Diurnal variability in a fish assemblage of a Bahamian coral reef

Synopsis Variation in the diurnal composition of a fish assemblage of a Bahamian coral reef was investigated by comparing visual counts of fishes taken along a 100 × 4 m wide fixed transect at four times: 0900, 1200, 1500 and 1800 hours during the summer of 1979. One sample per day was obtained at each time period over a span of 20 consecutive days. Forty-two species were recorded in these samples, with 25 occurring frequently enough to permit statistical analysis of diurnal variations in abundance. Of the 25 species compared, nearly one fourth (6 species) displayed significant variation in abundance patterns among the four time periods tested. It is suggested, because of the strong possibility of bias which might otherwise be introduced because of these variations, that repetitive quantitative visual censusing of coral reef fishes be undertaken at about the same time each day.     

Strong density-dependent survival and recruitment regulate the abundance of a coral reef fish

Debate on the control of population dynamics in reef fishes has centred on whether patterns in abundance are determined by the supply of planktonic recruits, or by post-recruitment processes. Recruitment limitation implies little or no regulation of the reef-associated population, and is supported by several experimental studies that failed to detect density dependence. Previous manipulations of population density have, however, focused on juveniles, and there have been no tests for density-dependent interactions among adult reef fishes. I tested for population regulation in Coryphopterus glaucofraenum, a small, short-lived goby that is common in the Caribbean. Adult density was manipulated on artificial reefs and adults were also monitored on reefs where they varied in density naturally. Survival of adult gobies showed a strong inverse relationship with their initial density across a realistic range of densities. Individually marked gobies, however, grew at similar rates across all densities, suggesting that density-dependent survival was not associated with depressed growth, and so may result from predation or parasitism rather than from food shortage. Like adult survival, the accumulation of new recruits on reefs was also much lower at high adult densities than at low densities. Suppression of recruitment by adults may occur because adults cause either reduced larval settlement or reduced early post-settlement survival. In summary, this study has documented a previously unrecorded regulatory mechanism for reef fish populations (density-dependent adult mortality) and provided a particularly strong example of a well-established mechanism (density-dependent recruitment). In combination, these two compensatory mechanisms have the potential to strongly regulate the abundance of this species, and rule out the control of abundance by the supply of recruits.     

Abundance–occupancy patterns in a riverine fish assemblage

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Spatial patterns of self‐recruitment of a coral reef fish in relation to island‐scale retention mechanisms

Oceanographic features influence the transport and delivery of marine larvae, and physical retention mechanisms, such as eddies, can enhance self‐recruitment (i.e. the return of larvae to their natal population). Knowledge of exact locations of hatching (origin) and settlement (arrival) of larvae of reef animals provides a means to compare observed patterns of self‐recruitment ‘connectivity’ with those expected from water circulation patterns. Using parentage inference based on multiple sampling years in Moorea, French Polynesia, we describe spatial and temporal variation in self‐recruitment of the anemonefish Amphiprion chrysopterus, evaluate the consistency of net dispersal distances of self‐recruits against the null expectation of passive particle dispersal and test the hypothesis that larvae originating in certain reef habitats (lagoons and passes) would be retained and thus more likely to self‐recruit than those originating on the outer (fore) reef. Estimates of known self‐recruitment were consistent across the sampling years (~25–27% of sampled recruits). For most (88%) of these self‐recruits, the net distance between hatching and settlement locations was within the maximum dispersal distance expected for a neutrally buoyant passive particle based on the longest duration of the larval dispersive phase and the average direction and speed of current flow around Moorea. Furthermore, a parent of a given body size on the outer (fore) reef of Moorea was less likely to produce self‐recruits than those in passes. Our findings show that even a simple dispersal model based on net average flow and direction of alongshore currents can provide insight into landscape‐scale retention patterns of reef fishes.     

Stochastic determinants of assemblage patterns in coral reef fishes: a quantification by means of two models

Synopsis One perspective emphasizing the importance of stochastic processes in determining coral reef fish assemblages implies that there is little organization in species richness, abundance structure, and spatial distribution. We examine the degree to which this perspective is correct by analyzing distribution of fishes on a collection of patch reefs (Discovery Bay, Jamaica). We ask the question whether these patches accumulate species and individuals in a manner consistent with stochastic expectations. To address this question we use two conceptual models, each permitting a different insight. One assumes that fish are distributed stochastically on patches while the other assumes presence of restrictions on fish distribution due to habitat structure. For each conceptual model we use two types of benchmark: we compare observed patterns to those predicted by theoretical models, and we also compare observed patterns to those obtained from a random reallocation of fish individuals to patches. We found that the conceptual model assuming stochastic processes appeared to provide weaker explanation of patterns than the conceptual model that includes restrictions due to habitat structure. Further, and more importantly, we found that (i) the community is shaped by a mixture of stochastic and non-stochastic mechanisms, and (ii) the stochastic assembly processes decrease in importance for species restricted to fewer microhabitat types and sites. Our study therefore indicates that patches do accumulate individuals and species in a manner consistent with stochastic expectations, however, this applies primarily to the habitat generalists (unrestricted species). By the same token, increased habitat specialization by some species imposes constraints on the stochastic model such that it eventually fails.     

Multi-scale recruitment patterns and effects on local population size of a temperate reef fish

Recruitment of the temperate reef fish Coris julis was studied across the Azores Archipelago (central North Atlantic), over four consecutive recruitment seasons and at three spatial scales: between islands (separated by 100s of km), sites within islands (separated by 10s of km) and transects within sites (separated by 10s of m). At the largest scale ( i.e . between islands) spatial recruitment patterns were highly variable, suggesting the influence of stochastic processes. Recruitment was spatially consistent within islands, even though magnitude was unpredictable between years, indicating that processes at meso-scales are probably more deterministic. Recruits settled randomly at the transect scale, probably reflecting habitat homogeneity. It was proposed that large and island-scale patterns reflect larval availability, driven by physical and biological processes occurring in the plankton. No evidence was found for a density-dependent relationship between newly settled and 2 week settled C. julis nor between cumulative recruitment and young-of-the-year. It appears that adult density is limited by larval supply (pre-settlement regulation) at low recruitment sites, and determined by post-settlement, density-dependent processes at high recruitment sites. This work is one of few to investigate multiple spatial and temporal scales of recruitment for a coastal fish species inhabiting isolated, temperate oceanic islands and hence, provides a novel comparison to the many studies of recruitment on coral reefs and other, more connected systems.     

Sexual selection explains sex-specific growth plasticity and positive allometry for sexual size dimorphism in a reef fish

In 1950, Rensch noted that in clades where males are the larger sex, sexual size dimorphism (SSD) tends to be more pronounced in larger species. This fundamental allometric relationship is now known as ‘Rensch''s rule’. While most researchers attribute Rensch''s rule to sexual selection for male size, experimental evidence is lacking. Here, we suggest that ultimate hypotheses for Rensch''s rule should also apply to groups of individuals and that individual trait plasticity can be used to test those hypotheses experimentally. Specifically, we show that in the sex-changing fish Parapercis cylindrica, larger males have larger harems with larger females, and that SSD increases with harem size. Thus, sexual selection for male body size is the ultimate cause of sexual size allometry. In addition, we experimentally illustrate a positive relationship between polygyny potential and individual growth rate during sex change from female to male. Thus, sexual selection is the ultimate cause of variation in growth rate, and variation in growth rate is the proximate cause of sexual size allometry. Taken together, our results provide compelling evidence in support of the sexual selection hypothesis for Rensch''s rule and highlight the potential importance of individual growth modification in the shaping of morphological patterns in Nature.     

Global warming may disproportionately affect larger adults in a predatory coral reef fish

Global warming is expected to reduce body sizes of ectothermic animals. Although the underlying mechanisms of size reductions remain poorly understood, effects appear stronger at latitudinal extremes (poles and tropics) and in aquatic rather than terrestrial systems. To shed light on this phenomenon, we examined the size dependence of critical thermal maxima (CTmax) and aerobic metabolism in a commercially important tropical reef fish, the leopard coral grouper (Plectropomus leopardus) following acclimation to current‐day (28.5 °C) vs. projected end‐of‐century (33 °C) summer temperatures for the northern Great Barrier Reef (GBR). CTmax declined from 38.3 to 37.5 °C with increasing body mass in adult fish (0.45–2.82 kg), indicating that larger individuals are more thermally sensitive than smaller conspecifics. This may be explained by a restricted capacity for large fish to increase mass‐specific maximum metabolic rate (MMR) at 33 °C compared with 28.5 °C. Indeed, temperature influenced the relationship between metabolism and body mass (0.02–2.38 kg), whereby the scaling exponent for MMR increased from 0.74 ± 0.02 at 28.5 °C to 0.79 ± 0.01 at 33 °C, and the corresponding exponents for standard metabolic rate (SMR) were 0.75 ± 0.04 and 0.80 ± 0.03. The increase in metabolic scaling exponents at higher temperatures suggests that energy budgets may be disproportionately impacted in larger fish and contribute to reduced maximum adult size. Such climate‐induced reductions in body size would have important ramifications for fisheries productivity, but are also likely to have knock‐on effects for trophodynamics and functioning of ecosystems.     

Spatial and temporal patterns in the coral assemblage at Clipperton Atoll: a sentinel reef in the Eastern Tropical Pacific

Coral Reefs - Isolated coral reef habitats are unique systems to study the natural dynamics of coral traits and their natural acclimatization, adaptation, and recovery from global-scale stressors...     

Distinct patterns of hybridization across a suture zone in a coral reef fish (Dascyllus trimaculatus)

Hybrid zones are natural laboratories for investigating the dynamics of gene flow, reproductive isolation, and speciation. A predominant marine hybrid (or suture) zone encompasses Christmas Island (CHR) and Cocos (Keeling) Islands (CKE), where 15 different instances of interbreeding between closely related species from Indian and Pacific Oceans have been documented. Here, we report a case of hybridization between genetically differentiated Pacific and Indian Ocean lineages of the three‐spot dascyllus, Dascyllus trimaculatus (Rüppell, 1829). Field observations indicate there are subtle color differences between Pacific and Indian Ocean lineages. Most importantly, population densities of color morphs and genetic analyses (mitochondrial DNA and SNPs obtained via RADSeq) suggest that the pattern of hybridization within the suture zone is not homogeneous. At CHR, both color morphs were present, mitochondrial haplotypes of both lineages were observed, and SNP analyses revealed both pure and hybrid genotypes. Meanwhile, in CKE, the Indian Ocean color morphs were prevalent, only Indian Ocean mitochondrial haplotypes were observed, and SNP analysis showed hybrid individuals with a large proportion (~80%) of their genotypes assigning to the Indian Ocean lineage. We conclude that CHR populations are currently receiving an influx of individuals from both ocean basins, with a greater influence from the Pacific Ocean. In contrast, geographically isolated CKE populations appear to be self‐recruiting and with more influx of individuals from the Indian Ocean. Our research highlights how patterns of hybridization can be different at scales of hundreds of kilometers, due to geographic isolation and the history of interbreeding between lineages.     

Effects of microhabitat characteristics on the settlement and recruitment of a coral reef fish at two spatial scales

Populations of fishes on coral reefs are replenished by the settlement of pelagic larvae to demersal populations. Recruitment varies spatially and temporally and can exert strong effects on the dynamics of reef fish populations. This study examined the effect of microhabitat characteristics on small-scale and large-scale recruitment variation in the three-spot damselfish, Stegastes planifrons (Cuvier). Comparison of 0.25-m² quadrats occupied by three-spots with randomly sampled null quadrats showed that three-spots quadrats contained a higher percent cover of the coral Montastrea annularis than would be expected at random. Manipulative experiments on three types of 1.0-m² patch reefs (living M. annularis, dead Porites Porites and dead Acropora palmata) patch reefs on showed that this non-random distribution was established by microhabitat choice during settlement and not by differential post-settlement survival. The presence of conspecific juveniles did not affect settlement. Recruitment was monitored at nine sites on three islands over 3 years. Recruitment showed no consistent pattern in the relative levels of recruitment among sites. Similarly, no consistent relationship emerged between recruitment levels and microhabitat characteristics at the nine sites. For example, at this large scale, the percent cover of M. annularis explained variation in recruitment in only 1 out of 3 years. These results suggest that small-scale recruitment patterns are influenced by microhabitat choice during settlement, but that these habitat effects do not scale up to influence large-scale variation in recruitment.     

Latitudinal patterns of egg size and maternal investment trade-offs in reef corals

Aim

The aims were to test the role of temperature in latitudinal patterns of egg size and investigate maternal investment trade-offs among coral taxa.

Location

Global, from 34° S to 34° N.

Time period

1981–2020.

Major taxa studied

Reef coral species from the order Scleractinia.

Methods

We compiled a comprehensive geo-referenced global dataset of egg sizes (diameter or volume) and fecundity (number of eggs per area) for colonial corals (Scleractinia; 123 species, 5359 observations and 39 localities), substantially enhanced by new field collections (>88% of observations). We used Bayesian phylogenetic multilevel models to test for Rass' rule (a hypothesized negative relationship between egg size and temperature); we also included other environmental variables and life history traits. We also tested whether a trade-off exists between egg size and fecundity in broadcast spawning hermaphroditic corals with horizontal symbiont transmission (HHT).

Results

We found a significant relationship between coral egg size and symbiont transmission. Eggs from coral species with vertical symbiont transmission were c. 18.8% smaller than those from species with horizontal symbiont transmission. We also found non-significant relationships between egg size and sea surface temperature (SST) for broadcast spawning corals and between egg size and fecundity specifically for HHT species.

Main conclusions

Contrary to recognized latitudinal patterns of egg size across taxa, our study does not provide support for Rass' rule in corals. Additionally, our findings do not support a maternal investment trade-off between egg size and fecundity for HHT species. Our study used a phylogenetic framework that should be a standard practice when studying interspecific variation, including investigation of maternal investment and identification of the influence of multiple predictors on larval fitness (egg size), in addition to trade-offs affecting propagule influx (fecundity). Both these functional traits are vital and have direct consequences for population maintenance and connectivity in sessile organisms, such as corals.     

Variation in fish density,assemblage composition and relative rates of predation among mangrove,seagrass and coral reef habitats

We tested the hypothesis for several Caribbean reef fish species that there is no difference in nursery function among mangrove, seagrass and shallow reef habitat as measured by: (a) patterns of juvenile and adult density, (b) assemblage composition, and (c) relative predation rates. Results indicated that although some mangrove and seagrass sites showed characteristics of nursery habitats, this pattern was weak. While almost half of our mangrove and seagrass sites appeared to hold higher proportions of juvenile fish (all species pooled) than did reef sites, this pattern was significant in only two cases. In addition, only four of the six most abundant and commercially important species (Haemulon flavolineatum, Haemulon sciurus, Lutjanus apodus, Lutjanus mahogoni, Scarus iserti, and Sparisoma aurofrenatum) showed patterns of higher proportions of juvenile fish in mangrove and/or seagrass habitat(s) relative to coral reefs, and were limited to four of nine sites. Faunal similarity between reef and either mangrove or seagrass habitats was low, suggesting little, if any exchange between them. Finally, although relative risk of predation was lower in mangrove/seagrass than in reef habitats, variance in rates was substantial suggesting that not all mangrove/seagrass habitats function equivalently. Specifically, relative risk varied between morning and afternoon, and between sites of similar habitat, yet varied little, in some cases, between habitats (mangrove/seagrass vs. coral reefs). Consequently, our results caution against generalizations that all mangrove and seagrass habitats have nursery function.     

Behavioral and energetic costs of group membership in a coral reef fish

Animals in social aggregations often spend more time foraging than solitary conspecifics. This may be a product of the relative safety afforded by aggregations: group members can devote more time to foraging and less time to antipredator behaviors than solitary animals (the “risk reduction” effect). All else being equal, risk reduction should result in higher food intake for grouped animals. However, intragroup competition may force group members to spend more time foraging in order to obtain the same food ration as solitary individuals (the “resource competition” effect). We compared these opposing explanations of foraging time allocation in a coral reef fish, bluehead wrasse (Thalassoma bifasciatum). Aggregations of juvenile bluehead wrasse experience safety-in-numbers, and preliminary observations suggested that juveniles in aggregations spent more time foraging for copepods in the water column than solitary juveniles. However, the risk reduction and resource competition hypotheses are indistinguishable on the basis of behavioral observations alone. Therefore, we collected behavioral, dietary, and growth data (using otolith growth rings) for bluehead wrasse at multiple reefs around a Caribbean island. Despite spending more time foraging in the water column, grouped fish did not capture more prey items and had slower growth rates than solitary fish. Thus, the increased foraging time of grouped fish appears to reflect resource competition, not risk reduction. This competition may limit the size and frequency of aggregations among juvenile bluehead wrasse, which have been shown to experience reduced mortality rates in larger groups. Bluehead wrasse recruits also spent less time foraging but grew faster at sites where planktonic copepod prey were more abundant. This suggests the possibility that large-scale spatiotemporal variability in the abundance of planktonic copepods over coral reefs may produce corresponding variability in the dynamics of reef fish populations. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Seascape continuity plays an important role in determining patterns of spatial genetic structure in a coral reef fish

Detecting patterns of spatial genetic structure (SGS) can help identify intrinsic and extrinsic barriers to gene flow within metapopulations. For marine organisms such as coral reef fishes, identifying these barriers is critical to predicting evolutionary dynamics and demarcating evolutionarily significant units for conservation. In this study, we adopted an alternative hypothesis‐testing framework to identify the patterns and predictors of SGS in the Caribbean reef fish Elacatinus lori. First, genetic structure was estimated using nuclear microsatellites and mitochondrial cytochrome b sequences. Next, clustering and network analyses were applied to visualize patterns of SGS. Finally, logistic regressions and linear mixed models were used to identify the predictors of SGS. Both sets of markers revealed low global structure: mitochondrial Φ_ST = 0.12, microsatellite F_ST = 0.0056. However, there was high variability among pairwise estimates, ranging from no differentiation between sites on contiguous reef (Φ_ST = 0) to strong differentiation between sites separated by ocean expanses ≥ 20 km (maximum Φ_ST = 0.65). Genetic clustering and statistical analyses provided additional support for the hypothesis that seascape discontinuity, represented by oceanic breaks between patches of reef habitat, is a key predictor of SGS in E. lori. Notably, the estimated patterns and predictors of SGS were consistent between both sets of markers. Combined with previous studies of dispersal in E. lori, these results suggest that the interaction between seascape continuity and the dispersal kernel plays an important role in determining genetic connectivity within metapopulations.     

Social and environmental influences on egg size evolution in frogs

The reproductive strategies of frogs are highly diverse, but analysis of these strategies in a phylogenetic context has lagged behind other taxa. Here we investigate associations between aspects of parental care and egg size in a phylogenetic context. We obtained data on egg size and parental care strategies in various species of frogs from the scientific literature. We developed a phylogenetic supertree of frogs by combining the results of multiple phylogenetic analyses using matrix representation parsimony. We used phylogenetic pairwise comparisons to investigate the correlation between various forms of parental care and egg size across the order Anura. We also investigated correlations between tadpole carnivory and egg size, and phytotelm breeding and egg size. We also investigated the association of egg size with several environmental factors. Parental care, male parental care, direct development, stream breeding and montane breeding habitats were all associated with large egg size. Female care (in species with trophic egg feeding), carnivory, use of small pools (phytotelmata) and use of temporary pools were not associated with egg size.     

Meta-population structure in a coral reef fish demonstrated by genetic data on patterns of migration,extinction and re-colonisation

Background  

Management strategies for coral reefs are dependant on information about the spatial population structure and connectivity of reef organisms. Genetic tools can reveal important information about population structure, however, this information is lacking for many reef species. We used a mitochondrial molecular marker to examine the population genetic structure and the potential for meta-population dynamics in a direct developing coral reef fish using 283 individuals from 15 reefs on the Great Barrier Reef, Australia. We employed a hierarchical sampling design to test genetic models of population structure at multiple geographical scales including among regions, among shelf position and reefs within regions. Predictions from island, isolation-by-distance and meta-population models, including the potential for asymmetric migration, local extinction and patterns of re-colonisation were examined.     

Predation risk assessment by olfactory and visual cues in a coral reef fish

Assessment of predation risk is vital for the success of an individual. Primary cues for the assessment include visual and olfactory stimuli, but the relative importance of these sources of information for risk assessment has seldom been assessed for marine fishes. This study examined the importance of visual and chemical cues in assessing risk for the star goby, Asterropteryx semipunctatus. Visual and chemical cue intensities were used that were indicative of a high threat situation. The behavioural response elicited by both the visual cues of a predator (the rock cod, Cephalopholis boenak) and the chemical alarm cues from conspecifics were similar in magnitude, with responses including a decrease in feeding strikes and moves. A bobbing behaviour was exhibited when the predator was visible and not when only exposed to the chemical alarm cue. When visual and chemical cues were presented together they yielded a stronger antipredator response than when gobies were exposed solely to conspecific alarm cues. This suggests additivity of risk assessment information at the levels of threat used, however, the goby’s response is also likely to depend on the environmental and social context of the predator–prey encounter. This study highlights the importance of chemical cues in the assessment of predation risk for a coral reef fish.     

Seascape and life-history traits do not predict self-recruitment in a coral reef fish

The persistence and resilience of many coral reef species are dependent on rates of connectivity among sub-populations. However, despite increasing research efforts, the spatial scale of larval dispersal remains unpredictable for most marine metapopulations. Here, we assess patterns of larval dispersal in the angelfish Centropyge bicolor in Kimbe Bay, Papua New Guinea, using parentage and sibling reconstruction analyses based on 23 microsatellite DNA loci. We found that, contrary to previous findings in this system, self-recruitment (SR) was virtually absent at both the reef (0.4–0.5% at 0.15 km²) and the lagoon scale (0.6–0.8% at approx. 700 km²). While approximately 25% of the collected juveniles were identified as potential siblings, the majority of sibling pairs were sampled from separate reefs. Integrating our findings with earlier research from the same system suggests that geographical setting and life-history traits alone are not suitable predictors of SR and that high levels of localized recruitment are not universal in coral reef fishes.