Killer responsible for sea urchin die-offs identified

Coral reefs, often referred to as the rainforests of the sea, are delicate ecosystems. They rely on a complex web of interactions among various organisms. One such critical species is the long-spined sea urchin, Diadema antillarum.

These urchins play a vital role in maintaining the health of coral reefs by consuming algae. This allows corals to flourish. A thriving coral population, in turn, supports healthy fish populations and has numerous positive impacts throughout the food chain.

In early 2022, scientists observed a sudden and alarming die-off of long-spined sea urchins in St. Thomas. Large numbers of urchins perishing at an alarming rate. Researchers quickly mobilized to identify the cause. They discovered that a microscopic parasite had invaded the urchins’ bodies and spines, consuming them from within.

The perpetrator turned out to be a scuticociliate, a microscopic organism most similar to the protozoan parasite Philaster apodigitiformis. This parasite rapidly spread throughout the Caribbean, decimating sea urchin populations. Within days of showing symptoms, the affected urchins succumbed to the infection. Within months, nine more locations across the Caribbean, including off the Florida coast, reported losses.

Why scientists consider this situation to be urgent

Don Behringer is a professor at the University of Florida and leader of a National Science Foundation RAPID grant. He explained the urgency of the situation. “The research team was still processing samples from the last site where a die-off occurred when we would get calls about a new location with dying urchins. It only took a couple weeks for the majority of the long-spined urchins to be wiped out at a specific site. Rapid-response funding like this allows us to go to locations to sample and assess environmental conditions quickly and learn from it.”

Mass mortality events of this magnitude can have devastating and long-lasting effects on marine ecosystems. The recent die-offs are reminiscent of a similar event in 1983. Within 13 months, sea urchin losses reached 98 percent during that year. Scientists never determined the cause of that die-off. This left open many questions unanswered about how to protect coral reefs from future catastrophes.

In the decades since the 1983 event, some coral reef systems have struggled to recover. Reports indicate that urchin populations at affected reefs have only reached 12 percent of their pre-die-off numbers. This demonstrates the lasting impact of such events on these fragile ecosystems.

Professor Ian Hewson of Cornell University emphasized the importance of swift action when dealing with mass die-offs: “We had to act very fast. You really have to act within a week or two, or you’ll lose your chance. These mass die-offs usually blow through extremely fast. Sometimes if you get there too late, you’ll only be left with diseased animals and won’t even know what ‘normal’ looks like.”

How researchers found the culprit

It didn’t take long for researchers to identify the microscopic parasite responsible for the alarming die-off of long-spined sea urchins in the Caribbean. The scientific community was quick to react. They conducted a series of experiments to validate their findings and better understand this deadly pathogen.

The researchers first discovered the parasite in fluid samples taken from the urchins’ bodies, similar to a blood sample. They then isolated the pathogen, allowed it to multiply, and sought to confirm its role in the urchins’ deaths in a controlled environment. 

Don Behringer, a key researcher in the study, said, “We were really lucky to have access to urchins that were raised in a controlled environment and that we knew had not been exposed to the ciliate.”

Challenges of raising sea urchins in captivity

Rearing urchins in aquaculture environments is a challenging task. However, UF/IFAS associate professor of restoration aquaculture Josh Patterson, in collaboration with The Florida Aquarium, has successfully hand-raised these animals. 

Patterson’s primary goal is to release urchins into the wild to help restore coral reefs. However, in this case, the healthy urchins proved invaluable in validating the researchers’ findings.

Patterson explained the importance of these lab-raised urchins: “When this disease went through the Caribbean, it was impossible to know which urchins pulled out of the water were exposed to the parasite. We had cultured urchins in a tank that were naïve, known to be uninfected, that could help confirm what was causing the mass deaths of urchins in the wild.”

Researchers took the healthy urchins to the University of South Florida. They were then infected with the ciliate. Within four days, they began showing signs of illness, confirming the parasite as the culprit. 

“Other parasites similar to this one are known to cause disease in other organisms but have not been implicated in urchin disease outbreaks, in the Caribbean or elsewhere,” said Behringer. “It appears to act in a micropredation mechanism where it swarms the urchins and starts multiplying and rapidly eating away at them.”

Where did this parasite come from?

The reason behind the parasite’s sudden emergence and voracious nature remains unknown, but researchers hope to answer these questions in future studies. The findings have raised further questions about the long-term effects of such die-offs on coral reefs.

Regarding the 1983 mass mortality event, no tissue or samples from the affected urchins are available for comparison. Despite this limitation, the knowledge gained from the 2022 die-off can help conserve populations in the future.

Behringer said, “We documented current algae coverage, urchin abundance, and other species present before, during, and after the die-offs. We can use this information as a baseline from which we can compare a year, two years, five years, ten years, and beyond. It helps us create a clearer picture of the impact urchin loss has on the condition of the reefs and the broader reef community. We’re fortunate we had the opportunity to collect the data we did.”

The day the die-offs stopped

By December 2022, the die-offs appeared to have stopped. People reported seeing new urchins in some areas, indicating recovery. However, recent reports of dying urchins in the Cayman Islands and the U.S. Virgin Islands have raised concerns. 

“We cannot say for sure if it is the return of the same parasite, but it appears ominous,” said Behringer, “The previous die-off was extremely consequential for the reefs that were impacted, and some never recovered. This time we know the culprit and are trying to figure out how and why it emerged.”

As scientists continue to investigate the cause of this mysterious and devastating event, the urgent need for effective conservation and management strategies becomes increasingly clear. The future of coral reefs – and the diverse array of species that depend on them – hangs in the balance.

This project would not have been possible without the support of many, including the funding agencies, the National Science Foundation, Florida Sea Grant, National Oceanic and Atmospheric Administration and the National Fish and Wildlife Foundation. Special thanks to the project partners including the University of South Florida, University of the Virgin Islands, Virgin Islands Government and many more.

More about sea urchins

Sea urchins are fascinating marine creatures belonging to the phylum Echinodermata, which also includes starfish, sea cucumbers, and sand dollars. 

Their radial symmetry characterizes these spiny organisms. This means their body parts are arranged around a central axis. Here are some interesting facts about sea urchins:

Anatomy

Sea urchins have a hard, round exoskeleton called a test. It is made up of calcium carbonate plates. Spines cover the test, serving as protection against predators and helping with movement.

Their mouth, known as Aristotle’s lantern, is located on the underside and has five sharp teeth for grazing on algae and other organic materials.

Habitat

Sea urchins inhabit oceans all around the world, from shallow coastal waters to deep-sea environments. They inhabit various types of seafloors, including rocky shores, coral reefs, and seagrass meadows.

Diet

Sea urchins primarily feed on algae, making them important grazers in marine ecosystems. Some species also consume detritus, dead animals, and even small invertebrates.

Reproduction

Sea urchins reproduce through external fertilization. Males and females release sperm and eggs into the water, where fertilization occurs. The fertilized eggs develop into free-swimming larvae, which eventually settle on the seafloor and metamorphose into adult sea urchins.

Ecological role

Sea urchins play a vital role in maintaining the health of coral reefs and other marine ecosystems by grazing on algae. This prevents algae from overgrowing and smothering corals, ensuring a balanced ecosystem.

Predators

Sea urchins have various natural predators, including sea otters, sunflower stars, fish, and humans. Some regions consider sea urchins a delicacy and harvest them for their edible roe (gonads).

Species diversity

There are about 950 species of sea urchins, varying in size, color, and spine length. Some species, like the long-spined sea urchin (Diadema antillarum), have long, venomous spines, while others, such as the green sea urchin (Strongylocentrotus droebachiensis), have shorter spines.

Lifespan

Sea urchins have a diverse range of lifespans, depending on the species. Some may live for only a few years, while others can live for several decades.

Regeneration

Sea urchins have remarkable regenerative abilities. They can regrow lost spines and damaged internal organs, and some species can even regenerate their entire bodies from just a small piece of tissue.

Symbiotic relationships

Sea urchins often engage in symbiotic relationships with other marine organisms, such as shrimps and crabs. These smaller creatures seek refuge among the urchin’s spines, gaining protection from predators, while the urchin benefits from the removal of parasites and debris.

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