Clarifying functional roles: algal removal by the surgeonfishes Ctenochaetus striatus and Acanthurus nigrofuscus

The lined bristletooth, Ctenochaetus striatus, and the brown surgeonfish, Acanthurus nigrofuscus, are among the most abundant surgeonfishes on Indo-Pacific coral reefs. Yet, the functional role of these species has been the focus of an ongoing debate lasting at least six decades. Specifically, to what extent are C. striatus herbivorous like the visually similar A. nigrofuscus? To address this question, we used natural feeding surfaces, covered with late successional stage reef-grown algal turfs, to examine turf algal removal by the two species. Surfaces exposed to C. striatus in laboratory experiments exhibited no significant reductions in turf length or area covered by turfing algae. In marked contrast, A. nigrofuscus reduced turf length by 51% and area covered by turfing algae by 15% in 1 h. The gut contents of specimens from the reef revealed that A. nigrofuscus predominantly ingests algae (the dominant item in 79.6-94.7% of gut content quadrats), while C. striatus ingests detritus and sediments (dominant in 99.6-100% of quadrats). Therefore, C. striatus ingests detritus and sediment, leaving mature algal turfs relatively intact, while A. nigrofuscus directly removes and ingests turf algae. The function of C. striatus differs from cropping herbivorous surgeonfishes such as A. nigrofuscus. On coral reefs, C. striatus brush detrital aggregates from algal turfs, removing microorganisms, organic detritus and inorganic sediment. Confusion over the functional role of C. striatus may stem from an inability to fit it into a single functional category

Site fidelity and homing in juvenile rabbitfishes (Siganidae)

The behaviour of juvenile fishes is critical in establishing the link between recruitment and subsequent adult populations. If juvenile fishes move, they can respond to variation in local conditions before adult home ranges are established. Alternatively, if juveniles establish fixed home ranges at settlement, their decisions may determine future population densities at small spatial scales. Field observations and translocations revealed that juvenile rabbitfishes (Siganus corallinus and S. doliatus) have small home ranges and strong homing abilities (covering 6 m in 1 h or 36 m within 24 h). Only four of 22 individuals failed to return; all were transferred up-current, suggesting that olfaction is important in homing. Small home ranges and strong homing tendencies in juvenile herbivores suggest that decisions made by recruits will impact the spatial extent of both adult fishes and the functional roles they play within ecosystems

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

A functional evaluation of feeding in the surgeonfish Ctenochaetus striatus: the role of soft tissues

Ctenochaetus striatus is one of the most abundant surgeonfishes on Indo-Pacific coral reefs, yet the functional role and feeding ecology of this species remain unclear. This species is reported to possess a rigid structure in its palate that is used for scraping, but some authors have reported that this element is comprised of soft tissue. To resolve the nature and role of this structure in the feeding ecology of C. striatus we examined evidence from anatomical observations, scanning electron microscopy, histology, X-ray micro-computed tomography scanning, highspeed video and field observations. We found that C. striatus from the Great Barrier Reef possess a retention plate (RP) on their palates immediately posterior to the premaxillary teeth which is soft, covered in a thin veneer of keratin with a papillate surface. This RP appears to be used during feeding, but does not appear to be responsible for the removal of material, which is achieved primarily by a fast closure of the lower jaw. We infer that the RP acts primarily as a 'dustpan', in a 'dustpan and brush' feeding mechanism, to facilitate the collection of particulate material from algal turfs

Biologically mediated sediment fluxes on coral reefs:\ud sediment removal and off-reef transportation by the\ud surgeonfish Ctenochaetus striatus

Off-reef sediment transport by the surgeonfish Ctenochaetus striatus (Acanthuridae) was quantified on the reef crest at Lizard Island, Great Barrier Reef. Three independent methods were implemented to estimate sediment ingestion rates. These considered (1) the bite rate and bite volume, (2) the defecation rate and faecal pellet size, and (3) the average gut contents and throughput rate. The 3 methods provided a broad range of estimates of sediment ingestion from 8.8 ± 2.4, to 66.1 ± 14.4 g fish–1 d–1 (mean ± SE). Nevertheless, these estimates were comparable to rates of sediment ingestion by parrotfishes (Labridae), the other major sediment-moving group on reefs. Overall, 36.5% of all sediment ingested was transported from the upper reef crest into deeper water, equating to a removal rate of 28.6 ± 6.2 kg 100 m–2 yr–1 at the study site. By brushing the reef, C. striatus reduces the sediment loading in the epilithic algal matrix (EAM) while causing little damage to the algal turf. Reducing sediments in EAMs provides favourable settlement surfaces for benthic organisms and increases the palatability of the EAM to herbivorous reef fishes, thus supporting reef resilience. The ecological importance of C. striatus, which is abundant on reefs throughout the Indo-Pacific, appears to have been underestimated, particularly when considering reef sediment dynamics

Fishes on coral reefs: changing roles over the past 240 million years

Key morphological traits reveal changes in functional morphospace occupation of reef fish assemblages over time. We used measurements of key functional attributes (i.e., lower jaw length and orbit diameter) of 208 fossil fish species from five geological periods to create bivariate plots of functional morphological traits through time. These plots were used to examine possible function and ecological characteristics of fossil reef fish assemblages throughout the Mesozoic and Cenozoic. A previously unknown trend of increasing orbit diameter over time became apparent. The Teleostei are the principal drivers of this change. The Eocene appears to mark a dramatic increase in two previously rare feeding modes in fishes: nocturnal feeding and high-precision benthic feeding. Interestingly, members of the Pycnodontiformes had relatively large eyes since the Triassic and appear to be the ecological precursors of their later teleost counterparts and may have been among the earliest nocturnal feeding fishes. Our results highlight potential changes in the roles of fishes on coral reefs through time

Composition and temporal stability of turf sediments on inner-shelf coral reefs

Elevated sediment loads within the epilithic algal matrix (EAM) of coral reefs can increase coral mortality and inhibit herbivory. Yet the composition, distribution and temporal variability of EAM sediment loads are poorly known, especially on inshore reefs. This study quantified EAM sediment loads (including organic particulates) and algal length across the reef profile of two bays at Orpheus Island (inner-shelf Great Barrier Reef) over a six month period. We examined the total sediment mass, organic load, carbonate and silicate content, and the particle sizes of EAM sediments. Throughout the study period, all EAM sediment variables exhibited marked variation among reef zones. However, EAM sediment loads and algal length were consistent between bays and over time, despite major seasonal variation in climate including a severe tropical cyclone. This study provides a comprehensive description of EAM sediments on inshore reefs and highlights the exceptional temporal stability of EAM sediments on coral reefs

The role of fishes as food: a functional perspective on predator–prey interactions

Every animal dies. In nature, mortality usually occurs due to predation by other animals. One of the fundamental consequences of mortality is the transfer of energy and nutrients from one organism (prey) to another (predator). On coral reefs, these key interactions and processes, that are essential for ecosystem functioning, are primarily mediated by fishes; up to 53% of fishes on coral reefs can be regarded as piscivorous. To date, piscivory on coral reefs has been primarily studied with regard to the species piscivores feed on, and how piscivores control populations. Consequently, understanding prey selectivity by piscivorous fishes has been a major goal. However, prey functional traits may also be important in understanding these ‘energy transactions’, especially in complex ecosystems such as coral reefs. Our goal, therefore, was to quantify—at a community level—functional traits of prey that have been shown to influence predator–prey interactions. We found that, on average, deep-bodied, social fishes occupy higher positions in the water column, whereas solitary species are usually elongate and more closely associated with the benthos. On closer examination, we found that solitary species have a size-dependent relationship, with substratum associations shifting to water column associations, at approximately 50 mm body length. Our results reveal three distinct prey functional groups: cryptobenthic substratum dwellers, solitary epibenthics and social fishes. These groups display significant differences in their morphologies and behaviours. Furthermore, based on a meta-analysis of published mortality rates of small-bodied (<100 mm TL) reef fishes, we show that the three groups display different mortality rates, possibly due to differential exposure to, and potential to be captured by, different predator types. Although fishes are widely available on coral reefs, they may not be equally available as prey to all piscivore types. Prey are not simply victims; they are capable of influencing potential predation through functional traits. A free Plain Language Summary can be found within the Supporting Information of this article

Low-quality sediments deter grazing by the parrotfish Scarus rivulatus on inner-shelf reefs

Elevated sediment loads in the epilithic algal matrix (EAM) deter grazing by herbivorous fishes and may compromise their critical roles on coral reefs. However, the properties of sediments that drive herbivore deterrence are unknown. Binary choice trials in aquaria were used to examine the effects of three sediment attributes—sediment source, grain size and organic load—on grazing by the abundant inner-shelf parrotfish, Scarus rivulatus. Fish were presented with a choice between EAM-covered rocks treated with (a) terrigenous or reefal sediments, (b) fine or coarse sediments or (c) sediments with high or low organic loads. Scarus rivulatus did not show a preference for sediments from different sources (terrigenous vs. reefal); however, a clear preference was evident for fine-grained sediments over coarse (109 % more bites) and sediments with high organic loads over low (147 % more bites). The avoidance of coarse sediments is likely to be a key factor driving the inhibition of grazing on mid-shelf reefs, which are dominated by coarse sediments. In contrast, on inner-shelf reefs, grazing by parrotfishes may be deterred primarily by high sediment loads, which reduce the proportional organic content in EAM sediments. Our study highlights the potential impact of sediments on critical ecological processes and the threats posed by changing sediment loads on inner-shelf reefs

Small cryptopredators contribute to high predation rates on coral reefs

Small fishes suffer high mortality rates on coral reefs, primarily due to predation. Although studies have identified the predators of early post-settlement fishes, the predators of small cryptobenthic fishes remain largely unknown. We therefore used a series of mesocosm experiments with natural habitat and cryptobenthic fish communities to identify the impacts of a range of small potential predators, including several invertebrates, on prey fish populations. While there was high variability in predation rates, many members of the cryptobenthic fish community act as facultative cryptopredators, being prey when small and piscivores when larger. Surprisingly, we also found that smashing mantis shrimps may be important fish predators. Our results highlight the diversity of the predatory community on coral reefs and identify previously unknown trophic links in these complex ecosystems