The colorful world of coral

A diversity of coral reef color leads to a diversity in fish color, and by extension, a diversity of fish species. With temperatures rising, the loss of colorful coral may lead to a loss of fish and an ocean in grayscale. Featured image source: Kyle Taylor

Reference: Hemingson, C. R., Mihalitsis, M., & Bellwood, D. R. (2022). Are fish communities on coral reefs becoming less colourful? Global Change Biology, 28, 3321– 3332.

Many organisms in the underwater world express and perceive color in ways that we can only dream of understanding. Humans have three receptors in our eyes that allow us to perceive all the colors in the “visible light” spectrum (colors visible to humans), but many marine animals have many more: the peacock mantis shrimp, for example, has twelve. As beautiful and brightly-colored as we might find it, one can only imagine what their vibrant exoskeletons look like to each other.

Coral reefs are among the most colorful habitats in the natural world, where the oranges and pinks of coral mix with the bright yellows and blues of ocean fish, inspiring everything from children’s books to Pixar movies (and Pixar movie sequels…). Recently, researchers working on Australia’s Great Barrier Reef published a study diving into where this abundance of color comes from, what the colors present in a coral reef community tell us about its health, and how climate change may influence the colors we see in the water.

A color study

Measurements like species richness (the amount of individuals in a habitat) and species diversity (the amount of species present) are very important in understanding the health of a given system. Healthier ecosystems generally have space and resources for more organisms and many types of organisms. Sometimes, though, this approach can make results challenging to interpret. Is a habitat healthier or more worthy of conservation efforts if it has a single “rare” species, versus a habitat that supports many more common ones?

To avoid some of the challenges that arise from that paradigm, researchers devised a system to quantify the colors of coral reefs and the fish species that call them home. This approach removes the bias that can enter when focusing on a few charismatic species and instead looks at what the whole community looks like. It’s an artful approach: with the right technology even two coral that look similar to the naked eye can still be differentiated by subtle color differences, and quantifying all the colors present can give a good idea on whether or not a given ecosystem can support a diverse array of wildlife.

But how do you quantify color? In short, all colors humans are able to see exist on the spectrum of visible light. The three receptors in the human eye process red, green, and blue light. Every color has some amount of one of these three colors (yellow, for example, has a lot of red and green light, but no blue). Computers are very good at splitting up these colors into exact values of each type of light, and image processing software gives precise values of the color of each pixel in an image. By taking many pictures of coral and fish, it is possible to determine the color signatures of each organism and how similar these signatures are to other things in their environment.

Colorful coral species like the Acropora in this photograph are temperature-sensitive, and a warming ocean might also be one without much color. Image source: USFWS / J. Maragos
What we know from color

The colors present in a community tell us many important things about the habitat itself. More colorful, complex coral structures allow a wider array of colorful fish species to thrive. Researchers looked at how community coloration responds to global bleaching events by applying their methods to a 27-year dataset that contained years before and after the 1998 global bleaching event which devastated coral reefs worldwide. When coral died and only a few species of similar color and structure survived, the colors present in the fish community also declined. However, community coloration recovered after a few years, signifying the return of many fish species.

Researchers also discovered a strong connection between the structure provided by the benthos (essentially, the ocean floor) and the diversity of fish colors present. This makes intuitive sense: fish that have evolved bright coloration, such as the flagtail surgeonfish in the photograph above, usually evolve these colors through sexual selection (flashy, colorful fish are more attractive to their mates). But the colors that make them stand out to other fish also make them more visible to predators. This isn’t a problem if there are lots of complex structures for fish to take refuge in, but when fish find themselves in flat areas without many places to hide, the need to blend in becomes much more important. Analyzing the coral and fish colors therefore gave insight onto whether sexual selection or predation was the greater pressure on certain fish species in areas with different benthic structures.

Additionally, coral reefs are a popular ecotourism destination, and people will spend a lot of money to travel to the most vibrant habitats. By developing a system to quantify color at the community level, scientists can monitor the health of an ecosystem as it relates to the sources of money that could provide critical financial support to protect it. Coral reefs have a deep cultural significance for many people, and preserving their vast landscapes of color maintains those cultural connections to our oceans.

Climate change and community color

With global ocean temperatures rising, many believe further coral bleaching events are imminent. Reefs are some of the most diverse ecosystems in the world, and losing these foundational species would not only result in the loss of many aquatic species that call the reefs home, but also the loss of the ocean’s colors.

Not all hope is lost: some coral species, such as those in the Porites genus, are able to withstand a greater range of temperatures and will likely be resilient in the face of ocean warming. However, these coral are associated with lower community coloration values. While coral reefs may persist, it could still spell the end of the breathtaking color of these habitats.

As the researchers point out, many reefs are at a critical transition point where, given the right time and attention, it may be possible to preserve the incredible diversity of life and color. Like so many issues brought on by climate change, whether or not we can protect these natural systems will depend on how many people work to make a difference.

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