Scientists catch the suspect in long-term marine murder mystery

STRI/DICYT In the early 1980’s marine biologists at the Smithsonian Tropical Research Institute (STRI) in Panama observed dead and dying sea urchins (Diadema antillarum) near the Caribbean entrance to the Panama Canal. They alerted other marine labs and dive shops across the region—by snail mail, because this was pre-internet—and it soon became evident that a massive sea urchin die-off was underway. The cause was never identified, but the impact was enormous, as around 90% of sea urchins in the Caribbean were killed.

Sea urchins eat algae on the surface of coral reefs—like lawn mower bots. When they died the ecology of reefs changed completely: from reefs covered in live coral to reefs covered in algae. 50 years later, these sea urchin numbers are still not back to their numbers in the early 1980’s.

In 2022, a second major die-off occurred. But this time, scientists were prepared with new tools to solve this whodunit mystery. Divers and researchers from across the Caribbean sent samples of healthy and diseased sea urchins to Cornell University. Researchers there looked for genes and gene products that were present in the infected animals but not in the healthy ones: and one gene sequence stood out. It belonged to a group of microscopic organisms called ciliates that swim through the water using tiny hairs (cilia) on their bodies.

“Why was the identification of the pathogen possible in 2022, but not in 1983? There are many reasons,” says Harilaos Lessios, staff scientist at STRI. “One is the more recent availability of molecular techniques. Another is the development of expertise in marine microbiology and the realization that disease is an important factor in the ecology of many marine organisms. The internet also helps make information about outbreaks available as they occur and facilitates the exchange of information between scientists from different countries and different disciples.”

Specifically, researchers examining sea urchin tissues found tiny scuticociliates, already a known cause of disease in some fish.

But finding a disease-causing microbe in a dying sea urchin is not enough to confirm that it is the culprit. Based on his work with tuberculosis and anthrax in humans, in 1890 Nobel laureate Robert Koch published a process for confirming if an organism really causes disease. To complete Koch’s postulates, researchers had to show that the ciliate was in the tissues of the dying sea urchins, then they had to isolate and culture it, make healthy urchin sick, and then re-isolate it from the affected animals. In experiments led by Ian Hewson, marine ecologist at Cornell University, about half of the healthy sea urchins lost 5 to 20 spines when they were exposed to the ciliates.

It took 48 authors from 31 institutions to trace the mortality and do this work. They named the new disease D. antillarum scuticociliatosis. They discovered that it affects the sea urchin’s tube feet first, so they tend to fall over or detach from the reef, and then it attacks the base of the spines, which fall off. Eventually the ciliates invade the whole body of an urchin and cause death.

Ciliates seem to thrive in nutrient-rich shoreline habitats. The disease seems to originate in calm water ports and harbors. Then it appears to spread through the water across a wide space, perhaps in currents, with floating vegetation or carried by migratory fish or sea birds.

So now we know that the 2022 die-off was caused by this ciliate and possibly the 1983 die-off as well. For many reefs the 1983 sea urchin mortality was the kiss of death, as they were already suffering from overfishing and climate change.

“That Diadema recovered so slowly from the 1982-3 mortality was a surprise, and so was that it should start dying again thirty years later,” said Lessios. That all this could be documented shows the value of maintaining permanent research facilities, such as those at STRI, close to coral reefs. Caribbean research labs and the long-term observations they permit will make it possible to obtain answers to the many questions that remain. Is the ciliate present in other organisms without causing harm, yet kills Diadema when it invades? Is it perhaps in other oceans and causes havoc when imported by ship ballast water? Why does it break out when it does? How does it spread over such wide distances?”