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Long-Spined Sea Urchin: The Unsung Hero of Healthy Coral Reef Ecosystems

long spined sea urchin

During the 1970s, if you swam, snorkeled, or dove in the Caribbean, it was common to find numerous reefs with colorful structures of various shapes. These structures, made of calcium carbonate, are nothing more than some of the longest-lived animals on our planet: corals. Some corals even began to form over 50 million years ago. The spectacular shades of color that we see when looking at a coral are actually translucent polyps that are in association (symbiosis) with billions of micro algae called zooxanthellae. These tiny algae provide food through the process of photosynthesis, which allows the coral to direct its resources toward building and accreting its calcium carbonate «skeleton». As it grows, the coral groups with other colonies to form large extensions of reefs, such as the Great Barrier Reef in Australia and the Mesoamerican Reef System (MRS) in the Caribbean Sea.

The Cost of Human Activities on the Environment

Unfortunately, since humans began to increase their population, the demand for food resources has grown, resulting in overfishing. There has also been an increase in the number of homes and coastal urban development, as well as a greater generation of organic and inorganic waste from livestock and agriculture. All of these factors have had a significant impact on the marine environment.

How? Due to the increased demand for resources and the generation of pollutants, these substances end up in rivers and bodies of water that flow into the coast and ultimately reach the sea. This has resulted in negative impacts on marine biota, including diseases caused by pathogens such as bacteria, fungi, and viruses. A well-known example of this was the massive mortality of the long-spined black sea urchin Diadema antillarum in the 1980s in Panama. Due to marine currents, the pathogen responsible for the mortality subsequently spread as far as the Bahamas in the North Atlantic, almost completely killing off the sea urchins. But why was their mortality so alarming? These sea urchins are echinoderms that live on Caribbean reefs and are capable of consuming enough macroalgae in a single day to prevent it from growing excessively. Macroalgae naturally compete for light and space with corals, so if there is no efficient herbivore on the reef, they can crowd out or even kill corals.

With the increase in the use of fertilizers, larger livestock areas, and the continuous burning of fossil fuels, the situation of climate change has been exacerbated, causing changes in marine circulation patterns, increases in the number and intensity of meteorological phenomena, and a rise in sea surface temperature. This has an impact on corals, subjecting them to stress and causing the expulsion of their small symbiotic algae, producing bleaching events. If these stress factors are continuous and constant, they can end up killing the corals without giving them the opportunity to recuperate.

The Sea Urchin That Keeps Reefs Alive

As for myself, I was interested in discovering ways to support the conservation of Caribbean reefs. When I had the opportunity to research coral reefs, I read about the Diadema antillarum sea urchin, and how its grazing action could help maintain reef health. In order to conduct my investigation, I first had to determine where I could locate these small gardeners. Once located, I had to count and measure them, and differentiate between adults and juveniles, as their herbivore rate increases with their size. However, due to a massive die-off event, only 1-2% of their population remained intact. The pathogen responsible for their death was never identified; what was clear was their ecological absence. Previously, up to 30 individuals per square meter were reported, but now only 1 or 2 urchins could be found in 10 square meters. The reef gardeners had all but disappeared. In addition to the anthropogenic impacts previously mentioned, the reefs had collapsed, and the coral colonies were covered with macroalgae that could no longer be controlled. The algae were constantly fed by excess nutrients, such as nitrogen and phosphorus, coming from the continent due to agricultural runoff.‍

Currently, the sea is an excess nutrient broth, which, added to the increase in temperature, creates circumstances that are detrimental to the survival of corals. In addition to this, «new» diseases have appeared, and reefs suffer from their own pandemics. Due to this, a part of my research involved relocating juvenile Diadema antillarum sea urchins that I was able to find in the surf zone of the reef, which is dominated by the remains and debris of dead corals. The aim was to reintroduce sea urchins to reef sites with a certain degree of degradation. The chosen site was a natural protected area in the southern part of the state of Quintana Roo in Mexico. In this case, the population density of these urchins was manually increased in these coral patches, which were monitored monthly for almost two years. The results showed control over the growth of some species of macroalgae. However, not all species are susceptible to the sea urchin herbivore. Despite this, we were able to demonstrate that if we return to a density of 1 to 4 sea urchins per square meter, we could help reefs recover their health, increasing their resilience to natural and anthropogenic disturbances.

The preceding task was not an easy one, as Diadema sea urchins are still very scarce. Thus, part of my research was to understand their life cycle, from their larval contribution through planktonic drift to their settlement rate using specialized panels. The majority of echinoderms have an indirect development, and this species is no exception. Its free-living larva is part of the zooplankton in the water column, going through a metamorphosis process where it develops structures that allow it to adhere and settle on the seafloor to continue its development. The small sea urchins, measuring 1 mm in diameter, seek refuge among the crevices of the reef, where they feed on encrusting coralline algae and macroalgae, until they reach a diameter of up to 10 cm in their adult stage. These urchins will later reproduce, generating a new larval contribution to the coral reefs.

In my research, the results were not very encouraging. The larval contribution was very low, and larvae were only recorded in the summer months. As for its settlement, it was also very low. The plates that we placed to catch recently settled sea urchins were not frequently visited. This tells us that the population still cannot recover its pre-mortality abundances, leaving coral reefs without enough herbivores.

On the other hand, densities reintroduced to reef sites were successful. Most of the individuals transferred to their new home managed to adapt successfully. By having enough macroalgae to feed on, as well as enough crevices to take refuge, they managed to survive. This gives us hope to contribute to improving the resilience of reefs. In other words, if we manage to locate adults of this species, it is possible to relocate them to places where their herbivore is necessary. In this way, we support the control of macroalgae.

In the Caribbean region, some countries are implementing this methodology by moving sea urchins from one reef to another, always supported by previous research or backed by academic institutions that have been working with the dynamics of coral reefs and their species for some time. Not only are sea urchins important, but also parrotfish, which are great consumers of macroalgae that help maintain reef health. However, on occasion, parrotfish also bite off part of the coral structure while consuming macroalgae, discarding it as sand, which becomes part of the Caribbean beaches.

We need to take action and make a difference

With all the aforementioned factors considered, we must acknowledge that we cannot continue at our current pace of life and population growth. We cannot ignore the ecological consequences that this brings to marine ecosystems. If we do not reduce our impact, coral reefs will disappear, regardless of the amount of research hours or economic resources invested in finding solutions for their recovery. It is imperative that we allow the planet to rest, starting with population growth planning and investing in education for the most vulnerable sectors of the population, such as coastal communities, who directly depend on the natural resources provided by the sea.

We must contribute to the dissemination of basic and applied science to raise awareness among all sectors of society. This includes the fisherman who depends on the reef for their livelihood, by showing them which species they can and cannot fish due to their ecological importance, as well as those directly involved in the degradation of ecosystems, such as hotel real estate companies in the Caribbean area. Regarding organisms, all are important, but some are considered keystone species that, if removed from their environment, can trigger cascade of unfavorable consequences for the proper functioning of the ecosystem. These species, such as the Diadema sea urchins and parrotfish, must be protected above all else, as they have no other species to replace their ecological functions. As a society, we must make corrections to our lifestyle, such as consuming less and being more aware of our actions in the short and medium term, which will bring long-term consequences that semirreversible, as reef degradation and species loss increase day by day. This is something to reflect on, as it is up to us to avoid adding more stressors to the environment and to let the reefs begin to recover naturally at their own pace.

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About the author

Picture of Julieta Maldonado

Julieta Maldonado

Julieta is a biologist with a specialty in Marine Biology. She obtained her PhD in Marine Sciences from the Center for Research and Advanced Studies of the National Polytechnic Institute (CINVESTAV, Mérida, Mexico). She specializes in the ecological role of sea urchins in the Caribbean reefs. Currently, she is doing a postdoctoral fellowship on the Phylogenetic Diversity of Corals in the Mesoamerican Reef System.