The reefs remained virtually unchanged over the next 10 years. Every dive into this world of beauty was rewarded with a visual display, through clear and clean water, of shoals of rainbow-coloured fish, healthy corals, and when lucky, a glimpse of an apex predator such as a Great Hammerhead or Tiger Shark, as well as schools of Lemon Shark. 

When the big event of 1983 first occurred, none of us realised. The late spring and summer days were marked by an intense greyish-yellow haze that would extend from horizon to horizon over the South Atlantic Ocean and the Caribbean Sea. We all thought it was the humidity of summer and didn’t really pay attention to the darkened hazy skies. 

Over the next few weeks and months, we divers began noticing that the Long Spined Black Sea Urchins were losing their spines. Then, over the next few weeks and months, we slowly noticed the gradual depletion. These Sea Urchins – once so abundant they appeared as vast carpets of spikes – were disappearing right before our eyes. At the time, we didn’t realise how serious this was or indeed quite how mysterious. Instead, we gave thanks for not having to deal with Diadema and their pesky spikes that would pierce our fingers. We were – back then – ignorant to the oncoming catastrophe this depletion was warning us of. 

Over time we began to notice that the once brilliant oranges and golds of the Staghorn and Elkhorn Corals had begun to fade. Before our eyes, they had started to turn a ghostly white. Bleached. Gradually after that, we observed broken coral branches and appendages begin to pile up. As these piles increased in size, the Elkhorn and Staghorn Corals disappeared. Concurrent to this massive destruction, the giant colonies of Brain Coral, with their brilliant greens, vibrant blues, and glowing purples, faded before, also, turning white. 

This – in 1983 – was the first wave of massive coral bleaching. It stretched across our Florida waters and into the vast Caribbean Sea. In response, the authorities began to throw around a collective of theories as to the cause, among them ‘excessive heat’, ‘pesticides’, ‘sun block’, and other pollutants. 

The main school of thought was that there was indeed a correlation between the yellow-grey haze that we were seeing daily, the resultant disappearance of the Long Spined Black Sea Urchins, and the consequential loss of sea fans, staghorn, elkhorn, brain corals and coral bleaching. It was hypothesized that the huge African dust clouds were loaded with pesticides (used to kill a mass migration of locusts in North Africa), as well as large amounts of nutrients including iron, phosphorous, and sulfates. These nutrients were considered responsible for massive algae overgrowths throughout the South Atlantic and Caribbean Seas. In addition, the huge waves of Sahara dust cause human respiratory issues, allergic reactions, and eye irritation.  

Diadema antillarum plays a vital role in the maintenance and health of Atlantic and Caribbean coral reefs. Diadema grazes on algae, which is the main competitor of coral for light and space on the reef. This critical grazing and consumption of algae clears algae substrate from the reef, allowing juvenile coral – floating in the currents – to settle and grow. Diadema is exceptionally efficient in grazing over the reefs and rasping away algae mats and preventing the algae from blocking sunlight and suffocating the slow-growing young coral. But where had the Diadema gone?  Why were they dying off and disappearing? Was there a virus, bacteria, mold, or some other pathogenic entity? 

Over the next 20 years, I noticed that with less Sahara dust days, and simultaneous growth of the antipollution movement, the reefs began to slowly appear more alive and once again began to grow healthier corals. There also appeared to be a slow comeback of Diadema, but only a small fraction of the abundance of the species seen prior to 1983. Visibility improved, the shoals of reef fish grew larger, and our waters seemed to be slowly self-healing and recycling. 

Then came 2022 and the return of massive clouds of Saharan dust. A mass mortality of Diadema was first documented in the United States Virgin Islands.  Within five months the die-off had spread for thousands of miles throughout the Caribbean Sea and Florida resulting in more than 95 per cent of Diadema populations dying within a few weeks. 

Within a few months, a large multidisciplinary group of scientists from around the world made an exciting finding. They discovered that a ciliated organism (covered with hair like structures), Philaster apodigitiformis was the culprit. It is a member of a large species of parasites – with some 8,000 members – called Ciliates. They are also a well-known fish parasite.

This Philaster infection first caused the Diadema to lose their spines before slowly disintegrating until they died. Once again, the coral reefs were left unprotected from the onslaught of algae and resulting suffocation.

To prove the relationship of the loss of Diadema on the reef secondary to infection with the disease-causing pathogen, the ciliates, scientists in Florida compared healthy and sick Diadema. They identified that the Philaster species was present within all the sick sea urchins and not present in any of the healthy ones. They then isolated the ciliate parasite and exposed healthy sea urchins to it. The newly exposed sea urchins demonstrated the same disease symptoms – the loss of spines, disintegration, and death. Scientists then identified the ciliates, Philaster apodigitiformis, infecting the previously healthy sea urchins. All of these Diadema died. 

It was the proof they needed that these ciliates were indeed the primary pathogen causing the massive die-offs of 2023. Perhaps they had even been a contributing factor of the 1983 die-off? Considering that biomolecular testing was not available in 1983, it remains something we can only hazard a guess at.

Now the identity of the infectious parasite is known, scientists in the United States – and those around the world – are working on how to manage the current crisis  to prevent future outbreaks in Long Spined Black Sea Urchin populations, the subsequent overgrowths of algae, and the resultant coral bleaching. We can all hope that, once again, we begin seeing carpets of Diadema spread out on our coral reefs and keeping them vibrant, colourful, and healthy.

Gary Rose MD has been a certified diver for over 52 years and is a PADI Open Water Instructor. As a Plastic Surgeon and former Associate Professor of Microbiology and Surgery at the College of Medicine at Florida Atlantic University, he has fulfilled his life passion as a marine biologist with his research on marine microorganisms, as well as large ocean apex predators.

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