Coral reef fish smell leaves to find island homes

Recent studies have shown that some coral reef fish larvae return to natal reefs, while others disperse to distant reefs. However, the sensory mechanisms used to find settlement sites are poorly understood. One hypothesis is that larvae use olfactory cues to navigate home or find other suitable reef habitats. Here we show a strong association between the clownfish Amphiprion percula and coral reefs surrounding offshore islands in Papua New Guinea. Host anemones and A. percula are particularly abundant in shallow water beneath overhanging rainforest vegetation. A series of experiments were carried out using paired-choice flumes to evaluate the potential role of water-borne olfactory cues in finding islands. Recently settled A. percula exhibited strong preferences for: (i) water from reefs with islands over water from reefs without islands; (ii) water collected near islands over water collected offshore; and (iii) water treated with either anemones or leaves from rainforest vegetation. Laboratory reared-juveniles exhibited the same positive response to anemones and rainforest vegetation, suggesting that olfactory preferences are innate rather than learned. We hypothesize that A. percula use a suite of olfactory stimuli to locate vegetated islands, which may explain the high levels of self-recruitment on island reefs. This previously unrecognized link between coral reefs and island vegetation argues for the integrated management of these pristine tropical habitats.     

Coral larvae settle at a higher frequency on red surfaces

Although chemical cues serve as the primary determinants of larval settlement and metamorphosis, light is also known to influence the behavior and the settlement of coral planulae. For example, Porites astreoides planulae settle preferentially on unconditioned red substrata. In order to test whether this behavior was a response to color and whether other species also demonstrate color preference, settlement choice experiments were conducted with P. astreoides and Acropora palmata. In these experiments, larvae were offered various types of plastic substrata representing three to seven different color choices. Both species consistently settled on red (or red and orange) substrata at a higher frequency than other colors. In one experiment, P. astreoides settled on 100% of red, plastic cable ties but failed to settle on green or white substrata. In a second experiment, 24% of larvae settled on red buttons, more than settled on six other colors combined. A. palmata settled on 80% of red and of orange cables ties but failed to settle on blue in one experiment and settled on a greater proportion of red acrylic squares than on four other colors or limestone controls in a second experiment. The consistency of the response across a variety of plastic materials suggests the response is related to long-wavelength photosensitivity. Fluorescence and reflectance spectra of experimental substrata demonstrated that the preferred substrata had spectra dominated by wavelengths greater than 550 nm with little or no reflection or emission of shorter wavelengths. These results suggest that some species of coral larvae may use spectral cues for fine-scale habitat selection during settlement. This behavior may be an adaptation to promote settlement in crustose coralline algae (CCA)-dominated habitats facilitating juvenile survival.     

Coral reef fish rapidly learn to identify multiple unknown predators upon recruitment to the reef

Organisms often undergo shifts in habitats as their requirements change with ontogeny.Upon entering a new environment, it is vitally important to be able to rapidly assess predation risk. Predation pressure should selectively promote mechanisms that enable the rapid identification of novel predators. Here we tested the ability of a juvenile marine fish to simultaneously learn the identity of multiple previously unknown predators. Individuals were conditioned with a 'cocktail' of novel odours (from two predators and two non-predators) paired with either a conspecific alarm cue or a saltwater control and then tested for recognition of the four odours individually and two novel odours (one predator and one non-predator) the following day. Individuals conditioned with the 'cocktail' and alarm cue responded to the individual 'cocktail' odours with an antipredator response compared to controls. These results demonstrate that individuals acquire recognition of novel odours and that the responses were not due to innate recognition of predators or due to a generalised response to novel odours. Upon entering an unfamiliar environment prey species are able to rapidly assess the risk of predation, enhancing their chances of survival, through the assessment of chemical stimuli.     

Coral reef fish communities: a compromise view

Synopsis The fishes associated with coral reefs offer excellent opportunities for the study of the factors that determine the species composition of complex, highly interactive communities. Amenable to sampling and direct observations many patch reefs are small enough to be studied as entire units and yet diverse enough to include a wide range of interactions.Coral reef fishes appear to be highly specialized in morphology, color, behavior and life cycles and yet colonization experiments and repeated censusing have shown a surprisingly high variation in the fishes that are associated in similar habitats or in the same habitat at different times. This has led to two different views: (i) the chaos view that holds that the species composition is due to random factors and chance colonization, and (ii) the order view that resource sharing adaptations determine which species can live together.This paper reviews some of the obvious adaptations of reef fishes. An examination of the maximum sizes reached by infaunal reef dwellers shows that the largest individual of each species differs by a constant proportion from the next larger and next smaller species. This suggests growth limitations by interspecies competition. A hypothetical model showing how this might work is offered and it is shown that as long as there are more species than can be accommodated in the community at any one time the number of combinations is great enough to give the appearance of randomness even though the individual species may have precise environmental requirements.This paper forms part of the proceedings of a mini-symposium convened at Cornell University, Ithaca, N.Y., 18–19 May 1976, entitled Patterns of Community Structure in Fishes (G. S. Hellman, ed.).     

Coral reef fish populations can persist without immigration

Determining the conditions under which populations may persist requires accurate estimates of demographic parameters, including immigration, local reproductive success, and mortality rates. In marine populations, empirical estimates of these parameters are rare, due at least in part to the pelagic dispersal stage common to most marine organisms. Here, we evaluate population persistence and turnover for a population of orange clownfish, Amphiprion percula, at Kimbe Island in Papua New Guinea. All fish in the population were sampled and genotyped on five occasions at 2-year intervals spanning eight years. The genetic data enabled estimates of reproductive success retained in the same population (reproductive success to self-recruitment), reproductive success exported to other subpopulations (reproductive success to local connectivity), and immigration and mortality rates of sub-adults and adults. Approximately 50% of the recruits were assigned to parents from the Kimbe Island population and this was stable through the sampling period. Stability in the proportion of local and immigrant settlers is likely due to: low annual mortality rates and stable egg production rates, and the short larval stages and sensory capacities of reef fish larvae. Biannual mortality rates ranged from 0.09 to 0.55 and varied significantly spatially. We used these data to parametrize a model that estimated the probability of the Kimbe Island population persisting in the absence of immigration. The Kimbe Island population was found to persist without significant immigration. Model results suggest the island population persists because the largest of the subpopulations are maintained due to having low mortality and high self-recruitment rates. Our results enable managers to appropriately target and scale actions to maximize persistence likelihood as disturbance frequencies increase.     

Caging experiment to examine mortality during metamorphosis of coral reef fish larvae

All previous attempts to estimate early postsettlement mortality of coral reef fishes using either caging experiments or disappearance of new recruits have examined fish that had already settled, and therefore did not include the metamorphosis process. Crest nets capture unharmed transparent larvae during their migration from the open ocean to lagoon reefs before metamorphosis. We released these presettlement larvae at night into cages surrounding patch reefs and measured larval survivorship after two nights. This caging experiment involved cages enclosing the natural resident fish fauna, including predators, and others cleared of fish before releasing the larvae. The analyses of variance showed that (1) there was no difference in survivorship between the seven trials, (2) there was a significant difference between cleared and uncleared cages, and (3) there were significant differences between larval species tested. For the seven species that had a significant difference in survivorship between cleared and uncleared cages, average mortality of the larvae was 14% (range 0-26%) in cleared cages and 67% (range 29-76%) in cages with predators. The difference in mortality between species was related to the size of the larvae, as larger species exhibited reduced mortality compared to smaller species. Mortality was related to the abundance of resident fish that could act as predators or competitors. Predation can have a significant impact on the survival of metamorphosing fish larvae on coral reefs.     

Coral reef fish assemblages: intra- and interspecific competition for shelter sites

Synopsis Observations were made on intra- and interspecific aggressive interactions among the fishes living in the rubble/sand coral reef habitat in St. Croix, U. S. Virgin Islands. Four species (beaugregory — Stegastes leucostictus; ocean surgeonfish — Acanthurus bahianus; doctor fish — A. chirurgus; common squirrelfish — Holocentrus rufus) which sheltered in holes on the reef all actively defended one to several shelter sites at dusk. Short-term shelter side fidelity was observed in three of these four species. Agonistic interactions over both food and shelter occurred during the daytime but much less frequently than agonistic interactions over shelter at dusk. Dominance in intraspecific aggression was determined almost completely by the relative sizes of the individuals involved, with the larger individuals dominating in 95–98% of all encounters. A similar, but less strong, relationship between size and dominance existed for interactions between closely related species. For aggressive encounters between unrelated species, however, both relative sizes and species identity determined the outcome. Species, both diurnal and nocturnal, which strongly defend several shelter sites may have a strong and disproportionate impact on the sheltering behavior of other fishes. Intraspecific and interspecific defense of shelter sites may determine the patterns of mortality that result from predation, thereby influencing population abundances and assemblage composition.     

Parasites of recruiting coral reef fish larvae in New Caledonia

Recruiting coral reef fish larvae from 38 species and 19 families from New Caledonia were examined for parasites. We found 13 parasite species (Platyhelminthes: Monogenea, Cestoda and Trematoda) but no acanthocephalan, crustacean or nematode parasites. Over 23% of individual fish were infected. Didymozoid metacercariae were the most abundant parasites. We conclude that most of the parasites are pelagic species that become ‘lost’ once the fish larvae have recruited to the reef. Larval coral reef fish probably contribute little to the dispersal of the parasites of the adult fish so that parasite dispersal is more difficult than that of the fish themselves.     

Strategies and trajectories of coral reef fish larvae optimizing self-recruitment

Like many marine organisms, most coral reef fishes have a dispersive larval phase. The fate of this phase is of great concern for their ecology as it may determine population demography and connectivity. As direct study of the larval phase is difficult, we tackle the question of dispersion from an opposite point of view and study self-recruitment. In this paper, we propose a mathematical model of the pelagic phase, parameterized by a limited number of factors (currents, predator and prey distributions, energy budgets) and which focuses on the behavioral response of the larvae to these factors. We evaluate optimal behavioral strategies of the larvae (i.e. strategies that maximize the probability of return to the natal reef) and examine the trajectories of dispersal that they induce. Mathematically, larval behavior is described by a controlled Markov process. A strategy induces a sequence, indexed by time steps, of "decisions" (e.g. looking for food, swimming in a given direction). Biological, physical and topographic constraints are captured through the transition probabilities and the sets of possible decisions. Optimal strategies are found by means of the so-called stochastic dynamic programming equation. A computer program is developed and optimal decisions and trajectories are numerically derived. We conclude that this technique can be considered as a good tool to represent plausible larval behaviors and that it has great potential in terms of theoretical investigations and also for field applications.     

When to press on or turn back: dispersal strategies for reef fish larvae

Events that occur during the pelagic larval stage are thought to be important determinants of reef fish population dynamics. Recent research contradicts the early paradigm of larvae being advected as passive propagules and indicates that many late stage larvae have well-developed sensory and locomotory capabilities. Whether and how larvae use these capabilities to influence their dispersal is unknown. We compare alternative hypotheses regarding larval behavior. Contrary to the trend in dispersal modeling, we focus on larval biology rather than physical oceanographic considerations. Specifically, we present two streams of models: one that describes a return-based strategy and one in which dispersal is a central component. The models depend on different sets of behavioral assumptions for a pomacentrid species and for acanthurids, two groups with contrasting early life histories. Whether dispersal or return-based strategies are favored depends on the efficiency and sustainability of larval swimming methods and the environmental conditions experienced during dispersal. We argue that dispersal models should consider a variety of behavioral hypotheses and that the sensitivity of results to the behavioral assumptions made should be quantified.     

Vertical and horizontal distributions of coral‐reef fish larvae in open water immediately prior to reef colonization

To explore the vertical and horizontal distributions of fish larvae near the end of their pelagic period, six light traps were set up over four lunar months at different depths (sub‐surface, midwater and bottom) and different habitat types (reef slope: 50 m horizontal distance from the reef crest; frontier zone: 110 m horizontal distance; sandy zone: 200 m horizontal distance) on the outer reef slope of Moorea Island, French Polynesia. The highest captures were in sub‐surface traps on the reef slope and the frontier zone, and in bottom traps on the sandy zone and the frontier zone. It is hypothesized that fish larvae move towards the surface near the reef slope to avoid reef‐based planktivores and to get into a favourable position for surfing over the reef crest.     

Coral reef fisheries stock assessment

Summary Although all fisheries are multispecies and spatially heterogeneous, coral reef fisheries are an extreme in both respects. The two main approaches to stock assessment have been either to consider individual species separately or to lump all species together. Both are limited in their predictive power. The lack of ecological knowledge and the large number of parameters required make methods based on single species often impractical or expensive. However, where the appropriate information is available, ecological studies do not provide significant improvements on yield-per-recruit or surplus yield models currently used.Alternative community models based on aggregates of species lack predictive power, empirical support and relative data. Models are further limited because they do not address various economic aspects, fish movement and recruitment, all of which must be spatially resolved. However, the ECOPATH model, based on trophic compartments, represents a new approach useful to multispecies assessment, and the only way at present to include predation in stock models. In practice the data available will be the most important factor in the choice of stock assessment model.The major concern for management of many coral reefs is conservation of the habitat and stocks. In some cases this has been achieved with little reference to stock assessment, by using community management and closed areas, which are receiving increasing support. However, once the objective of conservation is achieved, stock assessment should have an important role in improving the economic performance of the fisheries. The wider problems of management have no simple solutions, but managers should look to adaptive management, designing their own experiments to choose between management models.     

A light trap design for stratum-specific sampling of reef fish larvae

The vertical distribution of late stage reef fish larvae may potentially influence their dispersal, recruitment success and energetic expenditure during the recruitment process. To date, methods of examining the vertical distribution of reef fish larvae either under-sample late stage individuals, or are incapable of discretely sampling the water column. The aim of this study was to develop a light trap able to sample a narrow depth range enabling fine scale patterns of vertical distribution to be examined. The experimental traps radiated light in a relatively narrow beam with a maximum vertical angle of radiation of 7.5°, indicating that the traps could be placed 4.8 m apart and still sample discrete depth strata. Their catch efficiency was similar to conventional light traps, indicating that they are adequate sampling units. Preliminary data showed that most families are more abundant near the surface, although many have significant numbers of individuals lower in the water column. Some families (inc. Apogonidae) occurred in higher abundance at greater depths. Our experimental light traps permit increased resolution of the vertical distribution of late stage larval reef fishes in the field.     

Coral bleaching, reef fish community phase shifts and the resilience of coral reefs

The 1998 global coral bleaching event was the largest recorded historical disturbance of coral reefs and resulted in extensive habitat loss. Annual censuses of reef fish community structure over a 12-year period spanning the bleaching event revealed a marked phase shift from a prebleach to postbleach assemblage. Surprisingly, we found that the bleaching event had no detectable effect on the abundance, diversity or species richness of a local cryptobenthic reef fish community. Furthermore, there is no evidence of regeneration even after 5–35 generations of these short-lived species. These results have significant implications for our understanding of the response of coral reef ecosystems to global warming and highlight the importance of selecting appropriate criteria for evaluating reef resilience.     

Coral reef bleaching: ecological perspectives

Coral reef bleaching, the whitening of diverse invertebrate taxa, results from the loss of symbiotic zooxanthellae and/or a reduction in photosynthetic pigment concentrations in zooxanthellae residing within the gastrodermal tissues of host animals. Of particular concern are the consequences of bleaching of large numbers of reef-building scleractinian corals and hydrocorals. Published records of coral reef bleaching events from 1870 to the present suggest that the frequency (60 major events from 1979 to 1990), scale (co-occurrence in many coral reef regions and often over the bathymetric depth range of corals) and severity (>95% mortality in some areas) of recent bleaching disturbances are unprecedented in the scientific literature. The causes of small scale, isolated bleaching events can often be explained by particular stressors (e.g., temperature, salinity, light, sedimentation, aerial exposure and pollutants), but attempts to explain large scale bleaching events in terms of possible global change (e.g., greenhouse warming, increased UV radiation flux, deteriorating ecosystem health, or some combination of the above) have not been convincing. Attempts to relate the severity and extent of large scale coral reef bleaching events to particular causes have been hampered by a lack of (a) standardized methods to assess bleaching and (b) continuous, long-term data bases of environmental conditions over the periods of interest. An effort must be made to understand the impact of bleaching on the remainder of the reef community and the long-term effects on competition, predation, symbioses, bioerosion and substrate condition, all factors that can influence coral recruitment and reef recovery. If projected rates of sea warming are realized by mid to late AD 2000, i.e. a 2°C increase in high latitude coral seas, the upper thermal tolerance limits of many reef-building corals could be exceeded. Present evidence suggests that many corals would be unable to adapt physiologically or genetically to such marked and rapid temperature increases.     

Directed motion in the sea: efficient swimming by reef fish larvae

Directed motion of marine organisms is examined with a focus on efficient behaviour, where efficient swimming minimizes either energetic expenditure or transit time. The swimming behaviour of late pelagic stage reef fish larvae is modelled to illustrate relevant concepts. To swim efficiently in the sea, an organism should exploit current-driven movements of the medium. Favourable currents should be ridden and unfavourable currents avoided. Relatively short movements to control advection can have a greater effect than longer swimming bouts used for independent horizontal locomotion. If larvae exploit the vertical structure of the water column, then the extent to which they can influence their dispersal will be substantially increased.     

Diel infestation dynamics of gnathiid isopod larvae parasitic on Caribbean reef fish

Infestation dynamics of parasitic gnathiid isopods on Caribbean reefs were studied throughout the 24-h diel cycle. Gnathiid infestation on caged longfin damselfish (Stegastes diencaeus) peaked strongly at dawn, remained low during the remainder of the day, and increased again at night until about midnight. Gnathiids were less abundant during the pre-dawn period. Peak loads on fish retrieved at dawn were the highest reported in any study thus far. The dawn peak consisted almost exclusively of individuals from the smallest size class, whereas nocturnal activity consisted almost exclusively of individuals of the largest size class. Because of the high rates of infestation at night and dawn, and the high variation in parasite loads on fish collected during that time, reduction of parasite infestation may play an important role in the selection of nocturnal and crepuscular shelter holes and settlement sites by reef fishes.     

Parasitic copepods affect morphospace and diet of larvae of a temperate reef fish

The effects of ectoparasites on larvae of the clingfish Gobiesox marmoratus were evaluated at the dietary and morphometric levels. The larvae and ectoparasites were collected by nearshore plankton samplings during October, November and December 2013 off El Quisco Bay, central Chile. The standardized abundance of total larvae and those ectoparasitized larvae (PL) was positively related and high parasite prevalence was found throughout the sampling period (up to 38%). Geometric morphometrics analyses indicate main changes in the shape through early ontogeny and subtle but significant variations between PL and non‐parasitized larvae (NPL). Prey composition varied between PL and NPL; small size (<6 mm standard length, L_S) parasitized larval G. marmoratus ate mostly gastropod larvae, whereas small non‐parasitized specimens ate mainly cirripede nauplii. All larger (>8 mm L_S), pre‐settlement stages parasitized by Trifur and, or Caligus copepods had content in their gut, suggesting that ectoparasites did not diminish prey capture in host with larger size. Morphometric and dietary changes occurring during larval development were decoupled, both for PL and NPL. The maintenance of a slender, more hydrodynamic body through pelagic development and the ingestion of less‐mobile prey in PL suggests non‐lethal effects of ectoparasitism on rocky‐reef fish larvae.