In the struggle to save corals from climate change, reef-dwelling animals don’t get much attention. Scientists tend to focus on the corals themselves — breeding more-resilient species, seeding new reefs, halting pollution and development that add to the stresses of high temperatures and acidifying ocean waters.
Yet animals can help protect reefs, too. To wit: Stegastes nigricans, known as the dusky farmerfish, a colony-dwelling species found in shallow-water reefs from the western Indian Ocean to the eastern Pacific. They eat algae growing in carefully tended plots — hence the name — and chase away all intruders, including coral-eating fish. Their territories become miniature sanctuaries.
“Farmerfish gardens,” write researchers in the journal Behavioral Ecology, “may serve as nursery habitats for branching coral and reestablishment nuclei following coral die-offs.” Following cyclones and outbreaks of invasive reef-scouring starfish, the presence of dusky farmerfish could help corals recover rather than collapse altogether.
Led by ecologist Jonathan Pruitt of the University of California Santa Barbara, the researchers studied how coral-protecting performance varied between farmerfish colonies off the French Polynesian island of Moorea. Some colonies are more aggressive than others; corals planted in their territories experienced 80% less skeletal loss than corals in less-aggressive colonies.
Moreover, other research suggests that, as warming waters increases the metabolic rate of cold-blooded fish, they become bolder and more aggressive. As oceans warm, then, dusky farmerfish may become even more effective reef guardians — and, as Pruitt and colleagues describe in a separate study published in Oikos, coral size also influences their aggressiveness. Larger corals provide substrate for farmerfish gardens and a barrier against predators; they become even more committed to protecting them.
The researchers don’t suggest that dusky farmerfish alone will save reefs from collapse. Their own focus is on how groups exhibit different personalities and how these differences then shape their environment. But the findings underscore the potential importance of these underwater guardians — and, says Pruitt, of other territorial, group-dwelling members of their damselfish family, who can be found in coral reefs around the world.
In many places, though, damselfish species are threatened or reduced. While there’s little commercial fishing for them, they’re frequently caught as bycatch. Many populations are exploited by the aquarium industry. And studies have found that, when large predators like grouper and snapper are caught, populations of medium-sized, damselfish-consuming predators explode, leading to dramatic declines in their numbers.
“Anything that impairs farmerfish populations is impairing the ecological services they provide,” says Pruitt. Perhaps protecting farmerfish and their relatives could become part of our strategy for saving coral reefs.
Sources: Kamath et al. “Potential feedback between coral presence and farmerfish collective behavior promotes coral recovery.” Oikos, 2018.
Pruitt et al. “Collective aggressiveness of an ecosystem engineer is associated with coral recovery.” Behavioral Ecology, 2018.
About the author: Brandon Keim is a freelance journalist specializing in animals, nature and science, and the author of The Eye of the Sandpiper: Stories From the Living World. Connect with him on Twitter, Instagram and Facebook.