"Can Herbivorous Crabs Assist in Saving Seaweed-Choked Corals?"

Within most relationships, a level of symbiosis between individuals is necessary for stability, meaning that both parties feel a similar or equal pull of benefit from their bond. This concept is one that not only extends endlessly to the human connection but to a world we often neglect to consider – that of marine life. Within their study highlighting coral reefs and their survival, two scientists, Angelo Spadaro and Mark Butler, note that, in many cases, corals and herbivorous fish share this symbiotic relationship. Part of this relationship entails the fact that these fish consume the fleshy seaweeds that can overpopulate a coral reef, seaweeds that can affect a coral’s growth, reproduction, recruitment, and ultimate survival of their ecosystems. Unfortunately, with the current abundance of seaweed and with a declining amount of fish the job is simply not being done fast

by Juliette Allayaud

California, USA enough or with enough efficiency. Thus, scientists have looked for solutions to cut back on this abundance; Spadaro and Butler being two of these scientists. Fortunately, they believe that they have found a potentially transformative one, an answer that involves the density of the Maguimithrax spinosissimus, or the Caribbean king crab.

When there is a shift in dominance, in this case by seaweeds, it can be ecologically altering and devastating. In many areas, such as the Caribbean, the proliferation of seaweed growth is caused by coastal eutrophication, when a plethora of nutrients often stemming from urban development leads to an increase in plant life and the

Spadaro and Butler illustrate that in order for a coral reef to survive, balance is key.

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overproduction of oxygen which can damage animal life. The loss of grazers, herbivore feeders, often due to overfishing and habitat loss, also contributes to the buildup of seaweed. What Spadaro and Butler have discovered is that the Caribbean king crab, the large, primarily herbivorous crab native to the Caribbean and Western Atlantic, is an incredibly effective consumer of seaweeds. In fact, it is so effective that it was believed that the imbalance between corals and seaweed could be restored by these crabs if their presence were to be increased significantly on degraded coral reefs where their current density is relatively low. This proposes a new, much needed, relationship between corals and crabs that could serve a similar purpose to the symbiotic one between corals and fish that has declined over time.

To test this hypothesis, they conducted a series of experiments from 2014 to 2015 on twelve isolated coral patch reefs in the middle of the Florida Keys. Three distinctly different treatments were applied on the twelve patches: reefs with no treatment, that is, unmanipulated and left as controls, reefs with crabs added to the natural population, and reefs scrubbed of their seaweed and then stocked with added crabs.

by Juliette Allayaud

California, USA Spadaro and Butler discovered that after a year had passed, nearly half of the eighty-four crabs released onto the reefs still remained inhabitants. Noting that initially 85% of the surface area of the reefs was covered in seaweed, they concluded that on the first treatment area, this percentage hardly shifted. However, for the second treatment area, the crabs reduced and maintained seaweed cover at 50% of that on the unmanipulated coral, and for the third seaweed area, the seaweed cover was reduced by 80%. From 2016 to 2017, thirteen km from the first location, the same experiment was carried out. Though the initial seaweed coverage was slightly

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different, the investigation yielded nearly identical results, which shows consistency in Spadaro and Butler’s results. Additionally, they found on both sites that with the asset of the crabs, the density of juvenile corals was more than twice as high on scrubbed reefs and almost four times higher on reefs to which crabs were added than the initial measurements. Not only was there a positive correlation between the crabs and coral growth, but the abundance and variety of coral reef fishes also felt the weight of this relationship. Seaweeds have been known to release chemicals that larval reef fishes avoid.

Therefore by limiting the amount of seaweed, Spadaro and Butler found a two to three fold increase in species richness and a three to five fold increase in the abundance of coral fishes. These numbers are only bolstered by the fact that the fishes have been more attracted to the reefs now that the shelter-providing crevices and holes had become more readily exposed. Furthermore, more mobile and larger fish were drawn to the reef communities with the prospect of the prey within these spaces.

With these extraordinary results that showed not only consistency but long-term positive effects, it becomes clear that utilizing the Caribbean king crab will be beneficial when it comes to restoring coral reefs. To unlock this asset,

by Juliette Allayaud

California, USA however, we first have to develop mariculture programs that will produce and grow crabs to a sufficient size and at sufficient numbers that allow them to combat their naturally low abundance and, unfortunately, high mortality rates.

Additionally, even with their striking success, it is essential to acknowledge that introducing or increasing a species into a specific area is a delicate process that must be implemented with strategic detail and management. If the Caribbean king crab is to be used as a method of restoration, it

needs to be done responsibly in a way that is both conducive to the corals, the crabs themselves, and the surrounding environment.

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Ultimately when it comes to the health of our coral reefs, we are facing an incredibly complex and global issue that, without immediate action, will continue to rapidly increase. Because of the complexity and overall broadness of the issue, it becomes vital that we approach the problem not only quickly but from all angles.

Thankfully, with scientists like Angelo Spadaro and Mark Butler working tirelessly on the situation, I am filled with hope for the fate of our oceans. Even with this hope, though, and this is a point that these two men express heavily, our dying reefs will require more than herbivores to keep them alive.

Overall it requires a combination of reversing the effects of global warming, ocean acidification, and cutting down on coastal pollution and overexploitation of coral reef fishes. This is a nuanced issue and, therefore, will require nuanced solutions. But why not start with adding a few crabs?

Works Cited/Image Credit

Spadaro, Angelo Jason, and Mark J. Butler, 4th. 2021.

“Herbivorous Crabs Reverse the Seaweed Dilemma on Coral Reefs. ” Current Biology: CB 31 (4): 853-859.e3. doi.org/10.1016/j.cub.2020.10.097

Figure from Spadaro and Butler; crab photograph by Angelo Sparado (www.eurekalert.org/multimedia/917800)

by Juliette Allayaud

California, USA