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.
Algal turf sediments across the Great Barrier Reef: Putting coastal reefs in perspective
Sediments trapped within algal turfs play a key role in mediating ecosystem processes on reefs. Despite this ecological importance, our understanding of turf-bound sediments on coastal reefs is limited, even though such reefs are heavily exposed to terrestrial sediments. We compared algal turf sediment loads, composition and grain size distributions among coastal reefs along the Great Barrier Reef (GBR) and compared these to turf-bound sediments on reefs across the GBR. Algal turf sediment loads on coastal reefs were on average 9.4 times higher than offshore reefs (range 2.2–16.1 times). Among coastal reefs there was marked variability in the loads (797.87–3681.78 gm⁻²), composition (organics 1.0–8.9%; silicates 27.4–93.0%) and grain size of sediments. Our data highlight the potential variability in physical and ecological factors that control sediment dynamics on coastal reefs, including proximity to rivers, and reveal that turfs on coastal reefs are heavily influenced by sediments.
Depth stratified light trap sampling reveals variation in the depth distribution of late-stage cryptobenthic reef fish larvae
Cryptobenthic fishes are abundant on coral reefs, and their larvae dominate the ichthyoplankton in near reef waters. However, we have a limited understanding of how pelagic and on-reef processes are linked, especially how late-stage cryptobenthic fish larvae use near reef waters. We therefore used depth-stratified light trap sampling from 2 to 27 m at Lizard Island, Great Barrier Reef. This revealed clear depth variation in late-stage larval fish assemblages. Gobiidae larvae characterised mid-depth (13 m) samples. By contrast, larval Apogonidae were only abundant in shallow samples. Deep samples were typifed by (non-target) adult apogonids. Contrary to expectations that poor-swimming cryptobenthic larvae would be flow-sheltering in deeper water, our results suggest that late-stage cryptobenthic larvae use large portions of the water column, although their preferred positions may be taxon-specific.
Shelter use by large reef fishes: long-term occupancy and the impacts of disturbance
Large fishes often shelter beneath structures on coral reefs. While avoidance of UV radiation has been proposed as the main driver of this behaviour, sheltering behaviour has only been studied during the day and over short timeframes. Here we applied passive acoustic telemetry techniques to continuously monitor shelter usage patterns by large reef fishes over a period of 7 months. For three sweetlip species (Haemulidae), one snapper species (Lutjanidae) and one surgeonfish species (Acanthuridae), diurnal shelter use was remarkably consistent, with occupation of shelters throughout the day, and under all weather conditions, suggesting that factors other than UV avoidance may be important in driving shelter use. Large-scale observations revealed that all fish species appeared to undertake long-distance migrations (>1 km) away from their shelter sites each night. With the exception of the surgeonfish Acanthurus dussumieri, all fishes returned to the same areas to shelter for the entire study period. Individuals of A. dussumieri, however, failed to return on the night of a severe tropical cyclone. They never reappeared at the shelter sites. The disappearance of this species suggests that A. dussumieri probably forage at night in a different location to the carnivorous haemulids and lutjanids. Overall, this study highlights the long-term importance of shelter structures for fishes that may range over large areas of coral reefs.
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, high-speed 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.
Algal turf sediments limit the spatial extent of function delivery on coral reefs
The presence of key organisms is frequently associated with the delivery of specific ecosystem functions. Areas with such organisms are therefore often considered to have greater levels of these functions. While this assumption has been the backbone of coral reef ecosystem-based management approaches for decades, we currently have only a limited understanding of how fish presence equates to function on coral reefs and whether this relationship is susceptible to stressors. To assess the capacity of a stressor to shape function delivery we used a multi-scale approach ranging from tens of kilometres across the continental shelf of Australia's Great Barrier Reef, down to centimetres within a reef habitat. At each scale, we quantified the spatial extent of a model function (detritivory) by a coral reef surgeonfish (Ctenochaetus striatus) and its potential to be shaped by sediments. At broad spatial scales, C. striatus presence was correlated strongly with algal turf sediment loads, while at smaller spatial scales, function delivery appears to be constrained by algal turf sediment distributions. In all cases, sediment loads above ~250-500 g m−2 were associated with a marked decrease in fish abundance or feeding activity, suggesting that a common ecological threshold lies within this range. Our results reveal a complex functional dynamic between proximate agents of function delivery (fish) and the ultimate drivers of function delivery (sediments), which emphasizes: a) weaknesses in the assumed links between fish presence and function, and b) the multi-scale capacity of algal turf sediments to shape reef processes. Unless direct extractive activities (e.g. fishing) are the main driver of function loss on coral reefs, managing to conserve fish abundance is unlikely to yield the desired outcomes. It only addresses one potential driver. Instead, management of both the agents that deliver functions (e.g. fishes), and the drivers that modify functions (e.g. sediments), is needed.
Collapsing ecosystem functions on an inshore coral reef
Ecosystem functions underpin productivity and key services to humans, such as food provision. However, as the severity of environmental stressors intensifies, it is becoming increasingly unclear if, and to what extent, critical functions and services can be sustained. This issue is epitomised on coral reefs, an ecosystem at the forefront of environmental transitions. We provide a functional profile of a coral reef ecosystem, linking time-series data to quantified processes. The data reveal a prolonged collapse of ecosystem functions in this previously resilient system. The results suggest that sediment accumulation in algal turfs has led to a decline in resource yields to herbivorous fishes and a decrease in fish-based ecosystem functions, including a collapse of both fish biomass and productivity. Unfortunately, at present, algal turf sediment accumulation is rarely monitored nor managed in coral reef systems. Our examination of functions through time highlights the value of directly assessing functions, their potential vulnerability, and the capacity of algal turf sediments to overwhelm productive high-diversity coral reef ecosystems.
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.
Algal Turf Sediments and Sediment Production by Parrotfishes across the Continental Shelf of the Northern Great Barrier Reef
Sediments are found in the epilithic algal matrix (EAM) of all coral reefs and play important roles in ecological processes. Although we have some understanding of patterns of EAM sediments across individual reefs, our knowledge of patterns across broader spatial scales is limited. We used an underwater vacuum sampler to quantify patterns in two of the most ecologically relevant factors of EAM sediments across the Great Barrier Reef: total load and grain size distribution. We compare these patterns with rates of sediment production and reworking by parrotfishes to gain insights into the potential contribution of parrotfishes to EAM sediments. Inner-shelf reef EAMs had the highest sediment loads with a mean of 864.1 g m⁻², compared to 126.8 g m⁻² and 287.4 g m⁻² on mid- and outer-shelf reefs, respectively. High sediment loads were expected on inner-shelf reefs due to their proximity to the mainland, however, terrigenous siliceous sediments only accounted for 13-24% of total mass. On inner-shelf reef crests parrotfishes would take three months to produce the equivalent mass of sediment found in the EAM. On the outer-shelf it would take just three days, suggesting that inner-shelf EAMs are characterised by low rates of sediment turnover. By contrast, on-reef sediment production by parrotfishes is high on outer-shelf crests. However, exposure to oceanic swells means that much of this production is likely to be lost. Hydrodynamic activity also appears to structure sediment patterns at within-reef scales, with coarser sediments (> 250 µm) typifying exposed reef crest EAMs, and finer sediments (< 250 µm) typifying sheltered back-reef EAMs. As both the load and grain size of EAM sediments mediate a number of important ecological processes on coral reefs, the observed sediment gradients are likely to play a key role in the structure and function of the associated coral reef communities.
Fine sediments suppress detritivory on coral reefs
Increasing sediment inputs are recognised as an important factor leading to coral reef degradation. However, the role of sediments in ecological processes is poorly understood. This study used paired-choice trials to quantify the effects of sediment grain size and chemical composition on feeding by the abundant detritivorous reef fish, 'Ctenochaetus striatus'. The size of sediments from algal turfs were also compared to those ingested by reef-dwelling 'C. striatus'. Algal turfs containing coarser sediments were preferred by 'C. striatus', while sediment composition (reefal carbonates vs. riverine silicates) had little effect. On the reef, 'C. striatus' ingested finer sediments than those present in algal turfs. 'C. striatus' appears to prefer algal turfs with coarser sediments as this facilitates ingestion of fine detrital particles, while finer sediments prevent selective feeding on detritus. These findings suggest that fine sediments from terrestrial runoff or dredging may be detrimental to feeding by detritivorous species.