- Once abundant in the Caribbean, long-spined sea urchins (Diadema antillarum) experienced a mass die-off in the early 1980s, contributing to coral reef deterioration.
- Another die-off occurred in 2022, leading to a further decline in coral reef health in some parts of the Caribbean.
- A new study has identified the culprit: a parasite called a ciliate that took over the sea urchins’ bodies and quickly killed them.
- While researchers are still trying to determine how this disease is transmitted, they say it’s possible that climate change played a role.
In the warm waters of the Caribbean, long-spined sea urchins — inky black creatures resembling pincushions stuck with knitting needles — gobble up algae with their claw-like mouths. But when the sea urchins aren’t around, this algae will smother coral reefs, eventually causing the corals to die.
That’s what happened in the early 1980s. A mass die-off of long-spined sea urchins (Diadema antillarum), caused by an unknown pathogen, led to a 98% decline in the species in the Caribbean, which, in turn, led to a deterioration of coral reefs. Over the next 40 years, the sea urchins slowly rebounded in some parts, although only up to about 15% of their original population. But then, in 2022, the sea urchins died off again — and scientists have finally figured out why.
In a new paper published in Science Advances, a team of international researchers posit that a single-celled organism known as a ciliate — Philaster apodigitiformis — is the parasite that sickened and killed the urchins in 2022. They even suggest this ciliate could have been responsible for the die-off in the 1980s.
The researchers used molecular analysis to compare samples of affected and unaffected urchins collected from 23 sites across the Caribbean. They found that the diseased sea urchins contained more abundant quantities of the parasite than the healthy ones. Lab tests showed similar results: when healthy lab-grown sea urchins were exposed to the same parasite, they developed the same symptoms as the wild sea urchins.
Sea urchins infected with the disease lost their spines and their ability to move with their tube-like feet, according to the study. Within days, they would die.
Study lead author Ian Hewson, a marine ecologist at Cornell University in the U.S., said the ciliates likely killed the sea urchins by taking over their bodies.
“What we know is that the ciliate appears to colonize the hollow spines of the urchin, then it can be found in the outer skin, followed by the inner part of the animal called the coelomic cavity,” Hewson told Mongabay in an email. “We do not yet understand how the ciliate gains entry to these tissues, but it is possible that the spines serve as enrichment sites for the species, then it has more or less direct entry into the soft tissues underneath. However, we do not have firm data as yet on mechanisms of pathogenesis.”
Hewson said the study’s findings were surprising for two reasons: first of all, ciliates are known pathogens that affect some fish and crustaceans, but have not previously been associated with mass die-offs. Second, he said, the findings are unexpected because ciliates can actually coexist with sea urchins, forming symbiotic relationships, and most of the time they don’t cause disease.
Hewson said he and his colleagues currently don’t have “any data to support mechanisms of transmission.” However, he said climate change might play a role.
“Climate change has potential to affect the interaction between host and pathogenic agent, for example by reducing host immunity to the pathogen, magnifying the pathogen in the environment, or changing the virulence of the pathogen itself so that it can infect more deleteriously,” he said.
Don Levitan, a professor at Florida State University who specializes in ecology and evolution of marine invertebrates, but was not involved in the research, said that before the sea urchin die-off in the 1980s, D. antillarum was the “most important grazer on most reefs in the Caribbean” because of its ability to create space on reefs for coral settlement, which supported coral growth.
While long-spined sea urchins never recovered in some parts of the Caribbean, areas in which sea urchins did rebound “were hit hard by this 2022 mass mortality,” Levitan said: “Diadema is now rare on these reefs, and the algal-free refuges for coral settlement have disappeared.”
Levitan praised the study for identifying the pathogen responsible for the mass die-off of long-spined sea urchins.
“By identifying this pathogen, it [creates] the potential to sample sea urchins and the local environment to see if outbreaks are likely to occur,” he said. “It would be important to collect more data on what environmental conditions are associated with this pathogen, and I am curious as to why [it] only seems to affect Diadema.”
While the transmission of the sea urchin disease is still being investigated, Hewson said he and his colleagues are looking into treatments for the condition.
“[This] will aid in addressing affected sites, and will be of particular interest to scientists who are involved in restocking efforts,” he said. “Finally, it should be noted that the disease has co-occurred with other severely detrimental coral diseases, which highlights the stress that climate change and wider anthropogenic impacts may affect the coral reef ecosystem. Hopefully, this is additional impetus to act to mitigate these impacts.”
Elizabeth Claire Alberts is a senior staff writer for Mongabay. Follow her on Twitter @ECAlberts.
Banner image caption: A juvenile long-spined sea urchin (Diadema antillarum). Image by August Rode via Flickr (CC BY-NC-SA 2.0).
Citation:
Hewson, I., Ritchie, I. T., Evans, J. S., Altera, A., Behringer, D., Bowman, E., … Breitbart, M. (2023). A scuticociliate causes mass mortality of Diadema antillarum in the Caribbean Sea. Science Advances, 9(16). doi:10.1126/sciadv.adg3200