Christopher H. R. Goatley

Quantifying Relative Diver Effects in Underwater Visual Censuses

Diver-based Underwater Visual Censuses (UVCs), particularly transect-based surveys, are key tools in the study of coral reef fish ecology. These techniques, however, have inherent problems that make it difficult to collect accurate numerical data. One of these problems is the diver effect (defined as the reaction of fish to a diver). Although widely recognised, its effects have yet to be quantified and the extent of taxonomic variation remains to be determined. We therefore examined relative diver effects on a reef fish assemblage on the Great Barrier Reef. Using common UVC methods, the recorded abundance of seven reef fish groups were significantly affected by the ongoing presence of SCUBA divers. Overall, the diver effect resulted in a 52% decrease in the mean number of individuals recorded, with declines of up to 70% in individual families. Although the diver effect appears to be a significant problem, UVCs remain a useful approach for quantifying spatial and temporal variation in relative fish abundances, especially if using methods that minimise the exposure of fishes to divers. Fixed distance transects using tapes or lines deployed by a second diver (or GPS-calibrated timed swims) would appear to maximise fish counts and minimise diver effects.

The Roles of Dimensionality, Canopies and Complexity in Ecosystem Monitoring

Canopies are common among autotrophs, increasing their access to light and thereby increasing competitive abilities. If viewed from above canopies may conceal objects beneath them creating a ‘canopy effect’. Due to complexities in collecting 3-dimensional data, most ecosystem monitoring programmes reduce dimensionality when sampling, resorting to planar views. The resultant ‘canopy effects’ may bias data interpretation, particularly following disturbances. Canopy effects are especially relevant on coral reefs where coral cover is often used to evaluate and communicate ecosystem health. We show that canopies hide benthic components including massive corals and algal turfs, and as planar views are almost ubiquitously used to monitor disturbances, the loss of vulnerable canopy-forming corals may bias findings by presenting pre-existing benthic components as an altered system. Our reliance on planar views in monitoring ecosystems, especially coral cover on reefs, needs to be reassessed if we are to better understand the ecological consequences of ever more frequent disturbances.

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

Abstract 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.

Fifty million years of herbivory on coral reefs: fossils, fish and functional innovations

The evolution of ecological processes on coral reefs was examined based on Eocene fossil fishes from Monte Bolca, Italy and extant species from the Great Barrier Reef, Australia. Using ecologically relevant morphological metrics, we investigated the evolution of herbivory in surgeonfishes (Acanthuridae) and rabbitfishes (Siganidae). Eocene and Recent surgeonfishes showed remarkable similarities, with grazers, browsers and even specialized, long-snouted forms having Eocene analogues. These long-snouted Eocene species were probably pair-forming, crevice-feeding forms like their Recent counterparts. Although Eocene surgeonfishes likely played a critical role as herbivores during the origins of modern coral reefs, they lacked the novel morphologies seen in modern Acanthurus and Siganus (including eyes positioned high above their low-set mouths). Today, these forms dominate coral reefs in both abundance and species richness and are associated with feeding on shallow, exposed algal turfs. The radiation of these new forms, and their expansion into new habitats in the Oligocene–Miocene, reflects the second phase in the development of fish herbivory on coral reefs that is closely associated with the exploitation of highly productive short algal turfs.

Sediment suppresses herbivory across a coral reef depth gradient

Sediments are a ubiquitous feature of all coral reefs, yet our understanding of how they affect complex ecological processes on coral reefs is limited. Sediment in algal turfs has been shown to suppress herbivory by coral reef fishes on high-sediment, low-herbivory reef flats. Here, we investigate the role of sediment in suppressing herbivory across a depth gradient (reef base, crest and flat) by observing fish feeding following benthic sediment reductions. We found that sediment suppresses herbivory across all reef zones. Even slight reductions on the reef crest, which has 35 times less sediment than the reef flat, resulted in over 1800 more herbivore bites (h−1 m−2). The Acanthuridae (surgeonfishes) were responsible for over 80 per cent of all bites observed, and on the reef crest and flat took over 1500 more bites (h−1 m−2) when sediment load was reduced. These findings highlight the role of natural sediment loads in shaping coral reef herbivory and suggest that changes in benthic sediment loads could directly impair reef resilience.

Evolution of long-toothed fishes and the changing nature of fish-benthos interactions on coral reefs.

Interactions between fishes and the benthos have shaped the development of marine ecosystems since at least the early Mesozoic. Here, using the morphology of fish teeth as an indicator of feeding abilities, we quantify changes over the last 240 million years of reef fish evolution. Fossil and extant coral reef fish assemblages reveal exceptional stasis in tooth design over time, with one notable exception, a distinct long-toothed form. Arising only in the last 40 million years, these long-toothed fishes have bypassed the invertebrate link in the food chain, feeding directly on benthic particulate material. With the appearance of elongated teeth, these specialized detritivores have moved from eating invertebrates to eating the food of invertebrates. Over evolutionary time, fishes have slid back down the food chain.

Ecological Consequences of Sediment on High-Energy Coral Reefs

Sediments are widely accepted as a threat to coral reefs but our understanding of their ecological impacts is limited. Evidence has suggested that benthic sediments bound within the epilithic algal matrix (EAM) suppress reef fish herbivory, a key ecological process maintaining reef resilience. An experimental combination of caging and sediment addition treatments were used to investigate the effects of sediment pulses on herbivory and EAMs and to determine whether sediment addition could trigger a positive-feedback loop, leading to deep, sediment-rich turfs. A 1-week pulsed sediment addition resulted in rapid increases in algal turf length with effects comparable to those seen in herbivore exclusion cages. Contrary to the hypothesised positive-feedback mechanism, benthic sediment loads returned to natural levels within 3 weeks, however, the EAM turfs remained almost 60% longer for at least 3 months. While reduced herbivore density is widely understood to be a major threat to reefs, we show that acute disturbances to reef sediments elicit similar ecological responses in the EAM. With reefs increasingly threatened by both reductions in herbivore biomass and altered sediment fluxes, the development of longer turfs may become more common on coral reefs.

The evolution of fishes and corals on reefs: form, function and interdependence.

Coral reefs are renowned for their spectacular biodiversity and the close links between fishes and corals. Despite extensive fossil records and common biogeographic histories, the evolution of these two key groups has rarely been considered together. We therefore examine recent advances in molecular phylogenetics and palaeoecology, and place the evolution of fishes and corals in a functional context. In critically reviewing the available fossil and phylogenetic evidence, we reveal a marked congruence in the evolution of the two groups. Despite one group consisting of swimming vertebrates and the other colonial symbiotic invertebrates, fishes and corals have remarkably similar evolutionary histories. In the Paleocene and Eocene [66–34 million years ago (Ma)] most modern fish and coral families were present, and both were represented by a wide range of functional morphotypes. However, there is little evidence of diversification at this time. By contrast, in the Oligocene and Miocene (34–5.3 Ma), both groups exhibited rapid lineage diversification. There is also evidence of increasing reef area, occupation of new habitats, increasing coral cover, and potentially, increasing fish abundance. Functionally, the Oligocene–Miocene is marked by the appearance of new fish and coral taxa associated with high‐turnover fast‐growth ecosystems and the colonization of reef flats. It is in this period that the functional characteristics of modern coral reefs were established. Most species, however, only arose in the last 5.3 million years (Myr; Plio–Pleistocene), with the average age of fish species being 5.3 Myr, and corals just 1.9 Myr. While these species are genetically distinct, phenotypic differences are often limited to variation in colour or minor morphological features. This suggests that the rapid increase in biodiversity during the last 5.3 Myr was not matched by changes in ecosystem function. For reef fishes, colour appears to be central to recent diversification. However, the presence of pigment patterns in the Eocene suggests that colour may not have driven recent diversification. Furthermore, the lack of functional changes in fishes or corals over the last 5 Myr raises questions over the role and importance of biodiversity in shaping the future of coral reefs.

The ontogeny of home ranges: evidence from coral reef fishes

The concept of home ranges is fundamental to ecology. Numerous studies have quantified how home ranges scale with body size across taxa. However, these relationships are not always applicable intraspecifically. Here, we describe how the home range of an important group of reef fish, the parrotfishes, scales with body mass. With masses spanning five orders of magnitude, from the early postsettlement stage through to adulthood, we find no evidence of a response to predation risk, dietary shifts or sex change on home range expansion rates. Instead, we document a distinct ontogenetic shift in home range expansion with sexual maturity. Juvenile parrotfishes displayed rapid home range growth until reaching approximately 100–150 mm length. Thereafter, the relationship between home range and mass broke down. This shift reflected changes in colour patterns, social status and reproductive behaviour associated with the transition to adult stages. While there is a clear relationship between body mass and home ranges among adult individuals of different species, it does not appear to be applicable to size changes within species. Ontogenetic changes in parrotfishes do not follow expected mass–area scaling relationships.

Morphological structure in a reef fish assemblage

Two key morphological traits, horizontal gape and eye diameter, were measured in a large representative group of coral reef fishes. These morphological traits were used concurrently to assess their utility in exploring abilities of coral reef fishes at an assemblage level. A total of 1,218 specimens from 181 species found on the Great Barrier Reef were examined. Cryptobenthic fishes were included to provide a broader representation of reef fish groups. In the analyses, a clear morphological distinction was found between nocturnal and diurnal fishes. Nocturnal fishes had larger relative horizontal gapes and relative eye diameters by factors of 1.6 and 1.5, respectively. A bivariate plot separated into quadrants was used to assess the implications of morphological variation. The morphological measures reflected distinct ecological traits in each quadrant. Whilst nocturnal fishes had large relative gapes and eye diameters, diurnal predators and detritivores had the same wide gapes, but small relative eye diameters. Highly selective, visual feeders such as the Chaetodontidae and Pseudochromidae had large eyes and small gapes, whilst non-selective feeders with low visual dependence such as the grazing herbivores (Acanthuridae, Siganidae, etc.) had both small eye diameters and gape sizes. The analysis proved to be robust enough to apply to a wide assemblage, but with enough subtlety to distinguish morphological differences within individual families. The methods used in this study may have broad applications to other fish assemblages, both fossil and extant.