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Parrotfishes (Scarini) are considered key agents in coral reef health and recovery, but the drivers of parrotfish–coral dynamics remain contentious. The prevailing view of parrotfishes as ecosystem engineers is based on the perceived removal of algal turf, macroalgae and sediment, but these are effects of feeding, not causes. The recent proposal that most parrotfishes are ‘microphages’ that target microscopic photoautotrophs (particularly cyanobacteria) identifies the need to resolve dietary targets at a microscopic scale. Here, we investigate parrotfish dietary targets by posing the following two questions: (1) are microscopic photoautotrophs the most consistent and dominant elements of the prey community, and (2) do the prey community and substratum taphonomy vary between parrotfish species? In order to identify and quantify dietary targets, five parrotfish species were followed until focused feeding was observed at Lizard Island on the Great Barrier Reef, Australia. Feeding sites were photographed in situ and extracted as substratum bite cores. Cores were analysed microscopically to identify and quantify all epilithic photoautotrophs. Endolithic photoautotrophs accessible to excavating parrotfish were also investigated by vacuum-embedding cores with epoxy resin followed by decalcification to expose endolith microborings. The dominant functional groups of epilithic biota on the cores were tufted cyanobacteria, turfing algae and crustose coralline algae (CCA). The only consistent feature across all cores was the high density of filamentous cyanobacteria, supporting the view that these parrotfishes target microphotoautotrophs. Macroalgae was absent or a minor component on cores, supporting the hypothesis that parrotfishes avoid larger algae. The microchlorophyte Ostreobium was the dominant photoautotrophic euendolith (true borer) in the cores of the excavating parrotfish Chlorurus microrhinos. Significant differences in CCA coverage, turf height and substrate taphonomy were found among the five parrotfish species, suggesting that interspecific resource partitioning is based on successional stage of feeding substrata.

The unique traits of large animals often allow them to fulfill functional roles in ecosystems that small animals cannot. However, large animals are also at greater risk from human activities. Thus, it is critical to understand how losing large animals impacts ecosystem function. In the oceans, selective fishing for large animals alters the demographics and size structure of numerous species. While the community-wide impacts of losing large animals are a major theme in terrestrial research, the ecological consequences of removing large animals from marine ecosystems remain understudied. Here, we combine survey data from 282 sites across the Caribbean with a field experiment to investigate how altering the size structure of parrotfish populations impacts coral reef communities. We show that Caribbean-wide, parrotfish populations are skewed toward smaller individuals, with fishes <11 cm in length comprising nearly 70% of the population in the most heavily fished locations vs. ~25% at minimally fished sites. Despite these differences in size structure, sites had similar overall parrotfish biomass. As a result, algal cover was unrelated to parrotfish biomass and instead, was negatively correlated with the density of large parrotfishes. To mechanistically explore how large parrotfishes shape benthic communities, we manipulated fishes' access to the benthos to create three distinct fish communities with different size structure. We found that excluding large or large and medium-sized parrotfishes did not alter overall parrotfish grazing rates but caused respective 4- and 10-fold increases in algal biomass. Unexpectedly, branching corals benefited from excluding large parrotfishes whereas the growth of mounding coral species was impaired. Similarly, removing large parrotfishes led to unexpected increases in coral recruitment that were absent when both large and medium bodied fishes were excluded. Our data highlight the unique roles of large parrotfishes in driving benthic dynamics on coral reefs and suggests that diversity of size is an important component of how herbivore diversity impacts ecosystem function on reefs. This study adds to a growing body of literature revealing the ecological ramifications of removing large animals from ecosystems and sheds new light on how fishing down the size structure of parrotfish populations alters functional diversity to reshape benthic reef communities.

Parrotfishes are a diverse group of herbivores that can influence benthic community dynamics and ecosystem function on coral reefs. Different species and size classes of parrotfishes vary in their feeding ecology and can impact reef ecosystems in distinct ways. We documented differences in the feeding ecology of 9 species of parrotfishes in the Florida Keys National Marine Sanctuary (FKNMS). Many of the key differences can be summarized by assigning species to functional groups (e.g. scrapers, excavators, croppers, macroalgae browsers), which are differentially responsible for carrying out specific ecological processes. For example, we found that Sparisoma viride, Scarus coelestinus, Sc. guacamaia, Sc. taeniopterus, and Sc. vetula feed on short turfs with few sediments, while Sp. aurofrenatum, Sp. chrysopterum, and Sp. rubripinne feed on longer sediment-laden turfs in addition to macroalgae. Further, parrotfishes use distinct bite types that indicate contrasting impacts on the benthos. Species that feed on short turfs scrape and excavate epilithic and endolithic algae, while species that feed on longer turfs and macroalgae tend to tear or crop algae from the reef. These distinct feeding behaviors result in different rates of algae removal, carbonate erosion, and sediment production. Recognizing that different species of parrotfishes interact with the benthos in fundamentally different ways will enable scientists and managers to better predict how changes in the structure of parrotfish assemblages may affect benthic communities and ecosystem processes.

Parrotfishes may affect the structure of benthic communities and reef ecosystem functioning. Despite extensive studies worldwide, parrotfishes in the southwestern Atlantic are relatively understudied, limiting our ability to propose effective management strategies. We assessed abundance, biomass and size class distribution of parrotfish assemblages in northeastern Brazil and identified habitat preferences based on reef attributes. Outer-shelf reefs sustained larger individuals and higher biomasses for all species (except Sparisoma radians). In contrast, inner-shelf reefs supported higher abundances of small individuals. Even though most species occurred across all areas, their abundances, biomass and size-class distributions were variable and related to their respective feeding modes and reef attributes. Benthic cover, reef structural complexity, depth and distance from the coast affected the composition of parrotfish assemblages, but had different effects on each species. The endemic greenbeak parrotfish Scarus trispinosus was more abundant on calcareous substrates and higher-complexity reefs. Sc. zelindae and Sp. amplum were more common in deeper biogenic reefs further from the coast, which were characterized by high abundances of sponges, stony corals and cyanobacterial mats. Sp. axillare and Sp. radians were more abundant on reefs that had high cover of large-bladed macroalgae, while Sp. frondosum was ubiquitous across all studied reefs. Such heterogeneity in habitat use is suggestive of functional complementarity rather than functional redundancy among parrotfish assemblages. Conservation of Brazilian endemic parrotfishes requires protecting reefs with diverse attributes and a better understanding of habitat connectivity and the role of different habitats in parrotfish reproduction and life cycle.

Cumulative anthropogenic stressors on tropical reefs are modifying the physical and community structure of coral assemblages, altering the rich biological communities that depend on this critical habitat. As a consequence, new reef configurations are often characterized by low coral cover and a shift in coral species towards massive and encrusting corals. Given that coral numbers are dwindling in these new reef systems, it is important to evaluate the potential influence of coral predation on these remaining corals. We examined the effect of a key group of coral predators (parrotfishes) on one of the emerging dominant coral taxa on Anthropocene reefs, massive Porites . Specifically, we evaluate whether the intensity of parrotfish predation on this key reef-building coral has changed in response to severe coral reef degradation. We found evidence that coral predation rates may have decreased, despite only minor changes in parrotfish abundance. However, higher scar densities on small Porites colonies, compared to large colonies, suggests that the observed decrease in scarring rates may be a reflection of colony-size specific rates of feeding scars. Reduced parrotfish corallivory may reflect the loss of small Porites colonies, or changing foraging opportunities for parrotfishes. The reduction in scar density on massive Porites suggests that the remaining stress-tolerant corals may have passed the vulnerable small colony stage. These results highlight the potential for shifts in ecological functions on ecosystems facing high levels of environmental stress.

Herbivorous and detritivorous fishes interact closely with the epilithic algal matrix (EAM) on coral reefs. While sediment and organic detrital loads within the EAM might influence this interaction, the responses of functionally distinct fishes to changing sediment and organic loads have not been investigated. Aquarium based feeding trials were performed to assess how different sediment and organic loads affected feeding by the highly abundant surgeonfishes, Ctenochaetus striatus, a detritivore, and Acanthurus nigrofuscus, a herbivore. C. striatus were highly sensitive to even small increases in sediment loads (of just 75 g m-2), displaying a significant decline in feeding rates as sediment loads increased. Although C. striatus is a specialised detritivore, changing organic loads had no effect and suggests that selection of feeding surfaces is primarily mediated by total sediment loads rather than organic loads. By contrast, A. nigrofuscus displayed no changes to its feeding behaviour regardless of sediment or organic load. These findings highlight the complex, species-specific way that sediments may mediate key ecological processes on coral reefs.

Understanding the trophic ecology of herbivorous and detritivorous fishes is essential for evaluating their ecological roles in coral reef ecosystems. In this study, we combined bulk stable isotope (δ 15 N and δ 13 C) and fatty acid analyses to investigate trophic partitioning and dietary resource use among herbivorous and detritivorous fishes from the Great Barrier Reef, Australia. Isotopic niches and fatty acid profiles confirmed significant trophic partitioning among algivores, detritivorous surgeonfishes, and parrotfishes. We also applied mixing models based on these ecological tracers to quantify the contributions of basal dietary sources to the fish. Our findings further support previous dietary knowledge for several species, including algivorous acanthurids, kyphosid chubs, and the rabbitfish Siganus doliatus . However, they also highlight trophic niche specializations within these groups, particularly in Naso unicornis , which assimilates substantial dietary protein from epiphytic cyanobacteria despite a macroalgal diet, and in the detritivorous Ctenochaetus striatus , which exhibited isotopic similarities to parrotfishes but differed in fatty acid composition, likely due to a higher intake of diatoms. Additionally, our analyses reinforce the distinctive dietary composition of parrotfishes, emphasizing the complexity of their feeding biology, in which microscopic photoautotrophs such as cyanobacteria and dinoflagellates play a key dietary role that has often been overlooked in previous studies on their nutritional ecology. Furthermore, these findings underscore the usefulness of multi-tracer approaches in refining our understanding of coral reef fish trophic ecology.

Building a comprehensive understanding of geo-ecological functions is viewed as a critical step in the quantification of carbonate and sediment budgets on coral reefs. Although parrotfishes are known to play key roles in such functions, the roles of surgeonfishes are less well understood. This is despite surgeonfishes being some of the largest and most abundant, nominally herbivorous fishes on reefs, with the potential to play a major role in sediment dynamics. Here, we quantified the role of four focal surgeonfish species across two functional groups (sediment suckers and brushers) in cross-habitat sediment dynamics by combining data on behavioural observations, fish abundance, gut content analysis, and published gut throughput rates. On average, these surgeonfish species together were estimated to rework and/or transport 71.4 ± 12.7 kg 100 m −2  year −1 of sediment. A single 30–35 cm surgeonfish was estimated to rework ~ 36 kg individual −1  year −1 , a value equivalent to that of large parrotfishes. However, the two functional groups of surgeonfishes contributed to sediment dynamics in fundamentally different ways. The brusher, Ctenochaetus striatus , was primarily involved in on-reef sediment reworking. In contrast, the three sediment suckers were primarily involved in off-reef sediment reworking and in driving a potential net flux of sediment onto the reef. These results highlight the importance of off-reef shallow sand habitats as key feeding grounds for some surgeonfishes, as well as the role of these surgeonfishes in connecting off-reef habitats to reefs via biological sediment fluxes.

Cyanobacteria are ubiquitous on coral reefs and perform many important ecosystem functions. Benthic cyanobacterial mats (BCMs) have become increasingly abundant on degraded reefs. Mat-forming benthic cyanobacteria have frequently been considered unpalatable to reef fishes. Regardless, recent studies have documented substantial grazing of BCMs by reef fishes, including parrotfishes. Here, we observed foraging in five Caribbean parrotfishes on the fringing coral reefs of Bonaire, Netherlands, to investigate BCM consumption relative to other benthic substrates. Three of our study species preferentially targeted conspicuous BCMs (i.e., macroscopic, cohesive colonies taxonomically composed primarily of Cyanobacteria and Proteobacteria), often taking several consecutive bites on them. Additionally, a high proportion of bites by all species targeted substrates characterized by filamentous turfs and crustose coralline algae. These substrates also contain diverse communities of epilithic and endolithic cyanobacteria and microalgae. Our work is, therefore, consistent with and provides direct evidence supporting the recently proposed trophic categorization of parrotfishes as microphages. Contrasting observations of reef fishes avoiding substrates dominated by BCMs on other reefs suggests variation in the palatability of BCMs to grazing reef fishes, or species-specific differences in preference for these potentially nutritional trophic resources.