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A Person (or Fish) on the Inside: Scientists Discover New Ally to Fight Coral Bleaching.

Paper: Chase TJ, Pratchett MS, Frank GE, Hoogenboom MO (2018). Coral-dwelling fish moderate bleaching susceptibility of coral hosts. PLoS ONE 13(12): e0208545. https://doi.org/10.1371/journal.pone.0208545

Coral in the Great Barrier Reef. When coral is alive, it maintains beautiful, vivid colors. When it bleaches, it becomes transparent to show its white calcium-carbonate skeleton underneath. Source: Kyle Taylor, Flickr.

Coral reefs are amazing staples to diverse and complex ecosystems, and sustain over 25% of all marine life. However, the same reefs which support so much in the ocean are also incredibly vulnerable to climate change, and are facing extreme danger. As one of the world’s largest carbon sinks, the ocean sucks up a massive portion of the influx of carbon dioxide which causes climate change. Absorbing so much excess CO2 throws our ocean off balance, quickly and drastically warming its waters and increasing its acidity. These climate change-byproducts have caused many of the world’s integral coral reefs to bleach and die (for more on the process of coral bleaching & the effects of ocean acidification, check out this piece). As our ocean continues to warm at a rate faster than anyone expected, coral reefs are facing an absolutely massive threat.

Zooxanthellae is a type of Symbiodinium, and is a microscopic algae that lives inside of coral. The algae has a symbiotic relationship with coral, in that it lives inside coral polyps in return for feeding these polyps through their photosynthetic activity. Source: Tim Wijgerde, Wikimedia Commons.

When coral bleaches and dies, it can no longer participate in many symbiotic relationships with different organisms in reef ecosystems, causing a chain reaction which disrupts the entire system. From the tiny Symbiodinium which live inside coral and provide them nutrients, to the larger predators which eventually see their prey disappearing, everything in a coral reef suffers when coral suffers. Impacts include loss of coral species diversity, weakened coral skeletons, and increase in harmful algal blooms.

Scientists are hard at work, attempting to solve the global crisis that is coral bleaching and death. But what if these scientists had someone on the inside? A helpful aid which could make coral more climate change-resilient and help them recover faster after bleaching events in the field and in real time?

As it turns out, we do: damselfish.

Many species of damselfish have a symbiotic relationship to climate change sensitive coral species. These coral provide a home for damselfish, who in turn give coral nutrients through their excrement. Additionally, the damselfish’s swimming activity inside and within coral increases water flow, which promotes gas exchange needed for more efficient photosynthesis (a process by which Symbiodinium like zooxanthellae both feed themselves and the coral they live in). Since damselfish stick around in their coral hosts for some time after bleaching events, they may actually help coral recover as well.

Whitetail dascyllus, otherwise more cutely named as the humbug damselfish. Source: Leonard Low da Australia, Flickr.

Chase et al. (2018) explore whether damselfish Dascyllus aruanus (or the whitetail dascyllus) can indeed increase coral bleaching tolerance during climate-change induced warming waters and aid in post-bleaching recovery. They split their study into an experiment, with manipulated conditions, and field observation in Lizard Island Lagoon on the northern Great Barrier Reef in Australia.

In their experiment, Chase et al. (2018) look at the effects of damselfish in ambient (no sudden warming temperatures) and heated (simulating climate change warming temperatures) conditions. Overall, the scientists found that the whitetail dascyllus greatly helped coral bleaching tolerance and recovery during their experiment. Both ambient and heated colonies with the whitetail dascyllus showed higher levels of Symbiodinium than colonies without the fish, indicating that the coral were less likely to expel their algal roommates with the help of damselfish. Chase et al. (2018) also saw increased chlorophyll in colonies with damselfish in both the ambient and heated conditions in the stress and recovery periods when compared to their no-fish counterparts. More chlorophyll content means Symbiodinium were able to photosynthesize to feed themselves and their coral hosts even during temperature increases, preventing the likelihood of extreme bleaching and lowering the chance of coral mortality.

Notice how when coral bleaches, it loses its vibrant colors. Essentially, little polyps on the coral skeleton will expel their Symbiodinium in moments of high stress, like the extreme warming rates seen in Chase et al.’s (2018) study, and starve. Source: Elapied, Wikimedia Commons.

To see if damselfish could help wild coral in the same way, Chase et al. (2018) examined coral colonies with and without whitetail dascyllus in the field. They noted that coral colonies with whitetail dascyllus had significantly higher concentrations of Symbiodinium than their unoccupied counterparts. Unfortunately, Chase et al. (2018) soon discovered that even damselfish couldn’t fully protect their coral friends from climate change. In 2016 during their observations, the Lizard Island Lagoon experienced a sudden exposure to temperatures greater than 33 degrees Celsius (over 91 degrees Fahrenheit). Because of this extreme warming, more than 90% of observed coral colonies both bleached and died, unable to recover from these devastating temperatures. The severity of the bleaching event rendered the studied damselfish unable to help their coral hosts recover, and the fish eventually disappeared.

Chase et al.’s (2018) study comes with an amazing finding: that the symbiotic relationship between damselfish and coral (and possibly many other coral-dwelling fishes) can increase coral’s tolerance to bleaching and enhance their post-bleaching recovery. However, the scientists’ study also shows that these fish cannot work alone. In the face of extreme temperature rise caused by climate change, damselfish simply cannot use their “magic” and help their coral friends survive. Chase et al.’s (2018) findings provide an extremely important message: none of us can work to save coral or fight climate change in isolation. While it is absolutely indispensable to discover the many ways in which ecosystems as a whole can adapt to climate change and heal together, we must also work to mitigate the severity of climate change on our end. Finding different ways to cut our carbon emissions, supporting policy which encourages clean energy, looking for other methods of carbon sequestration which takes some pressure off the ocean: these are all ways that we humans can help our new ally on the inside fight the byproducts of climate change.


2 Responses to “A Person (or Fish) on the Inside: Scientists Discover New Ally to Fight Coral Bleaching.”

  1. Great article :) very informative

    Posted by Margaux | January 23, 2019, 5:46 pm

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