Judging a Coral Reef by Its…Parrotfish?
By Jay Chen
What’s a parrotfish?
It’s a parrot? No, it’s a fish! It’s a parrotfish! What’s this cool creature and what does it have to do with coral reefs?
As you might have guessed from their name, parrotfish (Scarus spp.) are a group of brightly colored fish, named for their parrotlike characteristics (Mark et al. 2024). Unlike parrots, they cannot fly in the air, though you could say they do “soar” through the water! Like parrots, they have strong mouths that resemble beaks. Unlike parrots, parrotfish are herbivores and only eat plants. Parrotfish live in the ocean, in coral reefs, and use their strong beak-like jaws to crush substrate in order to feed on microbes and cyanobacteria living throughout and on the reefs. With this unique feeding method, they play important roles in coral reefs.
With the continued growth and development of coastal cities, coastal measures and modifications are put in place to protect coastal urban areas. One of these includes sea walls, which are essentially large man-made walls constructed in the ocean to reinforce coasts and prevent flooding and erosion. Though beneficial to humans, these sea walls can have unintended consequences on the marine ecosystems near them, such as coral reefs.
Fishing for Data
Rachel Y. Y. Mark, alongside a team of fellow researchers, surveyed sea wall and coral reef habitats in the waters of two islands near Singapore: Pulau Hantu and Kusu Island. The researchers followed parrotfish around, observed their grazing behavior, and noted the result of any areas they grazed, which they termed “grazing scars.” Because of the unique way parrotfish graze by crushing substrate, the scientists could identify spots that had been grazed by parrotfish. Using this information, the scientists calculated the amount and intensity of grazing performed by the parrotfish in the different habitats, coral reefs versus seawalls. They also collected other measurements, such as amounts and type of sediment and volume of cyanobacteria, as these are related to parrotfish grazing.
What did they find?
The researchers found low densities of grazing in coral reef habitats, or even no grazing activity in some parts, but high densities of grazing in the seawall habitats. This was attributed to more suitable grazing substrate on the seawalls with lower sediment levels and shorter algae heights on surfaces due to the seawalls having more disturbance and exposure. It’s easier to graze on a consistent surface with no little rocks or big sediments in the way and nice short algae than a very densely vegetated area with lots of sediment in the way. The sea walls also had more cyanobacteria on average compared to the coral reefs nearby; why feed in an area with less food? Instead of grazing mostly in coral reefs, these parrotfish have been grazing off man-made seawalls.
Mark and fellow researchers also found that reduced grazing by parrotfish in coral reefs was paired with increased grazing on seawalls. This could possibly reduce the important impacts parrotfish have on coral reef ecosystems. The unique destructive feeding method of parrotfish aids in the cycling of carbonate, a material used by coral and other animals like crabs and clams to build their hard “skeletons” or shells. The parrotfish break down materials, allowing them to cycle through the system as smaller sediments. This grinding mechanism also makes parrotfish responsible for the generation of a substantial amount of sand in coral reefs. Additionally, grazing scars left behind by parrotfish clear new space for new organisms or new coral to settle and colonize the area. Not only this, but if other coral reef grazing organisms cannot compensate for reduced parrotfish grazing, there might be an imbalance with too much cyanobacteria and algae in the system.
Knowing this, what now?
It’s clear that parrotfish and coral reefs share an important relationship. Coral reef systems are also incredibly important ecosystems for us and others. They support many species of fish and other marine organisms (which also provide food for us), reduce coastal storm severity (that’s one of the reasons behind sea walls), provide building materials, generate billions of dollars through tourism, and many other benefits (Woodhead et al. 2019). Parrotfish help maintain, reshape, and support coral reefs. We’ve disrupted this system as we continue to develop coastal cities, overfish parrotfish, and build structures near coral reefs. We need to be wary of the coastal infrastructure we put into place to protect ourselves, because we may be harming the many organisms and the habitats nearby. We may also be harming ourselves as systems such as coral reefs provide us with so many benefits. This study brings up interesting considerations when it comes to parrotfish, sea walls, and coral reefs. On a broader scale, it brings up many questions. How will ecosystems and organisms respond to human modifications? How might or do specific behaviors influence ecosystems and habitats? How do we balance our immediate needs while protecting the organisms and ecosystems we inevitably interact with?
References
Mark, Rachel Y. Y., Daisuke Taira, & Peter A. Todd. (2024). Shoreline Armoring Potentially Diminishes the Ecological Roles of Parrotfishes in Urban Coral Reefs. Ecology, e4250. https://doi.org/10.1002/ecy.4250
Woodhead AJ, Hicks CC, Norström AV, Williams GJ, Graham NAJ. (2019). Coral reef ecosystem services in the Anthropocene. Funct Ecol, 33, 1023–1034. https://doi.org/10.1111/1365-2435.13331
