Sea Urchins: Dying off?

If you have ever snorkeled or visited the touch tank at your local aquarium, you've likely seen a sea urchin. They have a spherical or slightly flattened body, which is enveloped in a hard exoskeleton. The entire surface of the sea urchin's exoskeleton is covered with numerous long, sharp spines, often arranged in a symmetrical pattern, radiating outwards from the center of the body. The spines can vary in color, including shades of brown, black, purple, green, or even red, depending on the species. Sea urchins typically inhabit shallow tropical waters and feed on algae that would otherwise destroy coral reefs and seagrass meadows.

In 1983-1984, there was a massive die-off event in which 98% of the long-spined sea urchin (Diadema antillarum) population was annihilated. Within just six months of this event, local coral reefs became dominated by algae. The cause of this mass extinction event has never been determined; coral reefs and sea urchin populations still have not yet fully recovered, even 40 years later. As recently as 2021, coral cover throughout the Caribbean was approaching the lowest values ever recorded in modern times.

In 2022, there was yet another mass fatality event, killing an estimated 95-98% of the long-spined sea urchin population in impacted areas. The area was widespread, starting in the U.S. Virgin Islands and stretching throughout the Caribbean, covering over 1,000 miles before finally reaching the Florida Keys. The affected sea urchins would lose their spines, and then in a matter of days, they would die.

Mya Briebart, a Distinguished Professor at University of Southern Florida's College of Marine Science, assembled a small team of scientists to search for why this was happening. Being able to quickly identify and treat the cause was imperative for preserving the remaining sea urchins and maintaining a balance in the coral reefs. Once the team examined sea urchins collected from 23 sites in the Caribbean, they then used a series of genomic and pathological techniques to confirm what has killed so many of these sea urchins. "At the time, we didn't know if this die-off was caused by pollution, stress, or something else - we just didn't know," said Ian Hewson, an expert in diseases that caused mass die-offs of starfish from Cornell University.

Briebart's team took just four months to determine that the cause of these deaths was a specific ciliate, called scuticociliate. So, what are ciliates? They are single-celled organisms that are covered in hair-like structures called cilia that help them move and eat. They are found almost anywhere there is water, and most are not disease-causing agents. This particular ciliate, the scuticociliate, has been implicated in die-offs of other marine species, such as sharks, in the past. The research team's findings were published in Science Advances.

While pleased with what they have learned, the team's list of remaining questions is still long. Christina Kellogg, a microbiologist from the USGS in St. Petersburg, FL, explained, "One theory we have is that the ciliate grew well under high-productivity conditions that were observed in the Caribbean when the die-off first started. We're also curious about the fact that there is some overlap in some geographic areas where this die-off occurred and where corals are declining from stony coral tissue loss disease." One large remaining question is if this ciliate can infect other sea urchins or other sea creatures.

Even though the culprit has been found, the full extent of the 2022 D. antillarum extinction event is not currently known. Without sufficient grazing pressure from sea urchins, we can expect to see coral reef degradation and loss of biodiversity, ultimately impacting the resilience and functionality of entire marine ecosystems.