The reef had a janitor. It died. Twice.

If you’ve dived the Caribbean in the last decade and thought there’s a lot of algae and not much coral — you weren’t wrong. That’s what those reefs look like today. And the reason isn’t the sunscreen you wore that morning. For years we have heard about oxybenzone causing the reefs to die. The reality is yes, sunscreen plays a role in coral bleaching - but it is not the only cause of death to the reef ecosystem. The reef is an ecosystem, it needs a diverse set of inhabitants. It needs herbivores in that ecosystem who clean algae off the reef and create space for coral to grow on the newly exposed surfaces.

The reef had a janitor. A sea urchin that goes by the name Diadema antillarum, long spines, mouth on the underside with limestone plates that scrape algae off hard surfaces. Alex Petrosino at the Florida Aquarium calls it the janitor of the reef and that’s the accurate job description. Before 1983, roughly one to two of these per square meter across healthy Caribbean reefs, grazing continuously, keeping algae in check and opening bare limestone where coral larvae could settle. That’s how the reef stayed growing, vibrant and resilient.

Then something killed them.

January 1983, Punta Galeta, on the Atlantic side of the Panama Canal. Diadema started dying. Over 13 months the die-off tracked ocean currents through 3.5 million square kilometers of Caribbean — Tobago, Bermuda, Jamaica, Florida, the Bahamas, Puerto Rico, all of it. Average mortality across the region: 98%! At some sites 99% of urchisn died! H.A. Lessios at Smithsonian Tropical Research documented the whole thing in Science in 1984. The pathogen was never identified. Forty years we still don’t know the exact cause of that urchin die off.

The populations never recovered. Thirty years late, average densities were only around 12% of pre-1983 numbers.

The Caribbean, unlike the Indo-Pacific, doesn’t have a large bench of backup grazers. Andy Bruckner at Florida Keys National Marine Sanctuary is direct about this — the Caribbean has far less redundancy. Parrotfish are the other major reef grazer, and by 1983 they were already overfished across most of the region. So the algae grew. It kept growing. Fleshy algae covers substrate faster than coral can grow, smothers surviving colonies, blocks new coral larvae from finding clean limestone. That radically transformed the reef ecosystem, the results are what we see today. The IUCN/GCRMN 2014 report — 35,000 surveys across 90 Caribbean sites over 40+ years — put coral cover down by more than half since the 1970s and named loss of grazers as the primary driver. Ahead of bleaching. Ahead of climate.

Then February 2022. A diver in St. Thomas posted photos of dying Diadema (urchins). Within months it had hit Barbados, Martinique, Dominican Republic, St. Lucia, Florida, Curaçao. Different pattern than 1983 — faster, geographically random, possibly moving in ship ballast water. This time researchers caught the pathogen: a single-celled parasitic ciliate, Philaster apodigitiformis, published by Hewson et al. in Science Advances in 2023. They named the disease D. antillarum scuticociliatosis. At one Saba site, 99% of urchins were dead within a month.

The math is brutal. 1983 dropped populations to about 2% of baseline. Forty years of poor recovery got most locations back to roughly 12% of pre-1983 levels. Then 2022 killed 95–99% of what remained. THe result is in most of the Caribbean the urchin population is effectively gone. There is almost nothing that can keep the algae at bay.

Which brings me to the thing that keeps getting written about Caribbean reef decline. The consumer-facing story is sunscreen, touching coral. But the actual reef story is a killer set of pathogens that destroyed the reef herbivores with virtually no backup herbivores in the ecosystem, on a reef system whose backup grazer was already fished out at industrial scale. That’s the shape of it. Two die-offs — one still unknown, one only just identified — and a decades-long algae takeover on a reef that had no herbivore redundancy to lose in the first place.

Sunscreen isn’t the only problem. We need to find ways to bring back balance and redundancy to the ecosystem.