Ecological speciation in Gambusia fishes

Although theory indicates that natural selection can facilitate speciation as a by-product, demonstrating ongoing speciation via this by-product mechanism in nature has proven difficult. We examined morphological, molecular, and behavioral data to investigate ecology's role in incipient speciation for a post-Pleistocene radiation of Bahamas mosquitofish (Gambusia hubbsi) inhabiting blue holes. We show that adaptation to divergent predator regimes is driving ecological speciation as a by-product. Divergence in body shape, coupled with assortative mating for body shape, produces reproductive isolation that is twice as strong between populations inhabiting different predator regimes than between populations that evolved in similar ecological environments. Gathering analogous data on reproductive isolation at the interspecific level in the genus, we find that this mechanism of speciation may have been historically prevalent in Gambusia. These results suggest that speciation in nature can result as a by-product of divergence in ecologically important traits, producing interspecific patterns that persist long after speciation events have completed.     

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.

     

Sympatric speciation in a genus of marine reef fishes

Sympatric speciation has been contentious since its inception, yet is increasingly recognized as important based on accumulating theoretical and empirical support. Here, we present a compelling case of sympatric speciation in a taxon of marine reef fishes using a comparative and mechanistic approach. Hexagrammos otakii and H. agrammus occur in sympatry throughout their ranges. Molecular sequence data from six loci, with complete sampling of the genus, support monophyly of these sister species. Although hybridization occurrs frequently with an allopatric congener in an area of slight distributional overlap, we found no F₁ hybrids between the focal sympatric taxa throughout their coextensive ranges. We present genetic evidence for complete reproductive isolation based on SNP analysis of 382 individuals indicating fixed polymorphisms, with no shared haplotypes or genotypes, between sympatric species. To address questions of speciation, we take a mechanistic approach and directly compare aspects of reproductive isolation between allopatric and sympatric taxa both in nature and in the laboratory. We conclude that the buildup of reproductive isolation is strikingly different in sympatric vs. allopatric taxa, consistent with theoretical predictions. Lab reared hybrids from allopatric species crosses exhibit severe fitness effects in the F₁ or backcross generation. No intrinsic fitness effects are observed in F₁ hybrids from sympatric species pairs, however these treatments exhibited reduced fertilization success and complete pre‐mating isolation is implied in nature because F₁ hybrid adults do not occur. Our study addresses limitations of previous studies and supports new criteria for inferring sympatric speciation.     

Ecological speciation in marine v. freshwater fishes

Absolute barriers to dispersal are not common in marine systems, and the prevalence of planktonic larvae in marine taxa provides potential for gene flow across large geographic distances. These observations raise the fundamental question in marine evolutionary biology as to whether geographic and oceanographic barriers alone can account for the high levels of species diversity observed in marine environments such as coral reefs, or whether marine speciation also operates in the presence of gene flow between diverging populations. In this respect, the ecological hypothesis of speciation, in which reproductive isolation results from divergent or disruptive natural selection, is of particular interest because it may operate in the presence of gene flow. Although important insights into the process of ecological speciation in aquatic environments have been provided by the study of freshwater fishes, comparatively little is known about the possibility of ecological speciation in marine teleosts. In this study, the evidence consistent with different aspects of the ecological hypothesis of speciation is evaluated in marine fishes. Molecular approaches have played a critical role in the development of speciation hypotheses in marine fishes, with a role of ecology suggested by the occurrence of sister clades separated by ecological factors, rapid cladogenesis or the persistence of genetically and ecologically differentiated species in the presence of gene flow. Yet, ecological speciation research in marine fishes is still largely at an exploratory stage. Cases where the major ingredients of ecological speciation, namely a source of natural divergent or disruptive selection, a mechanism of reproductive isolation and a link between the two have been explicitly documented are few. Even in these cases, specific predictions of the ecological hypothesis of speciation remain largely untested. Recent developments in the study of freshwater fishes illustrate the potential for molecular approaches to address specific questions related to the ecological hypothesis of speciation such as the nature of the genes underlying key ecological traits, the magnitude of their effect on phenotype and the mechanisms underlying their differential expression in different ecological contexts. The potential provided by molecular studies is fully realized when they are complemented with alternative (e.g. ecological, theoretical) approaches.     

Patterns of distribution and processes of speciation in Brazilian reef fishes

Aim To evaluate the state of knowledge about the biogeography of Brazilian reef fishes and propose processes that lead to the observed distribution patterns. Location The tropical western Atlantic. Methods The geological history of the Amazon barrier was obtained from the literature, and its potential influence on speciation of reef fishes was analysed. Species distributions were analysed based on literature records and material deposited at Brazilian and American collections. Results Recent estimates indicate that about 12% of the Brazilian reef fish species are endemic, and most ichthyologists agree that this endemism is generated by the barrier formed by the freshwater and sediment discharge of large rivers in north‐eastern South America, mainly the Amazon, Orinoco and their tributaries. However, little is known about the dynamics of this barrier, and recent studies have demonstrated that it can be crossed through deep sponge bottoms on the outer continental shelf off north‐eastern South America. Moreover, the recent discovery of species regarded as Brazilian endemics in the extreme southern Caribbean is showing that the Amazon barrier is weaker than previously thought. Main conclusions The Amazon freshwater and sediment outflow is a strong barrier to shallow water reef fish and other organisms, and it is probably responsible for most of the endemism found in Brazilian coastal habitats. However, sea‐level fluctuations influence the effectiveness of such barrier and may play a large role in the generation of diversity in the western tropical Atlantic. Alternatively, larval exchange between Brazil and the Caribbean is small but constant, and contrasting selection pressures in divergent environments (continental Brazil vs. insular Caribbean) may be the central force driving speciation.     

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     

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.     

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.     

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.     

Diversity of settlement-stage reef fishes captured by light-trap in a tropical south-west Atlantic Ocean coastal reef system

This study reports the results of 5 years of monitoring reef fish post-larvae using light traps in the Bay of Tamandaré, north-east Brazil. An annotated checklist of pre-settlement fish species, their frequency of occurrence and taxonomic characteristics are provided. In total, 4,422 post-larval fishes belonging to 36 families, 56 genera and 76 species were captured. The most species-rich families were Carangidae (7), Lutjanidae (6) and Pomacentridae (4), while the families Gerreidae (30.47%), Holocentridae (16.54%), Blenniidae (12.01%), Labrisomidae (8.36%), Lutjanidae (8.29%) and Acanthuridae (5.95%) were the most abundant. This is the first study of the taxonomic diversity and assemblage structure of settlement-stage reef fishes in the tropical south-west Atlantic Ocean. Although a few common species were not captured due to selectivity of light traps, the composition and taxonomic diversity of this first collection suggests that light traps are useful for studies of the early life history of a wide range of pre-settlement reef fishes.     

The biogeography of tropical reef fishes: endemism and provinciality through time

The largest marine biodiversity hotspot straddles the Indian and Pacific Oceans, driven by taxa associated with tropical coral reefs. Centred on the Indo‐Australian Archipelago (IAA), this biodiversity hotspot forms the ‘bullseye’ of a steep gradient in species richness from this centre to the periphery of the vast Indo‐Pacific region. Complex patterns of endemism, wide‐ranging species and assemblage differences have obscured our understanding of the genesis of this biodiversity pattern and its maintenance across two‐thirds of the world's oceans. But time‐calibrated molecular phylogenies coupled with ancestral biogeographic estimates have provided a valuable framework in which to examine the origins of coral reef fish biodiversity across the tropics. Herein, we examine phylogenetic and biogeographic data for coral reef fishes to highlight temporal patterns of marine endemism and tropical provinciality. The ages and distribution of endemic lineages have often been used to identify areas of species creation and demise in the marine tropics and discriminate among multiple hypotheses regarding the origins of biodiversity in the IAA. Despite a general under‐sampling of endemic fishes in phylogenetic studies, the majority of locations today contain a mixture of potential paleo‐ and neo‐endemic fishes, pointing to multiple historical processes involved in the origin and maintenance of the IAA biodiversity hotspot. Increased precision and sampling of geographic ranges for reef fishes has permitted the division of discrete realms, regions and provinces across the tropics. Yet, such metrics are only beginning to integrate phylogenetic relatedness and ancestral biogeography. Here, we integrate phylogenetic diversity with ancestral biogeographic estimation of lineages to show how assemblage structure and tropical provinciality has changed through time.     

Behavioral thermoregulatory abilities of tropical coral reef fishes: a comparison with temperate freshwater and marine fishes

Synopsis Eight species in six different families of tropical marine reef fishes from the Indo-West Pacific region (Naso lituratus, Zebrasoma.flavescens, Balistes fuscus, B. vidua, Forcipiger longirostris, Echidna zebra, Cromileptes altivelis, Canthigaster jactator) were tested for ability to thermoregulate behaviorally in electronic shuttleboxes. All of these species preferred mean temperatures between 20 and 30°C, but differed considerably in thermoregulatory precision. All species avoided lethal high or low temperatures (i.e., they did not die during the tests), and some species thermoregulated as precisely as temperate species. Some temperate species prefer higher temperatures (above 30°C) than do these tropical reef species.     

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.     

How might recruitment research on coral-reef fishes help manage tropical reef fisheries?

Abstract ‘Ecologic’ reef fishes (basic research subjects) and ‘Economic’ reef fishes (exploited by humans) share fundamental early life-history attributes of small, widely dispersed planktonic eggs, larvae, and (for some species in both groups) pelagic juveniles. These attributes predispose the open populations of species in both groups to limitation resulting from environmentally induced fluctuations in recruitment from planktonic/pelagic to benthic stages. Rates of movement within and among reefs, one of several postrecruitment processes likely to be subject to density-dependent regulation, may differ between Ecologies (mostly small-bodied) and Economics (generally larger-bodied). This is because of differences between species in the two groups in size-related differences in the home ranges of individuals. Existing data, however, neither support the notion that natural growth and mortality rates basically differ between the adults of Ecological and Economic species, nor that the generally larger home ranges of larger-bodied adult Economics are more subject to density-dependent control. Further, the small-bodied young-of-year juveniles of both groups on average probably have similar growth and mortality rates and small individual home ranges that are equivalently affected by density dependence. In conclusion I argue that, because of fundamental similarities in the sizes and durations of planktonic propagules and spawning periodicities, certain Ecologies and Economics may comprise a single recruitment guild. Coefficients of growth and mortality for postsettlement Ecologies also may resemble, and be applied as preliminary proxies for, analogous coefficients for Economic species. The efficacy of management strategies such as harvest refugia may differ for Ecological and Economic species, however, depending on whether the refugia are used to counter growth or recruitment overfishing.     

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.