Biology of Parrotfishes

Parrotfish are found on almost every coral reef in the world. This ubiquity and uniqueness of their feeding action make them one of the most important groups of fishes within coral reef ecosystems. But why, exactly, are parrotfish so important to reefs? Can the evolution of a particular jaw morphology and feeding action really have had such a large impact on the health and functioning of the world's coral reefs? This book introduces the reader to this fascinating group of fishes (Labridae, Scarinae), from the morphological innovation of a jaw that has the power to bite through solid calcium carbonate, to the threats currently faced by parrotfish populations around the world. It contains new insights into their diet and food processing ability, and lifehistories, and concludes with an overview of emerging and future research directions

Parrotfish corallivory on stress-tolerant corals in the Anthropocene

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

Sediments and herbivory as sensitive indicators of coral reef degradation

Around the world, the decreasing health of coral reef ecosystems has highlighted the need to better understand the processes of reef degradation. The development of more sensitive tools, which complement traditional methods of monitoring coral reefs, may reveal earlier signs of degradation and provide an opportunity for pre-emptive responses. We identify new, sensitive metrics of ecosystem processes and benthic composition that allow us to quantify subtle, yet destabilizing, changes in the ecosystem state of an inshore coral reef on the Great Barrier Reef. Following severe climatic disturbances over the period 2011-2012, the herbivorous reef fish community of the reef did not change in terms of biomass or functional groups present. However, fish-based ecosystem processes showed marked changes, with grazing by herbivorous fishes declining by over 90%. On the benthos, algal turf lengths in the epilithic algal matrix increased more than 50% while benthic sediment loads increased 37-fold. The profound changes in processes, despite no visible change in ecosystem state, i.e., no shift to macroalgal dominance, suggest that although the reef has not undergone a visible regime-shift, the ecosystem is highly unstable, and may sit on an ecological knife-edge. Sensitive, process-based metrics of ecosystem state, such as grazing or browsing rates thus appear to be effective in detecting subtle signs of degradation and may be critical in identifying ecosystems at risk for the future

Seaweed-coral interactions: variance in seaweed allelopathy, coral susceptibility, and potential effects on coral resilience.

Tropical reefs are in global decline with seaweeds commonly replacing corals. Negative associations between macroalgae and corals are well documented, but the mechanisms involved, the dynamics of the interactions, and variance in effects of different macroalgal-coral pairings are poorly investigated. We assessed the frequency, magnitude, and dynamics of macroalgal-coral competition involving allelopathic and non-allelopathic macroalgae on three, spatially grouped pairs of no-take Marine Protected Areas (MPAs) and non-MPAs in Fiji. In non-MPAs, biomass of herbivorous fishes was 70-80% lower, macroalgal cover 4-9 fold higher, macroalgal-coral contacts 5-15 fold more frequent and 23-67 fold more extensive (measured as % of colony margin contacted by macroalgae), and coral cover 51-68% lower than in MPAs. Coral contacts with allelopathic macroalgae occurred less frequently than expected by chance across all sites, while contact with non-allelopathic macroalgae tended to occur more frequently than expected. Transplants of allelopathic macroalgae (Chlorodesmis fastigiata and Galaxaura filamentosa) against coral edges inflicted damage to Acropora aspera and Pocillopora damicornis more rapidly and extensively than to Porites cylindrica and Porites lobata, which appeared more resistant to these macroalgae. Montipora digitata experienced intermediate damage. Extent of damage from macroalgal contact was independent of coral colony size for each of the 10 macroalgal-coral pairings we established. When natural contacts with Galaxaura filamentosa were removed in the field, recovery was rapid for Porites lobata, but Pocillopora damicornis did not recover and damage continued to expand. As macroalgae increase on overfished tropical reefs, allelopathy could produce feedbacks that suppress coral resilience, prevent coral recovery, and promote the stability of algal beds in habitats previously available to corals

Size-dependent variation in the functional role of the parrotfish Scarus rivulatus on the Great Barrier Reef, Australia

Biodiversity loss and fishing-induced changes in the size distributions of fishes can impact ecosystem function on coral reefs. These changes have led to an urgent need for studies on the particular roles of species to underpin effective coral reef management. The present study focuses on the feeding ecology of 6 size classes (from 2.5 to 30 cm total length) of Scarus rivulatus (family Labridae), one of the most abundant parrotfishes on the Great Barrier Reef, Australia. Individuals in all 6 size classes strongly selected the epilithic algal matrix for foraging and rejected other substratum types, including coral, macroalgae and sand. The 6 size classes also had similar feeding rates (bites min–1) and diel feeding patterns, with higher feeding activity during the afternoon. However, the size of grazing scars by S. rivualtus differed significantly among the 6 size classes, with small individuals scraping a greater substratum area per unit biomass and larger individuals taking a greater volume of material per unit biomass. Thus, biomass cannot be not be viewed as a proxy for ecosystem impact; different sized individuals of S. rivulatus, and probably other parrotfish species, have a markedly different impact on the reef substratum. Selective harvesting of large individuals will therefore change the functional role of this species. These results emphasize the importance of considering the size of individuals when evaluating the role of reef species in ecosystem process. These intraspecific functional differences will be critical when formulating reef management strategies and evaluating the impact of fishing activity on reef ecosystems

Frequency of corals with macroalgal contact causing no damage, bleaching, mortality, or mortality with algal overgrowth in three study sites along Coral Coast, Fiji (n≥15 for each coral-macroalga species pair).

<p>Frequency of corals with macroalgal contact causing no damage, bleaching, mortality, or mortality with algal overgrowth in three study sites along Coral Coast, Fiji (n≥15 for each coral-macroalga species pair).</p

Percent coral and macroalgal cover (mean+SE) in Marine Protected Areas (MPAs) and adjacent non-MPAs associated with three sites along the Coral Coast.

<p>* = p<0.05, ** = p<0.01.</p

Relationship between colony size, as projected surface area (cm²), vs. damage size (cm²) of five coral species after 49 days of contact with <i>Chlorodesmis fastigiata</i> or <i>Galaxaura filamentosa</i>.

<p>Relationship between colony size, as projected surface area (cm²), vs. damage size (cm²) of five coral species after 49 days of contact with <i>Chlorodesmis fastigiata</i> or <i>Galaxaura filamentosa</i>.</p

Study sites at Votua, Vatu-o-lailai, and Namada villages along the Coral Coast of Viti Levu, Fiji.

<p>Dotted and squared areas correspond to Marine Protected Areas (MPAs) and non-MPAs, respectively, at each site.</p

Biomass of herbivorous fishes in MPAs and non-MPAs at three sites along the Coral Coast, Fiji.

<p>Letters indicate significant groupings (p<0.05).</p