Diversity and Structure of Parrotfish Assemblages across the Northern Great Barrier Reef

Quantifying production rates and size fractions of parrotfish-derived sediment: A key functional role on Maldivian coral reefs

Coral reef fish perform numerous important functional roles on coral reefs. Of these, carbonate sediment production, as a by-product of parrotfish feeding, is especially important for contributing to reef framework construction and reef-associated landform development. However, only limited data exist on: (i) how production rates vary among reef habitats as a function of parrotfish assemblages, (ii) the relative importance of sediment produced from eroded, reworked, and endogenous sources, or (iii) the size fractions of sediment generated by different parrotfish species and size classes. These parameters influence not only overall reef-derived sediment supply, but also influence the transport potential and depositional fate of this sedimentary material. Here, we show that parrotfish sediment production varies significantly between reef-platform habitats on an atoll-margin Maldivian reef. Highest rates of production (over 0.8 kg m-2 year-1) were calculated in three of the eight platform habitats; a rubble-dominated zone, an Acropora spp. dominated zone, and a patch reef zone. Habitat spatial extent and differences in associated parrotfish assemblages strongly influenced the total quantities of sediment generated within each habitat. Nearly half of total parrotfish sediment production occurred in the rubble habitat, which comprised only 8% of the total platform area. Over 90% of this sedimentary material originated from eroded reef framework as opposed to being reworked existing or endogenously produced sediment, and comprised predominantly coral sands (predominantly 125-1000 µm in diameter). This is comparable to the dominant sand types and size fractions found on Maldivian reef islands. By contrast, nearly half of the sediment egested by parrotfish in the Acropora spp. dominated and patch reef habitats resulted from reworked existing sediments. These differences between habitats are a result of the different parrotfish assemblages supported. Endogenous carbonate production was found to be insignificant compared to the quantity of eroded and reworked material. Our findings have important implications for identifying key habitats and species which act as major sources of sediment for reef-island systems.

Spatial distribution of parrotfishes and groupers in an Okinawan coral reef: size-related associations in relation to habitat characteristics

Parrotfishes (Labridae: Scarini) and groupers (Epinephelidae) are important fish groups that are regarded as the fisheries targets of primary importance in coral reefs. In order to establish ecosystem-based management of these two fish groups, clarifying the spatial distribution relative to habitat characteristics is of central importance. The present study investigated the spatial distributions of 12 parrotfishes species and seven groupers species in relation to environmental characteristics in an Okinawan coral reef. Ten out of the 12 parrotfish species and all seven grouper species showed species-specific spatial distributions. Four substrate types in the inner reefs (branching Acropora, bottlebrush Acropora, dead branching Acropora, and dead bottlebrush Acropora), three substrate types in the exposed reefs (massive coral, other coral, and calcium carbonate substratum), and water depth showed significant associations with the spatial distribution of fishes. Among the 12 parrotfish species, two species (Scarus spinus and S. forsteni) and four species (S. psittacus, S. hypselopterus, S. dimidiatus and S. ghobban) were primarily found in exposed reefs and inner reefs, respectively. Among the seven grouper species, two species (Cephalopholis argus and C. urodeta) and two other species (C. miniata and Epinephelus ongus) were primarily found in exposed reefs and inner reefs, respectively. Size-related spatial distribution was also found for three parrotfish species (Chlorurus microrhinos, Scarus rivulatus and S. hypselopterus), indicating that smaller-sized and larger-sized individuals were respectively found at sites with greater coverage of substrates with fine structure (live bottlebrush Acropora and dead bottlebrush Acropora) and coarse structure (live branching Acropora, dead branching Acropora and calcium carbonate substratum). The present study suggested that the spatial distribution of parrotfishes and groupers is not necessarily associated with the higher coverage of living corals, but positively associated with high substrate complexity. Thus, actual spatial distributional patterns of species should be considered to select candidate sites for protection and conservation for the two fish groups.

Critical Review and Conceptual and Quantitative Models for the Transfer and Depuration of Ciguatoxins in Fishes

We review and develop conceptual models for the bio-transfer of ciguatoxins in food chains for Platypus Bay and the Great Barrier Reef on the east coast of Australia. Platypus Bay is unique in repeatedly producing ciguateric fishes in Australia, with ciguatoxins produced by benthic dinoflagellates (Gambierdiscus spp.) growing epiphytically on free-living, benthic macroalgae. The Gambierdiscus are consumed by invertebrates living within the macroalgae, which are preyed upon by small carnivorous fishes, which are then preyed upon by Spanish mackerel (Scomberomorus commerson). We hypothesise that Gambierdiscus and/or Fukuyoa species growing on turf algae are the main source of ciguatoxins entering marine food chains to cause ciguatera on the Great Barrier Reef. The abundance of surgeonfish that feed on turf algae may act as a feedback mechanism controlling the flow of ciguatoxins through this marine food chain. If this hypothesis is broadly applicable, then a reduction in herbivory from overharvesting of herbivores could lead to increases in ciguatera by concentrating ciguatoxins through the remaining, smaller population of herbivores. Modelling the dilution of ciguatoxins by somatic growth in Spanish mackerel and coral trout (Plectropomus leopardus) revealed that growth could not significantly reduce the toxicity of fish flesh, except in young fast-growing fishes or legal-sized fishes contaminated with low levels of ciguatoxins. If Spanish mackerel along the east coast of Australia can depurate ciguatoxins, it is most likely with a half-life of ≤1-year. Our review and conceptual models can aid management and research of ciguatera in Australia, and globally.

Diel vertical movements and feeding behaviour of blue humphead parrotfish Scarus ovifrons in a temperate reef of Japan

The feeding ecology of scarinine parrotfishes on tropical coral reefs has received considerable attention in the past few decades; nonetheless, relatively few studies have been conducted in high-latitude reefs. Among the Indo-Pacific Scarus species, Scarus ovifrons is unique, being largely restricted to the warm temperate waters of Japan. Nonetheless, there is very little information available on the feeding ecology of this species. In this study, the authors used acoustic telemetry to detect the diel vertical movement patterns of S. ovifrons, video survey to detect its feeding depths and substrata and focal follow survey and genetic analysis to identify algae composition on the feeding scars at Kashiwajima Island, southwestern Japan (32° 46' N, 132° 38' E). Acoustic telemetry revealed that S. ovifrons spent most of its time in shallow water (<10 m) during the day and slept in deeper water (10-15 m) at night. Video and focal follow surveys revealed that most fishes of various sizes regularly took bites on epilithic algae and detrital materials on rocky substrata at depths of <10 m, but large fishes (>40 cm total length) sometimes took bites directly on live corals (Acropora solitaryensis) at the 5 m depth zone where live tabular corals dominated the benthos. Molecular phylogenetic analyses revealed that epilithic algae collected from feeding scars were mainly composed of Rhodophyta, and coralline algae were less often targeted. Overall, this study revealed that S. ovifrons feeds mostly at depths <10 m, and the feeding algae substrata of the species are similar to those of tropical coral reef parrotfishes.

The origin of the parrotfish species Scarus compressus in the Tropical Eastern Pacific: region-wide hybridization between ancient species pairs

Background

In the Tropical Eastern Pacific (TEP), four species of parrotfishes with complex phylogeographic histories co-occur in sympatry on rocky reefs from Baja California to Ecuador: Scarus compressus, S. ghobban, S. perrico, and S. rubroviolaceus. The most divergent, S. perrico, separated from a Central Indo-Pacific ancestor in the late Miocene (6.6 Ma). We tested the hypothesis that S. compressus was the result of ongoing hybridization among the other three species by sequencing four nuclear markers and a mitochondrial locus in samples spanning 2/3 of the latitudinal extent of the TEP.

Results

A Structure model indicated that K = 3 fit the nuclear data and that S. compressus individuals had admixed genomes. Our data could correctly detect and assign pure adults and F1 hybrids with > 0.90 probability, and correct assignment of F2s was also high in some cases. NewHybrids models revealed that 89.8% (n = 59) of the S. compressus samples were F1 hybrids between either S. perrico × S. ghobban or S. perrico × S. rubroviolaceus. Similarly, the most recently diverged S. ghobban and S. rubroviolaceus were hybridizing in small numbers, with half of the admixed individuals assigned to F1 hybrids and the remainder likely > F1 hybrids. We observed strong mito-nuclear discordance in all hybrid pairs. Migrate models favored gene flow between S. perrico and S. ghobban, but not other species pairs.

Conclusions

Mating between divergent species is giving rise to a region-wide, multispecies hybrid complex, characterized by a high frequency of parental and F1 genotypes but a low frequency of > F1 hybrids. Trimodal structure, and evidence for fertility of both male and female F1 hybrids, suggest that fitness declines sharply in later generation hybrids. In contrast, the hybrid population of the two more recently diverged species had similar frequencies of F1 and > F1 hybrids, suggesting accelerating post-mating incompatibility with time. Mitochondrial genotypes in hybrids suggest that indiscriminate mating by male S. perrico is driving pre-zygotic breakdown, which may reflect isolation of this endemic species for millions of years resulting in weak selection for conspecific mate recognition. Despite overlapping habitat use and high rates of hybridization, species boundaries are maintained by a combination of pre- and post-mating processes in this complex.

Inter-Habitat Variability in Parrotfish Bioerosion Rates and Grazing Pressure on an Indian Ocean Reef Platform

Parrotfish perform a variety of vital ecological functions on coral reefs, but we have little understanding of how these vary spatially as a result of inter-habitat variability in species assemblages. Here, we examine how two key ecological functions that result from parrotfish feeding, bioerosion and substrate grazing, vary between habitats over a reef scale in the central Maldives. Eight distinct habitats were delineated in early 2015, prior to the 2016 bleaching event, each supporting a unique parrotfish assemblage. Bioerosion rates varied from 0 to 0.84 ± 0.12 kg m−2 yr−1 but were highest in the coral rubble- and Pocillopora spp.-dominated habitat. Grazing pressure also varied markedly between habitats but followed a different inter-habitat pattern from that of bioerosion, with different contributing species. Total parrotfish grazing pressure ranged from 0 to ~264 ± 16% available substrate grazed yr-1 in the branching Acropora spp.-dominated habitat. Despite the importance of these functions in influencing reef-scale physical structure and ecological health, the highest rates occurred over less than 30% of the platform area. The results presented here provide new insights into within-reef variability in parrotfish ecological functions and demonstrate the importance of considering how these interact to influence reef geo-ecology.

Synchronous biological feedbacks in parrotfishes associated with pantropical coral bleaching

Biological feedbacks generated through patterns of disturbance are vital for sustaining ecosystem states. Recent ocean warming and thermal anomalies have caused pantropical episodes of coral bleaching, which has led to widespread coral mortality and a range of subsequent effects on coral reef communities. Although the response of many reef-associated fishes to major disturbance events on coral reefs is negative (e.g., reduced abundance and condition), parrotfishes show strong feedbacks after disturbance to living reef structure manifesting as increases in abundance. However, the mechanisms underlying this response are poorly understood. Using biochronological reconstructions of annual otolith (ear stone) growth from two ocean basins, we tested whether parrotfish growth was enhanced following bleaching-related coral mortality, thus providing an organismal mechanism for demographic changes in populations. Both major feeding guilds of parrotfishes (scrapers and excavators) exhibited enhanced growth of individuals after bleaching that was decoupled from expected thermal performance, a pattern that was not evident in other reef fish taxa from the same environment. These results provide evidence for a more nuanced ecological feedback system-one where disturbance plays a key role in mediating parrotfish-benthos interactions. By influencing the biology of assemblages, disturbance can thereby stimulate change in parrotfish grazing intensity and ultimately reef geomorphology over time. This feedback cycle operated historically at within-reef scales; however, our results demonstrate that the scale, magnitude, and severity of recent thermal events are entraining the biological responses of disparate communities to respond in synchrony. This may fundamentally alter feedbacks in the relationships between parrotfishes and reef systems.

Cross-Shelf Differences in the Response of Herbivorous Fish Assemblages to Severe Environmental Disturbances

Cross-shelf differences in coral reef benthic and fish assemblages are common, yet it is unknown whether these assemblages respond uniformly to environmental disturbances or whether local conditions result in differential responses of assemblages at different shelf positions. Here, we compare changes in the taxonomic and functional composition, and associated traits, of herbivorous reef fish assemblages across a continental shelf, five years before and six months after two severe cyclones and a thermal bleaching event that resulted in substantial and widespread loss of live hard coral cover. Each shelf position maintained a distinct taxonomic assemblage of fishes after disturbances, but the assemblages shared fewer species among shelf positions. There was a substantial loss of species richness following disturbances within each shelf position. Total biomass of the herbivorous fish assemblage increased after disturbances on mid- and outer-shelf reefs, but not on inner-shelf reefs. Using trait-based analyses, we found there was a loss of trait richness at each shelf position, but trait specialisation and originality increased on inner-shelf reefs. This study highlights the pervasiveness of extreme environmental disturbances on ecological assemblages. Whilst distinct cross-shelf assemblages can remain following environmental disturbances, assemblages have reduced richness and are potentially more vulnerable to chronic localised stresses.