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.     

Dispersal in the spiny damselfish,Acanthochromis polyacanthus,a coral reef fish species without a larval pelagic stage

The spiny damselfish, Acanthochromis polyacanthus, is widely distributed throughout the Indo‐Australian archipelago. However, this species lacks a larval dispersal stage and shows genetic differentiation between populations from closely spaced reefs. To investigate the dispersal strategy of this unique species, we used microsatellite markers to determine genetic relatedness at five dispersal scales: within broods of juveniles, between adults within a collection site (~30 m²), between sites on single reefs, between nearby reefs in a reef cluster, and between reef clusters. We sampled broods of juveniles and adults from seven reefs in the Capricorn‐Bunker and Swain groups of the Great Barrier Reef. We found that extra‐pair mating is rare and juveniles remain with their parents until fledged. Adults from single sites are less related than broods but more related than expected by chance. However, there is no evidence of inbreeding suggesting the existence of assortative mating and/or adult migration. Genetic differences were found between all of the reefs tested except between Heron and Sykes reefs, which are separated only by a 2‐km area of shallow water (less than 10 m). There was a strong correlation between genetic distance, geographical distance and water depth. Apparently, under present‐day conditions spiny damselfish populations are connected only between sites of shallow water, through dispersal of adults over short distances. Assuming that dispersal behaviour has not changed, the broad distribution of A. polyacanthus as a species is likely based on historical colonization patterns when reefs were connected by shallow water at times of lower sea levels.     

Temperature influences swimming speed, growth and larval duration in coral reef fish larvae

The effects of temperature on growth, pelagic larval duration (PLD) and maximum swimming speed were compared in the tropical fish marine species Amphiprion melanopus, to determine how temperature change affects these three factors critical to survival in larvae. The effects of rearing temperature (25 and 28 °C) on the length of the larval period and growth were examined in conjunction with the effects of swimming temperature (reared at 25 °C, swum at 25 and 28 °C, reared at 28 °C, swum at 25 and 28 °C) on critical swimming speed (U-crit). Larvae reared at 25 °C had a 25% longer pelagic larval duration (PLD) than larvae reared at 28 °C, 12.3 (±0.3) days compared with 9 (±0.6) days at 25 °C. To offset this effect of reduced developmental rate, growth and U-crit were measured in larvae reared at 28 and 25 °C at the same absolute age (7 days after hatching (dah)) and same developmental age (7 dah at 28 °C cf. 11 dah at 25 °C), corresponding to the day before metamorphosis. Larvae reared at 25 °C were smaller than larvae reared at 28 °C at the same absolute age (7 dah at 25 °C cf. 7 dah at 28 °C), yet larger at similar developmental age (11 dah at 25 °C cf. 7 dah at 28 °C) when weight and standard length were compared. This stage-specific size increase did not result in better performance in larvae at the same developmental age, as there was no difference in U-crit in premetamorphic larvae reared at either temperature (7 dah at 28 °C c.f 11 dah at 25 °C). However, U-crit was considerably slower in 7-day-old larvae reared at 25 °C than larvae of the same absolute age (7 dah) reared at 28 °C. Swimming temperature controls demonstrated that a change in temperature immediately prior to swimming tests did not effect swimming performance for larvae reared at either temperature.A decreased in rearing temperature resulted in longer larval durations, reduced growth rates and slower swimming development in larvae. However, the magnitude of the response of each of these traits varied considerably. As such, larvae reared at the lower temperature were a larger size at metamorphosis but had poorer relative swimming capabilities. This study highlights the importance of measuring a range of ecologically relevant traits in developing larvae to properly characterise their relative condition and performance in response to environmental change.     

Evolution of cannibalism in the larval stage of pelagic fish

Larvae of several ocean pelagic fish species, such as tunas and marlins, have been known to have large jaws, but the ecological significance of this unique morphological character has been hardly analyzed in evolutionary ecology. Pelagic spawners produce small and nutrition-poor ova, and spawning and nursery grounds of the open ocean migratory fishes are oligotrophic. We hypothesize that cannibalism would be a possible life style in the larval period and the large mouth gape would be an adaptive morphological characteristic for a cannibal in the oligotrophic pelagic environment. We showed that mouth gape size of the open ocean pelagic fish is significantly larger than that of offshore/coastal pelagic fish in larval period. A mathematical model demonstrated that cannibalism would tend to evolve in high sea environment. Our findings suggest an evolutionary pattern of cannibalism trait in the larval stage of pelagic fishes. This revised version was published online in July 2006 with corrections to the Cover Date.     

Nocturnal orientation to reefs by late pelagic stage coral reef fishes

 The nocturnal orientation behaviour of the late pelagic stages of two reef fish families (Apogonidae and Pomacentridae) was examined using behavioural cages deployed in the field. The behavioural cages enabled the fish to choose between swimming towards or away from the reef in response to natural cues. Overall, 55% of fish displayed a choice in the experiments, however, the proportion varied between the two families, with 67% of pomacentrids and 27% of apogonids displaying a choice. In both families, of the fish which displayed a choice, the proportion of fish swimming towards the reef was significantly greater than 50%, as random movement would predict (64% of pomacentrids and 67% of apogonids swam towards the reef). This proportion did not vary significantly among four field sites with different current regimes and geographic locations. The results suggest that the late pelagic stages of reef fish display nocturnal orientation behaviour, possibly in response to sound, which may aid in their settlement on reefs. Accepted: 18 August 1997     

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.     

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.     

Spatial connectivity in an adult‐sedentary reef fish with extended pelagic larval phase

Understanding the spatial scale of demographic connectivity in marine reef fishes dispersing pelagic larvae is a challenging task because of the technical difficulties associated with tagging and monitoring the movements of progeny at early life stages. Several studies highlighted a strong importance of local retention with levels of dispersal of ecological significance restricted to short distances. To date little information is available in species where pelagic dispersal lasts for long periods of time. In this work, population structure and connectivity were studied in the grey triggerfish, Balistes capriscus. Grey triggerfish larvae and juveniles remain associated with floating Sargassum sp. beds for an estimated period of 4–7 months before settling on benthic habitats where they remain sedentary as adults. Analysis of genetic variation among populations along the continental shelf of the northern Gulf of Mexico and U.S. east coast, encompassing over 3,100 km of coastline, revealed homogeneous allele frequencies and a weak isolation‐by‐distance pattern. Moment and maximum‐likelihood estimates of dispersal parameters both indicated occurrence of large neighbourhoods with estimates of the dispersal distribution parameter σ of 914 and 780 km, respectively. Simulated distributions of dispersal distances using several distribution functions all featured substantial fractions of long‐distance dispersal events with the 90% percentiles of travel distance prior to settlement averaging 1,809 km. These results suggest a high dependency of local recruitment on the output of nonlocal spawning stocks located hundreds of kilometres away and a reduced role of local retention in this species.     

Effects of parasites on larval and juvenile stages of the coral reef fish Pomacentrus moluccensis

The ecological role of parasites in the early life-history stages of coral reef fish is far from clear. Parasitism in larval, recently settled and juvenile stages of a coral reef fish damselfish (Pomacentridae) was therefore investigated by quantifying the ontogenetic change in parasite load and comparing the growth rates of parasitized juvenile fish to those of unparasitized ones. Parasite prevalence in two lunar pulses of Pomacentrus moluccensis was 4 and 0% for larval stage fish, 34 and 56% for recently settled fish and 42 and 49% for juveniles. A significant increase in parasite prevalence with age group was found; the most marked increase occurred immediately after larval fish had settled. Standard length did not model prevalence well; as length is a proxy for age, this indicates that the higher prevalence in recently settled and juvenile fish compared with larvae was not a simple result of parasites accumulating with age. In one of three cohorts, there was some evidence that parasitism affected the growth rate of juveniles, as measured by otolith width. The study suggests that settling on the reef exposes young fish to potentially harmful parasites. This supports the idea that the pelagic phase may have the effect of reducing the exposure of young fish to the debilitating effects of parasites.     

Survival dynamics of reef coral larvae with special consideration of larval size and the genus Acropora

Temporal dynamics of larval survival were examined in vitro in four broadcast-spawning reef coral species, Acropora hyacinthus, A. japonica, A. solitaryensis, and Goniastrea pectinata. Larval size was treated as an important characteristic that may relate to larval lifespan. Two patterns were observed in larval survival dynamics between the three Acropora species (mean initial larval size; 0.05-0.08 mm(3)) and G. pectinata (0.02 mm(3)), based on the timing of a sharp drop in larval survival rates (ca. > 50% reduction over a 1-2 week period). Consequently, the majority of larvae of the three Acropora species had a lifespan of less than 2-3 weeks, whereas those of G. pectinata were extended a further 2-3 weeks despite the smaller larval size. No significant relationship was detected between the initial larval size and larval lifespan in any of the four reef coral species. These results suggest that (1) larval dispersal of spawning Acropora species may be on a more local scale than that of G. pectinata and most other reef coral species previously reported, and (2) larval size is not a good estimator of larval lifespan in reef coral species.     

The influence of adult density on larval settlement in a coral reef fish Coryphopterus glaucofraenum

 For marine species with open populations, patterns of larval settlement can have important consequences for performance and abundance at later life-stages. In this study, I tested whether larvae of a reef-dwelling goby (Coryphopterus glaucofraenum) settled differentially to reefs occupied by varying numbers of adults. I monitored settlement daily to reefs on which the density of adult gobies varied naturally, or was manipulated experimentally. Rates of settlement were constant across a broad range of adult densities, suggesting that larvae do not choose settlement sites based on the number of adults in their immediate vicinity. Accepted: 30 October 1998     

Terrestrial chemical cues help coral reef fish larvae locate settlement habitat surrounding islands

Understanding the degree of connectivity between coastal and island landscapes and nearby coral reefs is vital to the integrated management of terrestrial and marine environments in the tropics. Coral reef fish are capable of navigating appropriate settlement habitats following their pelagic larval phase, but the mechanisms by which they do this are unclear. The importance of olfactory cues in settlement site selection has been demonstrated, and there is increasing evidence that chemical cues from terrestrial sources may be important for some species. Here, we test the olfactory preferences of eight island-associated coral reef fish recruits and one generalist species to discern the capacity for terrestrial cue recognition that may aid in settlement site selection. A series of pairwise choice experiments were used to evaluate the potential role that terrestrial, water-borne olfactory cues play in island-reef recognition. Olfactory stimuli tested included near-shore water, terrestrial rainforest leaf litter, and olfactory cues collected from different reef types (reefs surrounding vegetated islands, and reefs with no islands present). All eight island-associated species demonstrated high levels of olfactory discrimination and responded positively toward olfactory cues indicating the presence of a vegetated island. We hypothesize that although these fish use a suite of cues for settlement site recognition, one mechanism in locating their island/reef habitat is through the olfactory cues produced by vegetated islands. This research highlights the role terrestrial olfactory cues play in large-scale settlement site selection and suggests a high degree of ecosystem connectivity.     

Probability of successful larval dispersal declines fivefold over 1 km in a coral reef fish

A central question of marine ecology is, how far do larvae disperse? Coupled biophysical models predict that the probability of successful dispersal declines as a function of distance between populations. Estimates of genetic isolation-by-distance and self-recruitment provide indirect support for this prediction. Here, we conduct the first direct test of this prediction, using data from the well-studied system of clown anemonefish (Amphiprion percula) at Kimbe Island, in Papua New Guinea. Amphiprion percula live in small breeding groups that inhabit sea anemones. These groups can be thought of as populations within a metapopulation. We use the x- and y-coordinates of each anemone to determine the expected distribution of dispersal distances (the distribution of distances between each and every population in the metapopulation). We use parentage analyses to trace recruits back to parents and determine the observed distribution of dispersal distances. Then, we employ a logistic model to (i) compare the observed and expected dispersal distance distributions and (ii) determine the relationship between the probability of successful dispersal and the distance between populations. The observed and expected dispersal distance distributions are significantly different (p < 0.0001). Remarkably, the probability of successful dispersal between populations decreases fivefold over 1 km. This study provides a framework for quantitative investigations of larval dispersal that can be applied to other species. Further, the approach facilitates testing biological and physical hypotheses for the factors influencing larval dispersal in unison, which will advance our understanding of marine population connectivity.     

Intraspecific variation in the pelagic larval duration of tropical reef fishes

Estimates of pelagic larval duration (PLD) for 10 species of Pomacentridae and two species of Gobiidae were made. In eight of the 12 species examined, within‐population mean PLDs differed between sampling times, locations within regions and among regions. In contrast, the range of these same PLD estimates overlapped at all spatial and temporal scales examined in 11 of the 12 species, but not between regions in one species ( Amphiprion melanopus ). Therefore, despite tight error estimates typically associated with estimates of PLD taken from a particular population at a particular time in some taxa, the overlapping ranges in PLD reported here indicate that the length of the pelagic larval phase is a much more plastic trait than previously appreciated. Within‐species variation in PLD has considerable potential to provide further insights into the ecology and evolution of tropical reef fishes.     

Communication in coral reef fish: the role of ultraviolet colour patterns in damselfish territorial behaviour

Many coral reef fish possess ultraviolet (UV) colour patterns. The behavioural significance of these patterns is poorly understood and experiments on this issue have not been reported for free-living reef fish in their natural environment. The damselfish Pomacentrus amboinensis has UV facial patterns, and spectroradiometric ocular media measurements show that it has the potential for UV vision. To test the potential behavioural significance of the UV patterns, I studied the response of males, in natural territories on the reef and in aquaria, to two conspecific intruders, one presented in a UV-transmitting (UV+) container and the other in a UV-absorbing (UV−) one. Territory owners attacked intruders viewed through UV+ filters significantly more often and for longer than intruders viewed through the UV− filter. In general, the results of the field experiment confirmed those of the laboratory experiment. The results support the hypothesis that P. amboinensis males are sensitive to UV light and that reflectance patterns, which appear in high contrast only in UV, modulate the level of aggressive behaviour. A recent survey showed that many predatory fish may not have UV vision and the use of UV colours in select species of reef fish may therefore serve as a ‘private communication channel’.     

Impact of variable pelagic environments on natural larval growth and recruitment of the reef fish Thalassoma bifasciatum

New recruits of the bluehead wrasse Thalassoma bifasciatum were censused and collected from nearshore reefs of Barbados, West Indies, every 2 weeks for 20 months. Their temporal coincidence with low salinity (<34·5) water during their pelagic larval stage was determined by comparing the otolith records of new recruits with conductivity and temperature records from a current meter moored 2 km off the west coast of the island. Larval residence in a low salinity North Brazil Current (NBC) ring appeared to have a negative impact on growth. Larvae that encountered a NBC ring for at least 7 days during either the first half of the larval period exhibited slower larval growth than those that did not encounter a ring for 7 days during any part of their larval period. As a result of this slower growth, larvae that encountered low salinity waters had a longer pelagic larval duration and were larger at the time of settlement. The magnitude of settlement was not distinctly related to the presence or absence of a NBC ring, but the largest settlement event occurred at the end of the longest ring event. Early juvenile growth did not vary between fish that had encountered a ring and those that did not, so size differences at settlement were propagated through the first week of life on the reef. The potentially opposing attributes of fast and slow‐growing larvae ( e.g . fast growing larvae with shorter larval stage duration but smaller size at settlement and higher susceptibility to reef predation), and the resulting differential mortality on the reef may promote the persistence of individuals in the population with contrasting life history traits, and contribute to the lack of a relationship between larval growth and recruitment success. Positive transport related effects of rings ( i.e . enhanced retention during some ring events) may further complicate matters by outweighing the negative impact of rings on larval growth.     

First description of the neuro‐anatomy of a larval coral reef fish Amphiprion ocellaris

The present study described the neuro‐anatomy of a larval coral reef fish Amphiprion ocellaris and hypothesized that morphological changes during the transition from the oceanic environment to a reef environment (i.e. recruitment) have the potential to be driven by changes to environmental conditions and associated changes to cognitive requirements. Quantitative comparisons were made of the relative development of three specific brain areas (telencephalon, mesencephalon and cerebellum) between 6 days post‐hatch (dph) larvae (oceanic phase) and 11 dph (at reef recruitment). The results showed that 6 dph larvae had at least two larger structures (telencephalon and mesencephalon) than 11 dph larvae, while the size of cerebellum remained identical. These results suggest that the structure and organization of the brain may reflect the cognitive demands at every stage of development. This study initiates analysis of the relationship between behavioural ecology and neuroscience in coral reef fishes.     

Experimental test of the effect of maternal hormones on larval quality of a coral reef fish

Maternal hormones can play an important role in the development of fish larvae. Levels of the stress hormone, cortisol, in females are elevated by social interactions and transferred directly to the yolk of eggs, where they may influence developmental rates. In some vertebrates, prenatal exposure to high levels of testosterone determine early growth rates, social status and reproductive success. The present study examined whether post-fertilization exposure of eggs of the tropical damselfish, Pomacentrus amboinensis (Pomacentridae), to natural levels of cortisol or testosterone directly affects larval morphology at hatching. Maternal and egg levels of cortisol and testosterone varied widely among clutches of eggs from local populations around Lizard Island on the Great Barrier Reef. The morphology of larvae produced by these local fish populations also varied widely and differed significantly among sites (e.g., standard length: 2.6–3.4 mm; yolk sac area: 0.01–0.13 × 10⁻² mm²). Laboratory experiments showed that elevated cortisol levels in the egg reduced larval length at hatching, while slight elevations in testosterone increased yolk sac size. The influence of testosterone, and to a smaller extent cortisol, on larval morphology differed among egg clutches. These differences were partly explained by differences in initial egg hormone levels. Morphological changes induced by experimental hormonal regimes encompassed the entire range of variability in body attributes found in field populations. It is unclear whether cortisol influences growth alone or development rate or both. Testosterone appears to influence yolk utilization rates, and has no significant effect on growth, in contrast to its role in later developmental stages. Maternally derived cortisol and testosterone are important in regulating growth, development, and nutritive reserves of the embryo and larvae of this fish species. Factors that influence the maternal levels of cortisol and testosterone may have a major impact on larval mortality schedules and, therefore, on which breeding individuals contribute to the next generation. Received: 19 August 1998 / Accepted: 16 November 1998     

A preliminary distributional study of fish larvae near a ribbon coral reef in the Great Barrier Reef

Fish larvae from horizontal plankton tows along a single transect near outer ribbon reefs of the Great Barrier Reef in spring 1979 and summer 1980 had persistent distributional patterns. Larvae were identified to family and divided into young (preflexion) and old (postflexion) larvae, thus giving 28 taxa abundant enough for analysis. Non-uniform larval distributions were found for 81% of the 16 reef fish taxa with non-pelagic eggs, but for only 17% of the six reef fish taxa with pelagic eggs. Most differences in larval concentration were between the lagoonal and seaward sides of the reef. Only tripterygiid larvae had highest concentration just seaward of the reef, while larvae of 12 reef and three oceanic fish taxa occurred in highest concentrations on the lagoonal side of the reef. In five taxa of reef fishes, higher larval concentrations were found in the lagoonal backreef compared with the mid-lagoon habitat; but the reverse was not found in any taxon. Eleven taxa had indeterminate distributions, (i.e. no difference in concentration between stations). Mechanisms responsible for the distribution remain unknown, but we suggest that the view which considers fish larvae to be passively-drifting particles is unjustified without more information on larval behaviour.