Tobias Grun and Michal Kowalewski from the Florida Museum searched the ocean floor for sea urchins throughout the summer of 2020 by diving into the shallow waters off the Florida Keys. Sand dollars, sea biscuits, and heart urchins were hidden just beneath the surface, but they were discovered by them thanks to telltale trails and dimples in the silt.
Grun and Kowalewski traveled a 20-mile stretch of the Florida Keys’ coast from August to April of the next year, stopping at 27 different locations. By the time they were done, their sea urchin survey was among the most thorough that had been done in the area in recent years, and their findings are somewhat encouraging.
The number and variety of sand dollars, sea biscuits, and heart urchins appear to have remained relatively stable since researchers started keeping track of their populations in the 1960s, according to an analysis of the survey the researchers published last week in the journal PeerJ.
“It was a pleasant surprise to find that they’re still widespread and abundant,” said study co-author Kowalewski, the Florida Museum Thompson Chair of Invertebrate Paleontology. “The Florida Keys are heavily impacted by human activity, with fishing, tourism, and diving all occurring on a massive scale. On top of that, coastal ecosystems are subject to climate change, increasingly strong hurricanes, and escalating stressors resulting from continuous urban development.”
Sea urchins are essential for healthy marine ecosystems
Echinoderms, or sea urchins, get their name from a combination of Greek and Latin words that imply “spiny skin.” They come in two varieties, regular and irregular, and are closely related to starfish, brittle stars, sea lilies, and sea cucumbers.
The first kind of sea urchins have a spherical shape and a strong spine network covering them, giving them the appearance of ancient morning stars. While they graze on algae in open seagrass meadows, mangrove shoals, and coral reefs, each spine can be pointed in the direction of a threat and offers some protection.
It was a pleasant surprise to find that they’re still widespread and abundant. The Florida Keys are heavily impacted by human activity, with fishing, tourism, and diving all occurring on a massive scale. On top of that, coastal ecosystems are subject to climate change, increasingly strong hurricanes, and escalating stressors resulting from continuous urban development.
Michal Kowalewski
The unsung Roombas of the seafloor are irregular echinoids, which include sand dollars, sea biscuits, and heart urchins. Most sand dollars and sea biscuits are burrowers, as opposed to their spiky surface-dwelling relatives, and they use their short, locomotive spines to crawl and deposit food in grooves along their skin that run like conveyer belts directly to their mouths. Others, such as the cake urchin (Meoma ventricosa), simply scoop up anything in their path.
In their never-ending search for food, they burrow through sand, silt, and mud, cleaning, ventilating, and enriching the sediment to make it more friendly to other creatures.
They serve as ecosystem engineers by modifying the landscape, according to the lead author Grun, a postdoctoral researcher at the Florida Museum. “They’re essential for maintaining healthy environments. They feed on detritus and help oxygenate the sediment, which allows microorganisms to degrade waste,” he said.
They are numerous as well. Irregular urchins are among the most numerous organisms on the seafloor by volume in some places. This is notably true in the Caribbean Sea and the Gulf of Mexico, where hundreds of species live on vast shelf platforms.
Sand dollars and heart urchins stand the test of time in deteriorating environments
When it comes to marine surveys, sea urchins frequently get overlooked despite their significance and abundance. In the denuded Florida Keys, considerable effort has been extended to document the decline of coral, fish, seagrass, and manatees, but only a handful of widescale sea urchin surveys have been carried out over the last 60 years.
According to a 2020 report by the National Oceanic and Atmospheric Administration, Florida’s coral reefs have become impaired in recent decades due to a combination of factors. Since 1987, South Florida has experienced six large-scale coral bleaching events as a result of rising global temperatures, and in 2014, reefs close to Miami were reported to be experiencing an outbreak of stony coral tissue loss disease. Since then, the condition has gotten worse every year to the point where it now affects the entire Florida barrier reef, from Martin County in the center of the state to the farthest point of the Florida Keys.
Delicate seagrass meadows are additionally reeling from the combined effects of climate change, pollution and the reduced influx of freshwater from the Everglades; Florida’s mangrove forests are at risk from increasingly intense tropical weather events; and a 2022 study determined that, of 15 grouper and snapper species popular among recreational fisheries, 85% were being harvested past the point of sustainability in the Florida Keys.
Given the dearth of information on sea urchins, it was difficult to predict how their populations may have fared in the face of the deterioration of the local ecosystems.
“One of the reasons we conduct these surveys is to get a better numerical understanding of how important and abundant these organisms are because right now, that documentation is spotty,” Kowalewski said.
Grun and Kowalewski caution that this survey offers only a small snapshot of sea urchin diversity in the Florida Keys. Sand dollars, sea biscuits, and heart urchins, on the other hand, appear to have largely avoided the deleterious effects of environmental change if their findings are at all representative of neighboring benthic environments.
The majority (63%) of the investigated areas, from deeper mudflats on the other side of the barrier reef to protected seagrass meadows along the shoreline, were home to irregular urchins. They frequently observed deceased urchins’ wafer-like discs dissolving in the sand when they discovered living ones, suggesting that populations may have remained in one spot for several generations.
Although Grun admits it’s difficult to pinpoint the precise reason sea urchin populations have remained unaffected, he surmises that a contributing factor may be people’s general disregard for them. “These sea urchins are neither of commercial nor of recreational interest, and their sandy habitats are not often visited by fishers or divers,” he said.
Uncertainty surrounds whether sea urchins in the Florida Keys will survive unharmed as temperatures rise and oceans become more acidic.
“We’re planning on looking more into the environmental factors that affect sea urchins, such as sediment and water composition, over the next years,” he said, stressing that the amount known about sea urchins is dwarfed by what’s left to be discovered. “We’re entering a new arena of research in which we’d really like to drive home the importance of these organisms and highlight their role as ecosystem engineers.”
