Parrotfish: Grazers of the Reef

Locally harvested parrotfish are displayed for sale at a fish market in Pohnpei, Caroline Islands, Micronesia.

Parrotfish are more than just pretty faces — they may be vital for healthy coral reefs. These "beaked" and brightly colored fish delight divers, but there is much more to them than their visible characteristics.

For example, the colors, patterns, shape and even sex of an individual parrotfish can vary as it moves through multiple life phases. Parrotfish are hermaphroditic, and many parrotfish species begin life as females with the ultimate goal of becoming "supermales." Another interesting parrotfish fact is that at night some species cover themselves in a mucous cocoon that they secrete from an organ on their head. The fish remain motionless inside the cocoon, which is thought to protect them from predators and/or parasites.1

Parrotfish teeth are fused into beaks that allow the fish to scrape food from the reef. Along with their food, the fish swallow limestone, which is ground up by the pharyngeal mill (teeth in the back of the throat) and excreted as fine sand. Most of the sand on the reef, and even on nearby beaches, is actually parrotfish poo.

What these fish ingest is also important. Parrotfish spend 90 percent of their time feeding,2 and many species of parrotfish are algal grazers. Herbivory is a key process on coral reefs that can assist reef-building corals, and more than 80 percent of herbivores on Caribbean reefs are parrotfish.3 Herbivores remove the algae that is in constant competition with corals. The clear tropical waters that create the perfect environment for corals also provide the perfect conditions for algal growth. Macroalgae and corals are the dominant benthic groups in coral reefs and compete intensively for the available space. When corals face any kind of disturbance (e.g., bleaching, disease, hurricanes), macroalgae quickly colonize the newly available space. Today we see increasingly more reports that indicate a phase shift from coral-dominated to algae-dominated reefs.

Macroalgae and dense algal turfs can affect coral growth, settlement and survival. Algae can outshade, overgrow and abrade nearby corals.4 Thick algal turfs can trap settlement, smothering corals.3 Some algae can even compete allelopathically, causing coral mortality via the production of harmful chemicals.4 Parrotfish grazing can facilitate coral recruitment by removing macroalgae. This creates space for growth of coral or encrusting coralline algae,3 which can promote coral larvae settlement and metamorphosis.5

In July 2014 the Global Coral Reef Monitoring Network (GCRMN) of the International Union for the Conservation of Nature (IUCN) released a report with data collected by 78 principal investigators at 90 Caribbean reef locations in 35,000 reef surveys over 42 years, from 1970 to 2012.6 Along with noting a 50 percent decline in living corals throughout the study period, the report also suggests that the loss of parrotfish and other grazers has had a greater negative impact on Caribbean reefs than climate change has. The study reported that the healthiest reef locations are where parrotfish are protected from overfishing (e.g., Flower Garden Banks National Marine Sanctuary, Bermuda and Bonaire). As for management, the study concluded that promoting parrotfish population growth would lead to greater resilience in coral reefs to disturbances such as warming temperatures and ocean acidification.

We have already seen how a lack of herbivores can affect our reefs. Coral cover began declining throughout much of the Caribbean in the early 1980s after multiple stressors — hurricanes, coral disease and, finally, macroalgae overgrowth with the loss of the herbivorous urchin Diadema antillarum.7 It is thought that the status of the reefs in the 1980s was made less resilient because many herbivorous fish had been heavily targeted by fishermen. The urchin was the last remaining key herbivore, resulting in the phase shift from coral to algal dominance.

Overfishing continues to be a huge threat to parrotfish populations in the Caribbean. Herbivorous fish, especially parrotfish, have been heavily fished in some locations of the Caribbean for thousands of years.8,9 The largest species in the Caribbean, including the rainbow parrotfish (Scarus guacamaia) and midnight parrotfish (Scarus coelestinus), are rare or absent on most Caribbean reefs.9 Furthermore, these species are most abundant in places with little or no fishing pressure.

The decline in parrotfish populations, particularly in the Caribbean, is a loss not only for the ecosystem but also for the economy. Caribbean coral reefs generate more than $3 billion annually from tourism and fisheries,6 benefitting 38 different countries. Many people who visit these countries want to swim with large, colorful fish.

Although restoring healthy parrotfish populations will make Caribbean reefs more colorful, it won't solve every problem they face. The good news, as far as parrotfish are concerned, is that overfishing is not a universal problem like ocean acidification or global climate change. Parrotfish populations can be managed locally.

Parrotfish use their beaklike teeth to graze on algae that can enshroud a coral reef.

A variety of herbivores is needed to help control algal growth. Herbivorous reef fishes are diverse with specific dietary preferences and assorted feeding techniques, which have different impacts on the reef. For example, parrotfish in genus Sparisoma prefer macroalgae, whereas those in genus Scarus target turf algae.9 Parrotfish tend to be "scrapers," while surgeonfish are "grazers." Scrapers, which remove portions of the underlying limestone as they feed, are important in clearing new space for colonization by coral or crustose coralline algae, but scrapers also bioerode the reef. There are many important invertebrate grazers as well, such as the famed Diadema urchin. Maintaining a diverse herbivore population is key for reef resilience.

Parrotfish predation may be a source of coral mortality.10 More research is necessary to determine if the positive effects of herbivory outweigh the negative effects of corallivory. A 2012 study revealed that parrotfish coral predation intensity could increase as coral density declines.11 Another study found that parrotfish eat polyps with the highest number of gonads and concluded "chronic grazing by parrotfishes has negative fitness consequences for reef-building corals."12 A contradicting study from the same year, however, stated, "Corallivory may constitute a source of acute mortality in coral recruits, but the available evidence implies that any negative impacts are outweighed by positive effects in removing algal competitors."3

Even the evidence that herbivorous fish can promote coral recovery on Caribbean reefs has been inconsistent. While the long-term GCRMN study found a strong correlation between healthy reefs and healthy parrotfish populations, not all studies have come to the same conclusion. Paulina Guarderas and colleagues at Oregon State University studied herbivorous fish within a marine protected area and found that protection of herbivorous fish was not associated with increases in coral cover when compared with a fished location.13

The "herbivore/algae/healthy coral" paradigm has many layers and is extremely context dependent. Reefs are unique in structure, species and stressors, and each factor influences a reef's resilience. Coral reefs are complex ecosystems, and the diversity that makes them such exciting places to spend a few hours are also important in keeping them healthy.

Each organism has a niche — a role it plays on the reef. Many of these niches, like that of herbivorous parrotfish, are not yet completely understood, and understanding them better is perhaps more important than ever as we place increasingly more stress on reef habitats.

Save a parrotfish, save a reef? Maybe, maybe not, but these colorful fish certainly play a significant role in the coral reef ecosystem.
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10. Rotjan RD, Lewis SM. (2005) Selective predation by parrotfishes on the reef coral Porites astreoides. Mar Ecol Prog Ser. 2005; 305:193-201.

11. Burkepile DE.(2012) Context-dependent corallivory by parrotfishes in a Caribbean reef ecosystem. Coral Reefs 2012; 31(1):111-20.

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13. Guarderas AP, Hacker SD, Lubchenco J. Ecological effects of marine reserves in Latin America and the Caribbean. Mar Ecol Prog Ser. 2011; 429:219-25.

© Alert Diver — Q3 Summer 2016