The relationship between pelagic larval duration and range size in tropical reef fishes: a synthetic analysis

We address the conflict in earlier results regarding the relationship between dispersal potential and range size. We examine all published pelagic larval duration data for tropical reef fishes. Larval duration is a convenient surrogate for dispersal potential in marine species that are sedentary as adults and that therefore only experience significant dispersal during their larval phase. Such extensive quantitative dispersal data are only available for fishes and thus we use a unique dataset to examine the relationship between dispersal potential and range size. We find that dispersal potential and range size are positively correlated only in the largest ocean basin, the Indo-Pacific, and that this pattern is driven primarily by the spatial distribution of habitat and dispersal barriers. Furthermore, the relationship strengthens at higher taxonomic levels, suggesting an evolutionary mechanism. We document a negative correlation between species richness and larval duration at the family level in the Indo-Pacific, implying that speciation rate may be negatively related to dispersal potential. If increased speciation rate within a taxonomic group results in smaller range sizes within that group, speciation rate could regulate the association between range size and dispersal potential.     

Ecological traits influencing range expansion across large oceanic dispersal barriers: insights from tropical Atlantic reef fishes

How do biogeographically different provinces arise in response to oceanic barriers to dispersal? Here, we analyse how traits related to the pelagic dispersal and adult biology of 985 tropical reef fish species correlate with their establishing populations on both sides of two Atlantic marine barriers: the Mid-Atlantic Barrier (MAB) and the Amazon–Orinoco Plume (AOP). Generalized linear mixed-effects models indicate that predictors for successful barrier crossing are the ability to raft with flotsam for the deep-water MAB, non-reef habitat usage for the freshwater and sediment-rich AOP, and large adult-size and large latitudinal-range for both barriers. Variation in larval-development mode, often thought to be broadly related to larval-dispersal potential, is not a significant predictor in either case. Many more species of greater taxonomic diversity cross the AOP than the MAB. Rafters readily cross both barriers but represent a much smaller proportion of AOP crossers than MAB crossers. Successful establishment after crossing both barriers may be facilitated by broad environmental tolerance associated with large body size and wide latitudinal-range. These results highlight the need to look beyond larval-dispersal potential and assess adult-biology traits when assessing determinants of successful movements across marine barriers.     

Egg size in relation to fertilization dynamics in free-spawning tropical reef fishes

In marine invertebrates that spawn by simply releasing their gametes into the water (free-spawning), fertilization success likely is often limited by low sperm concentrations, due to dispersion of mates and dispersal of gametes by water movements. Production of large, low density eggs might be advantageous when sperm concentrations consistently are low, because large target size might increase egg/sperm encounters, and more low than high density eggs could be produced per clutch. Although average fertilization success in the labrid Thalassoma bifasciatum is 95% in both group spawns (in which multiple males compete for fertilizations by producing large quantities of sperm) and pair (mono-male) spawns, it is slightly lower in pair spawns, due to low level sperm limitation that arises because pair-spawning males release near the minimum number of sperm necessary for maximum fertilization. I examined whether variation in egg size and content in T. bifasciatum and other free-spawning fishes is related to variation in spawning mode, to assess whether compensatory production of large, low-density eggs might be contributing to high fertilization success in pair spawns. I found no difference between the volume or density of eggs of (1) pair- and group-spawning females of T. bifasciatum, or (2) pair-and group-spawning congeneric species of labrids, scarids, and serranids, or (3) labrids and scarids with vigorous, rapid spawning movements (which could turbulently diffuse gamete clouds) and those with slow movements. Further, egg density does not decline with increasing egg volume among those fishes. Assuming that egg size can affect fertilization success, then sperm limitation seems unlikely to represent a significant problem for pair-spawning T. bifasciatum, probably because mates place their vents close together during gamete release. The situation regarding sperm limitation in other fishes, and effects of environmentally generated water turbulence on it, are less clear. Interspecific variation in the size and content of these fishes' eggs may relate to provisioning of offspring for different larval life-histories.     

Ecological speciation in tropical reef fishes

The high biodiversity in tropical seas provides a long-standing challenge to allopatric speciation models. Physical barriers are few in the ocean and larval dispersal is often extensive, a combination that should reduce opportunities for speciation. Yet coral reefs are among the most species-rich habitats in the world, indicating evolutionary processes beyond conventional allopatry. In a survey of mtDNA sequences of five congeneric west Atlantic reef fishes (wrasses, genus Halichoeres) with similar dispersal potential, we observed phylogeographical patterns that contradict expectations of geographical isolation, and instead indicate a role for ecological speciation. In Halichoeres bivittatus and the species pair Halichoeres radiatus/brasiliensis, we observed strong partitions (3.4% and 2.3% divergence, respectively) between adjacent and ecologically distinct habitats, but high genetic connectivity between similar habitats separated by thousands of kilometres. This habitat partitioning is maintained even at a local scale where H. bivittatus lineages are segregated between cold- and warm-water habitats in both Bermuda and Florida. The concordance of evolutionary partitions with habitat types, rather than conventional biogeographical barriers, indicates parapatric ecological speciation, in which adaptation to alternative environmental conditions in adjacent locations overwhelms the homogenizing effect of dispersal. This mechanism can explain the long-standing enigma of high biodiversity in coral reef faunas.     

Diel feeding migrations in tropical reef fishes

Summary 1. Many tropical reef fishes that feed during the day rest at night, whereas many that feed at night rest during the day. The feeding grounds of many are some distance from their resting grounds. Thus they migrate between these two locations during twilight as part of a general changeover between diurnal and nocturnal situations.2. At least many of these migrations are predictable, both as to time and to the route taken. The distances traveled vary between species, ranging from just a few meters, to more than several kilometers.3. The pattern of migrations is strongly influenced by the relative threat from predators at different periods of the diel cycle.4. During the day, migrations of reef fishes are limited to intra-reef movements: short vertical movements by certain plankton feeders, and lateral excursions from one part of the reef to another by certain herbivores and plankton feeders. Movements into the open regions that lie adjacent to many reefs are not adaptive in daylight due largely to a danger from predators.5. Despite constant threat from predators during the day, smaller reef fishes remain relatively secure during most of this period by staying close to shelter, or by schooling. However, these defenses are less effective during twilight, when the danger from predators intensifies. The diurnal migrators return to the shelter of their resting places prior to that part of evening twilight when danger is greatest, and the nocturnal migrators usually do not expose themselves for their nightly foraging until after the period of maximum danger has passed. During morning twilight the sequence is reversed.6. The major mechanisms whereby smaller reef fishes reduce predation during the day — schooling and staying close to shelter — are less evident at night. Not only do reef fishes range freely at night into the open regions that are avoided in daylight, but their schools are more loosely defined, and many are active as solitary individuals or in small groups. The tendency for looser associations and ranging farther afield increases on darker nights.7. Most predators that threaten reef fishes are visual feeders whose mode of attack loses effectiveness when light falls below a certain level. Although they operate to some extent under moonlight, they threaten small reef fishes less at night than during the day.8. In addition to whatever other ways a shcool may be adaptive, by reducing variable behavior among its members the school is especially important to migrating species. Responses to the various cues that mark the migration routes may be refined to within acceptable limits for the population as a whole only by coordinated group action.9. Submarine topographical features are important reference points for migrating reef fishes.

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Tägliche Futtersuchwanderungen bei tropischen Riffischen

Kurzfassung Dämmerungszeiten wandern viele tropische Fische zwischen ihren Ruheplätzen auf dem Riff und ihren Futterplätzen. Diese Wanderungen, die sowohl von tag- als auch von nachtaktiven Fischen durchgeführt werden, sind bedingt durch den Licht-Dunkel-Wechsel, und für viele Arten können sogar Zeit und Weg der Wanderung vorhergesagt werden. Das Muster der tagesrhythmischen Wanderungsbewegungen wird vor allem durch das Ausmaß von Bedrohungen durch Raubfeinde beeinflußt. Bewegungen in ungeschützte Gebiete, die vom Riff wegführen, sind am Tage nicht vorteilhaft. Bei Einbruch der Dunkelheit sind diese offenen Gebiete hingegen ein Konzentrationspunkt der Wanderungen aus dem Riff, wobei sich auch der Zusammenhalt der Fischschwärme verringert. Da Schwarmverhalten und Schutzsuchen die beiden wichtigsten Verteidigungseinrichtungen gegenüber Räubern darstellen, bedeutet das Aufgeben dieser Verhaltensweisen zu Beginn der Dämmerung, daß während der Nacht kleinere Riffische weniger bedroht sind als während des Tages. Für wandernde Schwarmfische dürfte eine geringe Variabilität der Verhaltensweisen von adaptiver Bedeutung sein. Nur durch ein koordiniertes Schwarmverhalten ist es offensichtlich möglich, innerhalb bestimmter Grenzen auf die verschieden optischen Marken, welche die Wanderwege kennzeichnen, zu reagieren. Vermutlich spielen die topographischen Gegebenheiten unter Wasser eine entscheidende Rolle für die Orientierung der Riffische.

     

Patterns of parental investment,dispersal and size among coral-reef fishes

Synopsis Coral-reef fishes have been selected to produce propagules for dispersal because they live in a patchy environment, and the adults cannot migrate between patches. For large species (>100 mm SL) and widely separated patches, numerous propagules are needed, often with specialized pelagic intervals. Individuals of small species are confined to portions of the reef. They are unable to produce enough eggs for effective longrange dispersal, and so they keep their vulnerable eggs and young out of the plankton until they are well developed enough to seek out and settle onto the appropriate habitat before dispersing.Guarded demersal eggs, requiring a greater individual investment by the small short-lived species, further reduces their individual fecundity. These costs to fecundity, and the reduction in vagile young appear to account for the lack of postzygotic investment in larger longlived species.All coral-reef fishes are selected to disperse, usually with a young planktonic propagule interval. Both large and small species produce a mixed outcome, with some propagules returned to, or retained at the home reef, while others disperse more widely. The smaller the species the greater the proportion of propagules retained.     

The adaptive significance of larval dispersal in coral reef fishes

Synopsis Coral reef fishes almost universally disperse over relatively great distances during a pelagic larval phase. Barlow (1981) suggested that this dispersal is adaptive because adult fishes inhabit a patchy, uncertain environment. This reiterated an older idea that the random extinction of local populations necessarily favours dispersal, since ultimately all populations of non-dispersers will disappear. Whereas this view is based on adult survival, we emphasize a less frequent view that substantial larval dispersal may be adaptive when offspring experience patchy and unpredictable survival in the pelagic habitat. We do not address the question of why these animals ‘broadcast’ rather than ‘brood’, but suggest that species committed to pelagic offspring will be under selection to disperse siblings to spread the risk of failure among members of a cohort. Our arguments are supported by a heuristic computer simulation.     

High-quality seed dispersal by fruit-eating fishes in Amazonian floodplain habitats

Seed dispersal is a critical stage in the life history of plants. It determines the initial pattern of juvenile distribution, and can influence community dynamics and the evolutionary trajectories of individual species. Vertebrate frugivores are the primary vector of seed dispersal in tropical forests; however, most studies of seed dispersal focus on birds, bats and monkeys. Nevertheless, South America harbors at least 200 species of frugivorous fishes, which move into temporarily flooded habitats during lengthy flood seasons and consume fruits that fall into the water; and yet, we know remarkably little about the quality of seed dispersal they effect. We investigated the seed dispersal activities of two species of large-bodied, commercially important fishes (Colossoma macropomum and Piaractus brachypomus, Characidae) over 3 years in Pacaya-Samiria National Reserve (Peru). We assessed the diet of these fishes during the flood season, conducted germination trials with seeds collected from digestive tracts, and quantified fruit availability. In the laboratory, we fed fruits to captive Colossoma, quantified the proportion of seeds defecated by adult and juvenile fish, and used these seeds in additional germination experiments. Our results indicate that Colossoma and Piaractus disperse large quantities of seeds from up to 35% of the trees and lianas that fruit during the flood season. Additionally, these seeds can germinate after floodwaters recede. Overexploitation has reduced the abundance of our focal fish species, as well as changed the age structure of populations. Moreover, older fish are more effective seed dispersers than smaller, juvenile fish. Overfishing, therefore, likely selects for the poorest seed dispersers, thus disrupting an ancient interaction between seeds and their dispersal agents. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Self-similarity and the relationship between abundance and range size

Patterns in the relationships among the range, abundance, and distribution of species within a biome are of fundamental interest in ecology. A self-similarity condition, imposed at the community level and previously demonstrated to lead to the power-law form of the species-area relationship, is extended to the species level and shown to predict testable power-law relationships between range size and both species abundance and area of census cell across scales of spatial resolution. The predicted slopes of plots of log(range size) versus log(abundance) are shown to be in good agreement with data from British breeding bird and mammal censuses and with data on the distribution of fern species in old-growth forest. The predicted slopes of plots of log(range size) versus log (area of census cell) are consistent with the limited available data for British plant species. Self-similarity provides a testable theoretical framework for a unified understanding of patterns among the range, abundance, and distribution of species.     

Larval retention and connectivity among populations of corals and reef fishes: history,advances and challenges

The extent of larval dispersal on coral reefs has important implications for the persistence of coral reef metapopulations, their resilience and recovery from an increasing array of threats, and the success of protective measures. This article highlights a recent dramatic increase in research effort and a growing diversity of approaches to the study of larval retention within (self-recruitment) and dispersal among (connectivity) isolated coral reef populations. Historically, researchers were motivated by alternative hypotheses concerning the processes limiting populations and structuring coral reef assemblages, whereas the recent impetus has come largely from the need to incorporate dispersal information into the design of no-take marine protected area (MPA) networks. Although the majority of studies continue to rely on population genetic approaches to make inferences about dispersal, a wide range of techniques are now being employed, from small-scale larval tagging and paternity analyses, to large-scale biophysical circulation models. Multiple approaches are increasingly being applied to cross-validate and provide more realistic estimates of larval dispersal. The vast majority of empirical studies have focused on corals and fishes, where evidence for both extremely local scale patterns of self-recruitment and ecologically significant connectivity among reefs at scales of tens of kilometers (and in some cases hundreds of kilometers) is accumulating. Levels of larval retention and the spatial extent of connectivity in both corals and fishes appear to be largely independent of larval duration or reef size, but may be strongly influenced by geographic setting. It is argued that high levels of both self-recruitment and larval import can contribute to the resilience of reef populations and MPA networks, but these benefits will erode in degrading reef environments.     

Life history,larval dispersal,and connectivity in coral reef fish among the Scattered Islands of the Mozambique Channel

The Western Indian Ocean harbors one of the world’s most diverse marine biota yet is threatened by exploitation with few conservation measures in place. Primary candidates for conservation in the region are the Scattered Islands (Îles Éparses), a group of relatively pristine and uninhabited islands in the Mozambique Channel. However, while optimal conservation strategies depend on the degree of population connectivity among spatially isolated habitats, very few studies have been conducted in the area. Here, we use highly variable microsatellite markers from two damselfishes (Amphiprion akallopisos and Dascyllus trimaculatus) with differing life history traits [pelagic larval duration (PLD), adult habitat] to compare genetic structure and connectivity among these islands using classic population structure indices as well as Bayesian clustering methods. All classical fixation indexes F _ST, R _ST, G′_ST, and Jost’s D show stronger genetic differentiation among islands for A. akallopisos compared to D. trimaculatus, consistent with the former species’ shorter PLD and stronger adult site attachment, which may restrict larval dispersal potential. In agreement with these results, the Bayesian analysis revealed clear genetic differentiation among the islands in A. akallopisos, separating the southern group (Bassas da India and Europa) from the center (Juan de Nova) and northern (Îles Glorieuses) islands, but not for D. trimaculatus. Local oceanographic patterns such as eddies that occur along the Mozambique Channel appear to parallel the results reported for A. akallopisos, but such features seem to have little effect on the genetic differentiation of D. trimaculatus. The contrasting patterns of genetic differentiation between species within the same family highlight the importance of accounting for diverse life history traits when assessing community-wide connectivity, an increasingly common consideration in conservation planning.

     

The effect of microhabitat patch size on settlement differs among co-occurring coral reef fishes

Over small spatial scales, coral reefs represent a mosaic of suitable settlement microhabitat patches of varying size for late-stage larval reef fishes. Few studies have specifically examined how variation in patch size influences density of recently settled coral reef fishes (recruits). Using standardized units of coral rubble settlement substrate deployed on sandy bottom, we monitored the concurrent settlement of three reef fish taxa onto differently sized patches (0.28–1.68 m²) at 5-d intervals during a lunar settlement peak. We found marked differences among taxa in how recruit density scaled with patch size. Recruit density of a damselfish and a parrotfish decreased and increased, respectively, with the increase in patch size, while that of a wrasse was similar among patch sizes. Our results highlight the importance of the interaction between taxon-specific settlement behaviour and patch size in establishing initial spatial differences in density within and among coral reef fish taxa in a heterogeneous landscape.

     

Theory of resource allocation strategy in territorial male tropical reef fishes

A mathematical model is proposed to explain energy resource allocation between sperm production and territoriality in male reef fishes (Labridae species) from the point of view of optimization. Labridae species are typically characterized by both TP (terminal phase) and IP (initial phase) males. The former are considered to release a lower amount of sperm but show aggressive territoriality. In the model, TP male reproductive success is considered as depending upon both fertilization probability (depending on sperm density) and the individual's own territorial activities. Between these factors, a trade-off exists by which the fertilization probability can be enhanced only by reducing territoriality. Therefore, the male has to decide how much of the total available energy resource should be allocated to each. The model showed that under high fertilization efficiency the male can achieve high success by spending less of the resource on sperm production and correspondingly more for territoriality. The TP male reproductive success increases with decreasing male density in the habitat. Nevertheless, when intruding males cannot be excluded completely by territorial behavior of the TP male, females prefer high male density. If females can control the number of intruding males to some degree, conflict may arise between the sexes. Received: December 8, 1999 / Accepted: May 24, 2000     

Prey nutritional quality and the effectiveness of chemical defenses against tropical reef fishes

Summary Many coral-reef seaweeds and sessile invertebrates produce both secondary chemicals and mineral or fibrous skeletal materials that can reduce their susceptibility to consumers. Although skeletal materials often have been assumed to function as physical defenses, their deterrent effectiveness may derive from their reduction of prey nutritional quality as well as from noxiousness of the skeletal material itself. To test the relative importance of prey nutritional quality and chemical defenses in susceptibility to predation, we offered reef fishes on Guam a choice of artificial foods varying in nutritional quality (4% versus 22% protein) and in secondary chemistry (spanning approximately natural concentration ranges). Field feeding assays were performed with pachydictyol A from the pantropical brown seaweed genus Dictyota, manoalide from the Micronesian sponge Luffariella variabilis, and a brominated diphenyl ether from the Micronesian sponge Dysidea sp. The results indicated that chemical defenses were less effective in high- than in low-quality foods. In paired assays with metabolite-free controls, all three compounds at natural concentrations significantly reduced feeding by reef fishes only in assays using low-quality food, and not in assays with high-quality food. When fishes were offered an array of artificial foods varying in both food quality and metabolite concentration, food quality significantly affected fish feeding in all three cases, while secondary chemistry was significant in only one. Thus differences in nutritional quality, within the natural range among reef organisms, can be comparable to or greater in importance than secondary chemistry in affecting feeding preferences of their consumers. Reduced nutritional quality may be an important selective advantage of producing indigestible structural materials, in addition to their roles as physical support and defense, in coral reef organisms.     

Non-random reef use by fishes at two dominant zones in a tropical,algal-dominated coastal reef

Habitat use and the processes which determine fish distribution were evaluated at the reef flat and reef crest zones of a tropical, algal-dominated reef. Our comparisons indicated significant differences in the majority of the evaluated environmental characteristics between zones. Also, significant differences in the abundances of twelve, from thirteen analyzed species, were observed within and between-sites. According to null models, non-random patterns of species co-occurrences were significant, suggesting that fish guilds in both zones were non-randomly structured. Unexpectedly, structural complexity negatively affected overall species richness, but had a major positive influence on highly site-attached species such as a damselfish. Depth and substrate composition, particularly macroalgae cover, were positive determinants for the fish assemblage structure in the studied reef, prevailing over factors such as structural complexity and live coral cover. Our results are conflicting with other studies carried out in coral-dominated reefs of the Caribbean and Pacific, therefore supporting the idea that the factors which may potentially influence reef fish composition are highly site-dependent and variable.     

Cleaning interactions at the southern limit of tropical reef fishes in the Western Atlantic

Cleaning associations are one of the most dynamic and complex mutualistic interactions of reef environments and are often influenced by local conditions. In the Western Atlantic (WE) most studies concentrate in tropical areas, with little attention to subtropical areas. We examined an assemblage of cleaner fish and their clients on the rocky reefs of the coast of Santa Catarina state, South Brazil, the southern limit of tropical reef fishes in the WE. We recorded 150 cleaning interactions, in which four fish species and one shrimp species acted as facultative cleaners. The grunt Anisotremus virginicus and the angelfish Pomacanthus paru serviced most clients. Fifteen fish species acted as clients, among which the most frequent was the planktivorous grunt Haemulon aurolineatum (31%). Cleaning interactions occurred mostly (87%) with non-carnivorous clients and the number of interactions was not related to the abundance of the species involved. The absence of dedicated cleaner fishes at the study sites and the replacement of their roles by facultative cleaners may be related to local conditions, including cold currents and reduction of rock cover. Under these circumstances, clients take advantage of the services offered by facultative cleaners, a characteristic of temperate areas.     

Competitive ability and the potential for lotteries among territorial reef fishes

Stegastes diencaeus and S. dorsopunicans are mutually territorial Caribbean damselfishes. S. diencaeus is larger, grows faster and lives longer than S. dorsopunicans. S. diencaeus is a habitat specialist that shares its primary habitat mainly with S. dorsopunicans. Field manipulations show that both S. diencaeus and S. dorsopunicans readily take over living space from smaller, but not larger, heterospecific neighbors. Natural changes in the use of living space by both species occur frequently and adult S. diencaeus often aggressively usurp the living areas of smaller S. dorsopunicans. Lunar and seasonal patterns of juvenile recruitment by S. diencaeus and S. dorsopunicans are similar. Large size bestows competitive superiority on S. diencaeus by giving its adults a superior ability to aggressively acquire living space, and by enabling its juveniles to quickly escape the period when they lack a size advantage. Hence they spend much of their lives as competitive dominants. There is no evidence that competitive advantages arising from large size are offset either by other adult attributes or by differences in temporal patterns of recruitment that affect priority of access to space. The lottery hypothesis for species coexistence relies on patterns of abundance being determined by patterns of recruitment to vacant space because different species have equal space-holding abilities. These data show that the existence of such a mechanism is doubtful.     

Assessing early recruitment dynamics and its demographic consequences among tropical reef fishes: Accommodating variation in recruitment seasonally and longevity

Abstract When settlement of pelagic juveniles of reef fishes is highly and predictably seasonal, annual, end-of-season surveys of surviving recruits (which are commonly used on the Great Barrier Reef) are useful for assessing recruitment dynamics and their demographic effects. However, when settlement is continuous or weakly seasonal, with patterns that vary both between species and within species among years, regular, sometimes year-round, recruitment surveys at intervals linked to short-term settlement dynamics are needed to quantify fluctuations in recruitment strength. Monthly recruitment surveys may be appropriate in the tropical northwest Atlantic, where settlement is often both lunar periodic, and broadly and variably seasonal. Use of a variety of recruit-census methods impedes comparisons of recruitment patterns and their demographic effects, because recruit densities and recruit:adult ratios cannot be directly compared when recruits (because they have widely varying post-settlement ages) have experienced very different levels of early post-settlement mortality. Examining the relationship between changes in adult populations and annual, end-of-season recruitment may be satisfactory for long-lived species with strong settlement seasonality and maturation times of approximately 1 year. However, it is inappropriate for short-lived, rapidly maturing species, particularly those that have broad and variable settlement seasons and whose populations fluctuate substantially throughout the year in response to short-term fluctuations in recruitment. Comparisons of demographic effects of recruitment among species with different longevity require the use of non-arbitrary time scales, such as the time to maturity and the adult half-life.