Body size determines eyespot size and presence in coral reef fishes

Numerous organisms display conspicuous eyespots. These eye-like patterns have been shown to effectively reduce predation by either deflecting strikes away from nonvital organs or by intimidating potential predators. While investigated extensively in terrestrial systems, determining what factors shape eyespot form in colorful coral reef fishes remains less well known. Using a broadscale approach we ask: How does the size of the eyespot relate to the actual eye, and at what size during ontogeny are eyespots acquired or lost? We utilized publicly available images to generate a dataset of 167 eyespot-bearing reef fish species. We measured multiple features relating to the size of the fish, its eye, and the size of its eyespot. In reef fishes, the area of the eyespot closely matches that of the real eye; however, the eyespots "pupil" is nearly four times larger than the real pupil. Eyespots appear at about 20 mm standard length. However, there is a marked decrease in the presence of eyespots in fishes above 48 mm standard length; a size which is tightly correlated with significant decreases in documented mortality rates. Above 75-85 mm, the cost of eyespots appears to outweigh their benefit. Our results identify a "size window" for eyespots in coral reef fishes, which suggests that eyespot use is strictly body size-dependent within this group

Subconscious biases in coral reef fish studies

In complex, diverse ecosystems, one is faced with an exceptionally challenging decision: which species to examine first and why? This raises the question: Is there evidence of subconscious biases in study species selection? Likewise, is there evidence of this bias in selecting methods, locations, and times? We addressed these questions by surveying the literature on the most diverse group of vertebrates (fishes) in an iconic high-diversity ecosystem (coral reefs). The evidence suggests that we select study species that are predominantly yellow. Reef fish studies also selectively examine fishes that are behaviorally bold and in warm, calm, attractive locations. Our findings call for a reevaluation of study species selection and methodological approaches, recognizing the potential for subconscious biases to drive selection for species that are attractive rather than important and for methods that give only a partial view of ecosystems. Given the challenges faced by high-diversity ecosystems, we may need to question our decision-making processes

Bio-physical determinants of sediment accumulation on an offshore coral reef: A snapshot study

Sediments are found on all coral reefs around the globe. However, the amount of sediment in different reservoirs, and the rates at which sediments move between reservoirs, can shape the biological functioning of coral reefs. Unfortunately, relatively few studies have examined reef sediment dynamics, and associated bio-physical drivers, simultaneously over matching spatial and temporal scales. This has led to a partial understanding of how sediments and living reef systems are connected, especially on clear-water offshore reefs. To address this problem, four sediment reservoirs/sedimentary processes and three bio-physical drivers were quantified across seven different reef habitats/depths at Lizard Island, an exposed mid-shelf reef on the Great Barrier Reef. Even in this clear-water reef location a substantial load of suspended sediment passed over the reef; a load theoretically capable of replacing the entire standing stock of on-reef turf sediments in just 8 h. However, quantification of actual sediment deposition suggested that just 2 % of this passing sediment settled on the reef. The data also revealed marked spatial incongruence in sediment deposition (sediment trap data) and accumulation (TurfPod data) across the reef profile, with the flat and back reef emerging as key areas of both deposition and accumulation. By contrast, the shallow windward reef crest was an area of deposition but had a limited capacity for sediment accumulation. These cross-reef patterns related to wave energy and reef geomorphology, with low sediment accumulation on the ecologically important reef crest aligning with substantial wave energy. These findings reveal a disconnect between patterns of sediment deposition and accumulation on the benthos, with the ‘post-settlement’ fate of sediments dependent on local hydrodynamic conditions. From an ecological perspective, the data suggests key contextual constraints (wave energy and reef geomorphology) may predispose some reefs or reef areas to high-load turf sediment regimes

Greater multihabitat use in Caribbean fishes when compared to their Great Barrier Reef counterparts

Organisms often utilise different habitats for different reasons, whether it be for acquiring resources, avoiding predation or for reproduction. In coastal tropical ecosystems, coral reefs, seagrass beds and mangrove forests frequently occur in a complex mosaic of intermixed habitats. One of the most commonly identified links between these habitats is the nursery function; that some fish species utilise mangroves and seagrass as juveniles before moving onto reefs as adults. We investigated whether this potential link between habitats is reflected in the similarity of their fish assemblages and if this similarity differs between major biogeographic realms. Visual surveys in the three focal habitats in the Caribbean and on the Great Barrier Reef (GBR) provided assemblage composition data. We found a near ten-fold increase in the percentage of species that occupy all three habitats in the Caribbean when compared to the GBR. When only assessing species that occurred in multiple habitats, the Caribbean displayed strong shifts linked to ontogeny and greater abundances of multihabitat species, supporting the proposed nursery function. GBR assemblages remained more distinct regardless of ontogeny, offering little support for the nursery function. Most multihabitat species in the Caribbean reach larger adult body sizes (>= 30 cm, e.g. Haemulon flavolineatum) vs. smaller bodied (<30 cm, e.g. Halicheores miniatus) fishes on the GBR. When placed in this context, Caribbean species have a greater capacity to move and utilise more habitats compared to the limited movement from the smaller GBR species. The Caribbean and GBR therefore differ not only in the extent of species overlap between the three habitats, but also in the level of ontogenetic connectivity. It is likely that biogeographic history, evolution and tides, amongst other factors, influence these patterns

Biogeographic patterns in major marine realms: function not taxonomy unites fish assemblages in reef, seagrass and mangrove systems

We examine the effects of different biogeographic histories on assemblage composition in three major marine habitats in two biogeographically distinct marine realms. Specifically, we quantify the taxonomic and functional composition of fish assemblages that characterise coral reef, seagrass and mangrove habitats, to explore the potential effects of biogeographic history and environment on assemblage composition. The three habitats were surveyed in the Caribbean and on the Great Barrier Reef using a standardised underwater visual census method to record fish size and abundance data. The taxonomic composition of assemblages followed biogeographic expectations, with realm-specific family-level compositions. In marked contrast, the functional composition of assemblages separated habitats regardless of their biogeographic locations. In essence, taxonomy characterises biogeographic realms while functional groups characterise habitats. The Caribbean and Indo-West Pacific have been separated for approximately 15 million years. The two realms have different taxonomic structures which reflect this extended separation, however, the three dominant shallow-water marine habitats all retain distinct functional characteristics: seagrass fishes are functionally similar regardless of their taxonomic composition or biogeographic location. Likewise, for coral reefs and mangroves. The results emphasise the advantages and limitations of taxonomic vs. functional metrics in evaluating patterns. Taxonomy primarily reflects biogeographic and evolutionary history while functional characteristics may better reflect ecological constraints

Are fish communities on coral reefs becoming less colourful?

An organism's colouration is often linked to the environment in which it lives. The fishes that inhabit coral reefs are extremely diverse in colouration, but the specific environmental factors that support this extreme diversity remain unclear. Interestingly, much of the aesthetic and intrinsic value humans place on coral reefs (a core ecosystem service they provide) is based on this extreme diversity of colours. However, like many processes on coral reefs, the relationship between colouration and the environment is likely to be impacted by global environmental change. Using a novel community-level measure of fish colouration, as perceived by humans, we explore the potential links between fish community colouration and the environment. We then asked if this relationship is impacted by human-induced environmental disturbances, e.g. mass coral bleaching events, using a community-level dataset spanning 27 years on the Great Barrier Reef. We found that the diversity of colours found within a fish community is directly related to the composition of the local environment. Areas with a higher cover of structurally complex corals contained fish species with more diverse and brighter colourations. Most notably, fish community colouration contracted significantly in the years following the 1998 global coral bleaching event. Fishes with colouration directly appealing to human aesthetics are becoming increasingly rare, with the potential for marked declines in the perceived colour of reef fish communities in the near future. Future reefs may not be the colourful ecosystems we recognize today, representing the loss of a culturally significant ecosystem service

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

Body size determines eyespot size and presence in coral reef fishes

Numerous organisms display conspicuous eyespots. These eye-like patterns have been shown to effectively reduce predation by either deflecting strikes away from nonvital organs or by intimidating potential predators. While investigated extensively in terrestrial systems, determining what factors shape eyespot form in colorful coral reef fishes remains less well known. Using a broadscale approach we ask: How does the size of the eyespot relate to the actual eye, and at what size during ontogeny are eyespots acquired or lost? We utilized publicly available images to generate a dataset of 167 eyespot-bearing reef fish species. We measured multiple features relating to the size of the fish, its eye, and the size of its eyespot. In reef fishes, the area of the eyespot closely matches that of the real eye; however, the eyespots "pupil" is nearly four times larger than the real pupil. Eyespots appear at about 20 mm standard length. However, there is a marked decrease in the presence of eyespots in fishes above 48 mm standard length; a size which is tightly correlated with significant decreases in documented mortality rates. Above 75-85 mm, the cost of eyespots appears to outweigh their benefit. Our results identify a "size window" for eyespots in coral reef fishes, which suggests that eyespot use is strictly body size-dependent within this group

Drivers of eyespot evolution in coral reef fishes

Evolution via natural selection has continually shaped the coloration of numerous organisms. One coloration of particular importance is the eyespot: a phylogenetically widespread, conspicuous marking that has been shown to effectively reduce predation, often through its resemblance to the eye. Although widely studied, most research has been experimental in nature. We approach eyespots using a comparative phylogenetic framework that is global in scope. Herein, we identify the potential drivers of eyespot evolution in coral reef fishes; essentially the rules that govern their appearance in this group of organisms. We surveyed 2664 reef fish species (42% of all described reef fish species) and found that eyespots are present in approximately one in every 10 species. Most eyespots occur in closely related species and have been present in some families for over 50 million years. Focusing on damselfishes (family: Pomacentridae) as a study group, we reveal that eyespots are rare in planktivorous species, which is likely driven by the predation risk associated with their feeding location. Using a heatmapping technique, we also show that the location of eyespots is fundamentally different in active fishes that swim above the benthos vs. cryptobenthic fishes that rest on the benthos. These location differences may reflect different functions of eyespots among reef fish species

How flexible are habitat specialists? Short-term space use in obligate coral-dwelling damselfishes

As habitats change, highly specialised species may die or be forced to relocate. However, some obligate coral-dwelling damselfishes appear to survive the localized extinction of their primary habitat, branching coral, caused by coral bleaching. To address this apparent paradox, we documented the spatial behaviour of obligate coral-dwellers in relation to habitat quality. Focussing on two obligate coral-dwelling damselfishes (Pomacentrus moluccensis and Chromis viridis), we used KUDs (Kernel Utilisation Distributions) to quantify fishes’ short-term space use (daily 5 min observations across 6 days) and related it to live coral cover and structural complexity derived from 3D photogrammetry. Specifically, we calculated movement extent (95% KUD), core areas (50% KUD) and the temporal consistency of occupied areas across consecutive days. Structural complexity had no effect on space use. The effect of live coral cover was significant but weak and dependent on fish body-size: core areas increased with decreasing live coral cover for large fishes; smaller fishes showed little response. In contrast to weak habitat effects, there were strong differences across sites. At one site, average core areas increased three-fold to 1.1 m2 for P. moluccensis and over 60-fold for C. viridis, from 1.14 m2 to an average core area of 92.34 m2 and a maximum recorded extent of 1471.4 m2. These findings may help explain these fishes’ apparent, unexpected resilience to habitat loss. Obligate coral-dwelling fishes may prefer branching live coral, but their ‘obligate’ dependence may be more flexible and context dependent. As ecosystems reconfigure, plasticity in fine-scale spatial behaviour may be critical for the persistence of fish populations