
The survival of these urchins is critical to the survival of coral, as the sea urchins provide the herbivorous control of algae that compete with the coral for habitat space, sunlight, and nutrients. The period of D. antillarum die-off was followed by a dramatic increase in the number of fleshy benthic algae. In Jamaican reefs, algae biomass increased 50% in just 2-weeks following the sea-urchin die-off, and led to an over-all 40% reduction in coral cover over the next 10 years.  While it is clear that this reduction in 'top-down" control of algae can have detrimental effects on coral reefs, the "bottom-up" impacts on algae can not be forgotten. Increased run-off from farming results in excessive nutrients entering ocean habitats, supplying algae with all the supplements they need to grow and thrive. While the disease outbreak on D. antillarum had a more dramatic and sudden impact on algae, and therefore coral, its survival is continually compounded by human pollution.
2022 Mass Mortality Event
In February of 2022, the Virgin Islands Department of Planning and Natural Resources reported that "the territory is experiencing an unusual die-off event of Long Spined Sea Urchins", based on reports from local dive shops.  As word spread, reports started coming in from other islands in the Caribbean. By April 2022, concern was growing about the unusual die off, and scientists expanded the die-off event to include other species of urchins, including the rock-boring urchin and the West indian sea egg.  Even more worrisome, the die-offs of D. antillarum were extremely rapid - on the order of 1-2 days.
An international team of scientists including the US Geological Survey (USGS) "investigated the cause of this mass mortality event through combined molecular, biological, and veterinary pathologic approaches to identify the pathogenic microbe." They found "that a scuticociliate that is most similar to Philaster apodigitiformis is the organism responsible for the mass mortality of these urchins." The team is now "actively testing potential treatments on the ciliate in the lab, working to develop field deployable options that reef managers can use."  Their findings are available in Science Advances at https://www.science.org/doi/10.1126/sciadv.adg3200#sec-3
Growth of the Crown-of-Thorns Starfish
The Crown of Thorns Seastar (COTS) is a ravenous invertebrate predator of the Pacific Coral Reef. The Crown of Thorns Seastar ranges from the Tropical Indian and Western Pacific Oceans to Panama and parts of the Gulf of California. COTS feed primarily on corals, destroying them with digestive enzymes. The manner of consumption displayed by COTS characterizes them as extraoral eaters. As extraoral eaters, COTS evert their stomach over their prey and release digestive enzymes to dissolve their food. COTS have a particular preference for branching corals, especially acropods. This predatory seastar is noted for its unrestricted, carnivorous diet. While COTS outbreaks have been characterized as “slow and methodical,” the starfish posed a major threat to an area known as the Coral Triangle in 2012, one of the richest regions in terms of biodiversity.
COTS are a free-spawning, sexually reproductive species. Sexual viability is largely temperature dependent. Under suitable conditions, COTS can produce between 12 million to 60 million eggs per spawning season. At the juvenile or larval stage, COTS spend weeks in plankton-rich environments. Agricultural runoff from crop and livestock industries increases the food supply for juvenile COTS. Increased terrestrial runoff leads to increased presence of phytoplankton nutrients, meaning better larval survival and increased COTS populations. At about 2 years, COTS reach sexual maturity and may begin to reproduce. In their adult stage COTS are typically 25cm to 25cm in diameter, and have anywhere from 7 to 21 arms. These arms are covered in toxic spines, and serve as a primary defense for the seastar.
COTS outbreaks first erupted in the 1950s, and have even had a significant impact on the Great Barrier Reef. From the 1960s to the 1980s, COTS outbreaks plagued regions ranging from the Red Sea to the Indian Pacific Ocean and French Polynesia. Over-collecting and over-fishing of the predators of COTS, as well as a decline in predator populations due to habitat destruction (mostly by human interference), are considered major contributors to COTS outbreaks. In order to manage outbreaks, a variety of interventions have been implemented. Manual removals, while labor-intensive, have proven successful. Starfish injections are another control measure that has been used to effectively reduce COTS populations. Introduction of predator species such as certain types of fish (some pufferfish, triggerfish, and the humphead wrasse), Triton’s trumpet (a large gastropod mollusk), and other creatures may also help mitigate COTS outbreaks. However, concerns have arisen about how the introduction of these predators could affect the ecosystem as a whole. Other measures to alleviate outbreaks include regulating and restricting industrial and agricultural development along the reef in order to reduce run-off pollution promoting phytoplankton as a source of food for COTS. Many advocates promote investing in programs such as Project Catalyst in an effort to further responsible and sustainable agricultural practices. 
Due to lack of standardized sampling measures, it is often difficult to determine the usual or normal population size of the COTS. However, COTS outbreaks are being closely monitored to assess the increase in population density. Surveillance is important in managing COTS populations. Although outbreaks vary in size, most led to a coral mortality of 50% and greater. A normal, individual Crown of Thorns Seastar consumes between 5m2 and 6m2 of coral per annum. In outbreak terms, this translates to 6km2 of reef being consumed per year. While COTS outbreaks impact community species composition, composition also affects the recovery process in the aftermath of an outbreak. Debates have sparked as to whether these outbreaks are naturally-occurring cycles of loss and recovery in the coral reef ecosystem- to be expected in competitive interactions and between predator and prey- or unnatural, adverse results of pollution and ecosystem disruption by humans.
- Loya, Yossi. Coral Health and Disease. Springer-Verlag; New York NY, 2004
- Birkeland, Charles. Life and Death of Coral Reefs. Chapman & Hall; London UK, 1997
- Engman, James. Mass Mortality in Diadema antillarum: A Large-Scale Natura. Henderson State University; Arkadelphia AK, 2002
- Staff writers. "USVI Experiencing "Unusual Die-Off" of Sea Urchins; DPNR Asking Public to Report Cases". The Virgin Islands Consortium. Published on 02/24/2022. Accessed online on 4/21/2022 at https://viconsortium.com/vi-community_center/virgin-islands-usvi-experiencing-unusual-die-off-of-sea-urchins-dpnr-asking-public-to-report-cases
- Staff writers. "Mass Sea Urchin Die-off In USVI, Several Other Caribbean Islands Not Seen Since 1983 is Threatening Already Fragile Coral Reefs, Scientists Warn". The Virgin Islands Consortium. Published on 04/18/2022. Accessed online at https://viconsortium.com/vi-environment/virgin-islands-mass-sea-urchin-die-off-in-usvi-several-other-virgin_islands-islands-not-seen-since-1983-is-threatening-already-fragile-coral-reefs-scientists-warn
- St. Petersburg Coastal and Marine Science Center. "USGS and international team identify the pathogen responsible for 2022 die-off of long-spined sea urchins (Diadema antillarum) across the Caribbean". USGS. April 19, 2023. https://www.usgs.gov/centers/spcmsc/news/usgs-and-international-team-identify-pathogen-responsible-2022-die-long-spined. Accessed 4/24/2023.
- De'ath, Glenn. The 27–year decline of coral cover on the Great Barrier Reef and its causeshttp://www.pnas.org/content/109/44/17995.full.pdf+html Proceedings of the National Academy of Sciences; Townsville Australia, 2012
- Starfish Outbreak Threatens Coralhttp://www.sciencedaily.com/releases/2008/01/080114112308.htm Science Daily-Wild Conservation Society; 2008
- Predator Crown-of-Thorns Starfish (Acanthaster planci) Outbreak, Mass Mortality of Corals, and Cascading Effects on Reef Fish and Benthic Communitieshttp://journals.plos.org/plosone/article?id=10.1371/journal.pone.0047363; 2012