The long-spined sea urchin, Diadema antillarum, is a marine invertebrate characterized by its distinctively long, slender spines that can extend up to 30 centimeters. Its test is typically black, though some may be lighter or white. Juvenile urchins are identifiable by their black and white banded spines. These urchins primarily inhabit shallow tropical waters, including coral reefs, seagrass beds, and rocky bottoms, across the Atlantic Ocean, particularly throughout the Caribbean Sea and Gulf of Mexico. They reside at depths of 1 to 10 meters, seeking shelter in reef crevices during the day and emerging at night to feed.
The Long-Spined Sea Urchin’s Vital Role
Diadema antillarum is important for Caribbean coral reef ecosystems. It is a primary herbivore, consuming various types of algae, including turf algae, macroalgae, and even endolithic algae that bore into coral skeletons. Their grazing prevents algae from overgrowing and smothering corals, which compete for light and space on the reef.
This grazing activity helps maintain clear substrate for new coral polyps to settle and grow, thereby supporting coral recruitment and overall reef resilience. A density of at least five Diadema antillarum per square meter is often needed to effectively control algal cover on coral reefs. The urchins’ feeding also contributes to carbonate bioerosion, a process that helps shape the reef structure.
Unraveling Recent Mass Die-Offs
The Diadema antillarum population has experienced mass mortality events, with significant consequences for coral reefs. The most widely recognized event occurred between 1983 and 1984, when an unknown pathogen caused a Caribbean-wide die-off, reducing populations by 94-99% in many areas within a few weeks. This loss of their primary grazer led to dramatic increases in algal cover, fundamentally altering reef structures from coral-dominated to algal-dominated states. For instance, coral cover in Jamaica declined from approximately 50% to 3%, while macroalgal cover surged from about 4% to 90%.
A more recent mass mortality event began in January 2022, first observed in the U.S. Virgin Islands and rapidly spreading across the Caribbean. The symptoms observed in 2022 were strikingly similar to those from the 1980s die-off, including loss of tube feet control, slow spine reaction followed by spine loss, and necrosis of the epidermis. Research has since identified a ciliate, Philaster apodigitiformis, as the likely causative agent for the 2022 mass mortality. This discovery represents a significant step toward understanding and potentially mitigating future outbreaks.
Efforts to Restore Diadema Populations
Various research and conservation initiatives are underway to restore Diadema antillarum populations. These efforts include aquaculture, where urchins are raised in controlled laboratory environments from larvae to juveniles, bypassing early life predation stages. This approach allows for year-round production of urchins, with optimal growth at temperatures between 25 and 28 degrees Celsius.
Once grown, these laboratory-reared urchins are outplanted onto degraded reefs to enhance natural recovery. Other strategies involve translocating healthy urchin populations to areas of decline and researching factors influencing larval settlement rates. Challenges remain in achieving widespread recovery, as factors such as habitat complexity and continued disease threats can impede restoration success.