The Hidden Parasite Wreaking Havoc on Caribbean Sea Urchins

A team of scientists have traced the cause of the widespread death of long-spined sea urchins in the Caribbean Sea to a specific parasite. The sea urchins are instrumental in marine ecosystems, keeping algae levels in check and preventing it from overwhelming and killing coral. This epidemic has led to a shocking 85%-95% decrease in the sea urchin population in affected areas. Although methods for treating the parasite are yet to be established, knowing its identity could inform strategies for health preservation of Diadema sea urchins reared for repopulation purposes.

The long-spined sea urchins, or Diadema antillarum, are vital for maintaining equilibrium in the marine ecosystem, primarily by grazing on algae which can suffocate and kill corals if uncontrolled. First reports of Diadema deaths were noted in St. Thomas, in the U.S. Virgin Islands, in late January 2022. By late March, similar incidents were reported in the Lesser Antilles, Jamaica, and the Mexican Caribbean. By June, the disease had spread to most of the Greater Antilles, Florida, and Curacao.

After an extensive investigation into the cause of the deaths, which saw the sea urchin population in affected areas decrease by 85%-95%, a group of 42 scientists from around the world were able to identify the parasite Philaster apodigitiformis as the culprit. This unicellular eukaryote belongs to a family of 8,000 species known as ciliates and is a recognized parasite in fish.

Ian Hewson, professor of microbiology at Cornell University and lead author of the study published in Science Advances, stated that this discovery provides a rare opportunity to understand marine disease events in detail, which can guide strategic health maintenance for Diadema sea urchins being bred for replenishment. He further mentioned that identifying the pathogen could help mitigate risks to unaffected Diadema.

During the 1980s, the long-spined sea urchin populations were almost completely eradicated in the Caribbean due to an unknown cause, resulting in nearly 98% decline. After three decades, their populations rebounded, but only by about 12% from pre-epidemic levels. This mass death led to the rapid degradation of many coral reefs across the region that persist today. Hewson and his colleagues may now investigate whether P. apodigitiformis could have been the cause of the 1980s outbreak.

In the current research, three types of Diadema samples were collected: visually abnormal, infected individuals; healthy individuals from the same site; and completely healthy individuals from an unaffected area. Quick sample collection from 23 sites was facilitated by the Atlantic and Gulf Rapid Reef Assessment program. Despite the complexities of customs and border regulations, these samples were sent to Hewson’s lab at Cornell, where advanced molecular biological and veterinary pathological techniques were used to identify viral or bacterial pathogens in the tissues.

Initially, the results did not reveal any unusual or candidate microorganisms. However, when Hewson decided to examine the genomic signals of eukaryotic microorganisms, such as fungi, ciliates, and dinoflagellates, the signal of the ciliate Philaster became clearly evident. Under a microscope, Hewson confirmed that the ciliate was highly abundant in fluid samples from infected Diadema but absent in control samples.

This is the first instance where P. apodigitiformis has been linked with mass deaths in an invertebrate. To further solidify their findings, the team designed an experiment to test Koch’s Postulates, the gold standard test to determine a microorganism’s association with a disease. Using fresh samples collected from infected Diadema and aquarium-raised Diadema (which had never been exposed to any pathogens), they infected the latter with ciliates isolated from the infected sea urchins. The infected Diadema exhibited symptoms of sickness and eventually died in 60% of the cases. They were then able to isolate the very same P. apodigitiformis ciliate from these newly diseased animals, thereby confirming it as the disease-causing agent.

The research was funded by the National Science Foundation, the Cornell Atkinson Center for Sustainability, the National Oceanic and Atmospheric Administration, and the National Fish and Wildlife Foundation.

For further details on the study:

Scientists Unmask the Microscopic Menace Behind Massive Sea Urchin Die-Off
Murder Mystery in Marine World: Unmasking the Sea Urchin Assassin

Reference: “A scuticociliate causes mass mortality of Diadema antillarum in the Caribbean Sea” by Ian Hewson, Isabella T. Ritchie, James S. Evans, Ashley Altera, Donald Behringer, Erin Bowman, Marilyn Brandt, Kayla A. Budd, Ruleo A. Camacho, Tomas O. Cornwell, Peter D. Countway, Aldo Croquer, Gabriel A. Delgado, Christopher DeRito, Elizabeth Duermit-Moreau, Ruth Francis-Floyd, Samuel Gittens, Leslie Henderson, Alwin Hylkema, Christina A. Kellogg, Yasunari Kiryu, Kimani A. Kitson-Walters, Patricia Kramer, Judith C. Lang, Harilaos Lessios, Lauren Liddy, David Marancik, Stephen Nimrod, Joshua T. Patterson, Marit Pistor, Isabel C. Romero, Rita Sellares-Blasco, Moriah L. B. Sevier, William C. Sharp, Matthew Souza, Andreina Valdez-Trinidad, Marijn van der Laan, Brayan Vilanova-Cuevas, Maria Villalpando, Sarah D. Von Hoene, Matthew Warham, Tom Wijers, Stacey M. Williams, Thierry M. Work, Roy P. Yanong, Someira Zambrano, Alizee Zimmermann, Mya Breitbart, 19 April 2023, Science Advances.
DOI: 10.1126/sciadv.adg3200

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• Science Advances Journal
• National Science Foundation
• Cornell Atkinson Center for Sustainability
• National Oceanic and Atmospheric Administration
• National Fish and Wildlife Foundation