This is why urchins are dying across the Caribbean

It has to do with a microorganism called a scuticociliate.
Loukia Papadopoulos
Sea urchins are dying.jpg
Sea urchins are dying.

Michael Ziegler/iStock 

Sea urchins have been mysteriously dying off across the Caribbean since early 2022 and scientists may now know the reason: a single-celled marine microorganism called a scuticociliate.

This is according to a report by ScienceNews published on Wednesday.

Ian Hewson, a marine microbial ecologist at Cornell University, told the new outlet that the discovery is a little surprising given that “ciliates are not normally seen as agents of mass mortality" but that all roads lead to the organism, Philaster apodigitiformis, infecting the urchins, he said.

“In all of my years of investigating marine diseases, this is the one which we are 100 percent confident about," he added.

Hewson’s team analyzed the entire set of active genes of healthy and sick urchins from 23 sites across the Caribbean and found that some of the active genes in the sick urchins’ transcriptomes weren’t from the urchins but from the microorganism.

To investigate their theory, the scientists put a group of healthy sea urchins grown in a laboratory in a tank with some of the parasites. Six of the 10 urchins in the tank died.

The scientists are now busy looking for ways to prevent future losses. They are hoping to understand how the parasites spread so they can stop them.

More questions than answers

However, for now, the researchers still have more questions than answers. For instance, it remains unclear what conditions may have allowed P. apodigitiformis to become so dangerous to the urchins or how the organism causes infection.

Despite the fact that there are currently no available treatments for the disease, understanding the pathogen’s identity may lead to the development of possible prevention options. 

Hewson’s finds were reinforced by teams in Florida and Hawaii that observed the scuticociliates in the tissues and fluids of sick urchins located particularly in their body walls and at the base of their spines, according to the report by ScienceNews..  

The study was published April 19 in Science Advances.

Study abstract:

Mass mortality events have wide-reaching effects on marine ecosystem function and food web structure. The long-spined sea urchin, Diadema antillarum, experienced mass mortality in the Caribbean from 1983 to 1984, leading to declines of ~98% compared to premortality population densities. Ecologically functional D. antillarum population recovery was prolonged and estimated at ~12% of pre-event densities 30 years later. The loss of this herbivore contributed to a change in the competitive relationships between stony corals and benthic algae. In conjunction with other stressors, this mass mortality led to the rapid degradation of many coral reefs across the region. The cause of the 1980s D. antillarum mass mortality was never determined, and no specimens of affected urchins from that time exist in museum collections or other sample repositories. However, the spread of the mass mortality in the 1980s was consistent with the hypothesis that the causative agent was dispersed by water currents over long distances. Molecular biological approaches for the study of potential pathogens were not available at the time. Some urchin and other echinoderm mass mortality events have been ascribed to potential pathogens, but they have poorly understood etiologies that center on environmental stress, microbial dysbiosis, or some combination of these. Koch’s postulates have rarely been fulfilled for the few microorganisms proposed as associates of disease in echinoderms or any other marine invertebrate.