By: Staff Writer
April 21, 2023
Scientists at the University of South Florida (USF) have discovered the cause of Caribbean sea urchins shedding their spines and dying off in Caribbean waters.
In a report published in the Journal of Science Advances, researchers at USF said a tiny single-celled parasite is to blame for the massive die-of.
“The case is closed,” said study author Mya Breitbart, a marine microbiologist at the University of South Florida.
In late January 2022, sea urchins, or Diadema antillarum, began experiencing a mass mortality event. The condition was first observed in St. Thomas in U.S. Virgin Islands (USVI) and, by late March, had been found at nine more insular Caribbean jurisdictions across the Lesser Antilles, Jamaica, and the Mexican Caribbean.
By June of 2022, the condition had also been reported in most of the Greater Antilles, Florida, and Curaçao. Gross signs in affected urchins began with detachment from vertical surfaces, loss of spine movement/reaction to stimuli, loss of tube foot control, using spines for locomotion instead of tube feet, and forming a symmetrical spine arrangement.
Within days, urchins with these signs, catastrophically lost spines leading to epidermal tissue loss and exposure of underlying test which progressed rapidly with two days to death. The loss of spines, in particular, led to predation by nearby fishes or invertebrates.
At sites in the USVI, large masses of turbid water accompanied the onset of the condition, and in at least four other sites (northern Puerto Rico, Aruba, Saba, and St. Eustatius), heavy rainfall preceded mass mortality.
For marine scientists, it was deja vu: Another die-off swept through the region in the 1980s and slashed sea urchin populations by around 98 percent.
That case was never solved. But this time, an international team of researchers jumped into action, taking samples from sick urchins and healthy ones across the Caribbean to look for genetic clues.
They didn’t see signs of viruses or bacteria, said study author Ian Hewson, who researches marine diseases at Cornell University. But they did spot traces of tiny single-celled organisms called ciliates, which only showed up in the sick urchins.
The report further warned, however, “Considering observations from natural infections, the transmission potential demonstrated in the challenge study, and observations in culture, we suggest that the Philaster-like ciliate represents a highly prolific pathogen capable of invading tissues of grossly normal urchins and causing mass morbidity and mortality under conditions that are not yet well understood.”
It’s possible that this same parasite also caused the die-off in the 1980s, but scientists can’t be sure, Breitbart said.
And they haven’t figured out a way to treat the diseased urchins. But they’re hopeful that knowing the source of the die-offs will help conserve the reefs, especially once they learn more about how the parasites spread, Breitbart said.
(A to D) Grossly normal specimens (A) exhibit upright posture and typically shelter beneath rock/coral (photo taken in Pope Point, St. John, April 2022). At condition onset, specimens fall over or stand on spines and exhibit drooping spines (B) (photo taken in Saba, Caribbean Netherlands, April 2022). Advanced stages present as spine loss with elevated predation by fish (C) (photo taken in Aruba, August 2022) and, eventually, death. These signs were also observed in specimens that were experimentally challenged with the Philaster culture FWC2 in aquaria (D). Photo credits: (A) and (D) I. Hewson, (B) A. Hylkema, and (C) D. Behringer.