Some Caribbean parrotfishes find benthic cyanobacterial mats particularly tasty

Manning, J.C. & McCoy, S.J. (2023). Preferential consumption of benthic cyanobacterial mats by Caribbean parrotfishes. Coral Reefs 42: 967-975.

Rise of benthic cyanobacterial mats

Cyanobacteria – bacteria that live in water and photosynthesize – are found everywhere in coral reefs. They play important roles as primary producers, which form the foundation of the food web. As coral reefs continually degrade from stressors like ocean warming and pollution, benthic cyanobacterial mats (BCMs) have become more abundant. BCMs are large aggregations of (mostly) cyanobacteria that grow in a mat-like structure on the bottom of the seafloor. They thrive in warm, nutrient-rich water. In degraded reefs, these mats can overgrow and smother bottom-dwelling sea life, including coral. In some instances, BCMs can also house harmful bacteria that causes coral viruses and sickness in marine animals.

Benthic cyanobacterial mat at a coral reef in the Red Sea, Egypt. Photo by Derek Keats, CC BY 2.0.

Due to their commonness and potential risks to sea life, scientists are eager to understand how BCMs fit into coral reef dynamics. Surprisingly, BCMs may be a tasty food source for parrotfishes, a species that eats primarily plant-matter, because they contain a high amount of nutrients. However, there has been some conflicting observations on this matter – some recent studies showed that a substantial portion of parrotfish bites were of BCMs, while other studies have found that BCMs are usually avoided by plant-eating fish including parrotfishes. Determining whether parrotfishes like to eat these mats is essential, because it could help keep their growth in check on reefs.

Are parrotfish picky eaters?

To determine if parrotfishes ate BCMs, researchers studied the foraging behavior of the following five Caribbean parrotfishes in coral reefs in Bonaire: Queen parrotfish (Scarus vetula), Princess parrotfish (Sc. taeniopterus), Striped parrotfish (Sc. iseri), Stoplight parrotfish (Sparisoma viride), and Redband parrotfish (Sp. aurofrenatum). The foraging behavior of 162 parrotfishes was observed. For each fish, researchers noted the size and species of the fish, the number of bites taken, and the food selection.

While all five parrotfish species fed on BCMs, three of the five species (Sc. iseri, Sc. taeniopterus, and Sc. vetula) preferred them to other food choices including turf algae, crustose coralline algae, macroalgae, coral, sediment, and sponge. They especially targeted BCMs that grew on sediment and hard reef structures.  In these three species that preferentially fed on BCMs, bites taken of BCMs made up 7-13% of their total bite count. For the other two species (Sp. viride and Sp. aurofrenatum), bites on BCMs were between 1.5-6%.

Parrotfish feeding on benthic cyanobacterial mats at the Cayman Islands. Photo by Sharife, CC BY 2.0.

One fish’s trash is another fish’s treasure

While past research on the consumption of BCMs by plant-eating fishes has been conflicting, this study provides evidence that three common parrotfish species preferentially feed on BCMs. This indicates that the tastiness of BCMs may vary based on 1) geographic location,  2) the types of molecules growing in the mats, and/or 3) the species of parrotfish. Additionally, this research shows that parrotfish grazing may help control the overgrowth of BCMs in reefs, providing further support that parrotfishes are critical components of reef ecosystems. The researchers note that despite this important finding, there is a knowledge gap surrounding BCMs and reef dynamics. They recommend that future work should focus on the difference in mat composition and how BCM grazing affects coral reef dynamics on a larger scale.

Cover photo from Wikipedia, CC BY-SA 3.0.

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