Social induction of maturation and sex determination in a coral reef fish

Labile maturation and sex determination should be advantageous where the probability of finding a mating partner is unpredictable. Here we tested the hypothesis that the presence of a potential mating partner induces maturation and sex determination in a coral-dwelling fish, Gobiodon erythrospilus. In natural populations at Lizard Island (Great Barrier Reef), single individuals were less likely to be mature than paired individuals and they matured at a larger size, indicating plasticity in the timing of maturation. By manipulating group structure we demonstrated that both the timing of maturation and the sex of maturing individuals are socially controlled. Single juveniles did not mature, but maturation was rapidly induced by the presence of an adult partner. In addition, sex determination was found to be labile, with juveniles maturing into the opposite sex of the partner encountered. To our knowledge, this is the first experimental demonstration of social induction of maturation in conjunction with labile sex determination at maturation in vertebrates. This flexibility enables individuals to maximize their reproductive success in an environment where the timing of mate acquisition and the sex of their future partner are unpredictable.     

Variability of genetic sex determination in poeciliid fishes

Poeciliids are one of the best-studied groups of fishes with respect to sex determination. They present an amazing variety of mechanisms, which span from simple XX-XY or ZZ-ZW systems to polyfactorial sex determination. The gonosomes of poeciliids generally are homomorphic, but very early stages of sex chromosome differentiation have been occasionally detected in some species. In the platyfish Xiphophorus maculatus, gene loci involved in melanoma formation, in different pigmentation patterns and in sexual maturity are closely linked to the sex-determining locus in the subtelomeric region of the X- and Y- chromosomes. The majority of traits encoded by these loci are highly polymorphic. This phenomenon might be explained by the high level of genomic plasticity apparently affecting the sex-determining region, where frequent rearrangements such as duplications, deletions, amplifications, and transpositions frequently occur. We propose that the high plasticity of the sex-determining region might explain the variability of sex determination in Xiphophorus and otherbreak poeciliids.     

New aspects of sex change among reef fishes: recent studies in Japan

New aspects of sex change in reef fishes are reviewed with special emphasis on recent studies in Japan. For protogyny, studies on both monandric and diandric species have been conducted, but the distinction of primary males from prematurational secondary males seems to need further examination. For protandry, detailed field studies on anemonefishes have revealed alternative life-history styles associated with movements between hosts before or after maturation. The most interesting new aspect has been the discovery of 2-way sex change within a species. Conditions for evolution of 2-way sex change are examined in relation to the size-advantage model and social control mechanisms. A fish may change sex when it becomes dominant in a mating group, but a dominant fish may also change sex in the reverse direction when its social status changes to subordinate through inter-group movement. Two-way sex change has hitherto been reported only from basically protogynous fishes (e.g., Gobiidae, Pomacanthidae, Cirrhitidae, Epinephelinae). Possibilities of the reverse sex change in the protandrous anemonefishes are discussed with data from some unpublished studies.     

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.     

Bi-directional sex change in coral reef fishes from the family Pseudochromidae: an experimental evaluation

Coral reef fishes exhibit a diversity of hermaphroditic strategies and comparisons among species with different ecological characteristics will help identify the underlying basis of this complexity. We used manipulative experiments to test the potential for bi-directional sex change in three species of Pseudochromis (Pseudochromidae): P. flavivertex, P. aldabraensis and P. cyanotaenia. The first two species are sexually monochromatic, whereas, P. cyanotaenia is sexually dichromatic. For each species, where two functional females were kept together, one individual in the pair changed sex to male. Where two functional males were kept together, one individual in the pair changed sex to female. In all three species, functional sex change from male to female (52-93 days) took longer than sex change from female to male (18-56 days). In the sexually dichromatic species, P. cyanotaenia, colour change accompanied adult sex change. Females that changed sex to male took on the bright colouration of males and males that changed sex to female took on the drab colouration of females. These results indicate that bi-directional sex change is probably widespread in the family Pseudochromidae and cannot be predicted by the presence or absence of secondary sexual characteristics.     

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.

     

Behavioral thermoregulation in three Hawaiian reef fishes

Synopsis The preferred temperature of three Hawaiian coral-reef fishes — Acanthurus triostegus sandvicensis, Abudefduf abdominalis, and Chaetodon multicinctus — were determined in two-chambered tanks that permitted the fish to regulate behaviorally tank temperature. Median selected temperatures over 72 h were as follows: adult C. multicinctus, 24.0° C; juvenile C. multicinctus, 27.0° C; adult Ab. abdominalis, 25.9° C; juvenile Ab. abdominalis, 30.2° C; adult Ac. triostegus, 29.2° C; juvenile Ac. triostegus, 29.3° C. Juvenile Ab. abdominalis selected significantly higher median temperatures than the adults. C. multicinctus and juvenile Ab. abdominalis selected higher median temperatures during the day than at night.Based on the senior author's dissertation submitted to the Graduate School of the University of Hawaii in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Oceanography.Contribution No. 555 from the Hawaii Institute of Marine Biology, University of Hawaii, Honolulu, Hawaii.     

The spawning seasons of Caribbean reef fishes

Observations are recorded on the time of spawning of 83 species of Caribbean reef fishes. Details of seasonal variations in the percentage of sexually active fishes are given for 35 species. The majority of species spawn mostly when water temperatures are minimal, in February, March and April. The expected biomass of reef fish eggs in the plankton is about twelve times greater in the above mentioned period than in the period from June to December. Normal recruitment to insular fisheries may be very greatly dependent upon the success of parent stocks spawning elsewhere in the Caribbean and the abundance of recruits to a particular area may be dependent upon the speed and direction of the prevailing currents which carry larvae and juveniles to the insular shelves.     

Nocturnal relocation of adult and juvenile coral reef fishes in response to reef noise

Juvenile and adult reef fishes often undergo migration, ontogenic habitat shifts, and nocturnal foraging movements. The orientation cues used for these behaviours are largely unknown. In this study, the use of sound as an orientation cue guiding the nocturnal movements of adult and juvenile reef fishes at Lizard Island, Great Barrier Reef was examined. The first experiment compared the movements of fishes to small patch reefs where reef noise was broadcast, with those to silent reefs. No significant responses were found in the 79 adults that were collected, but the 166 juveniles collected showed an increased diversity each morning on the reefs with broadcast noise, and significantly greater numbers of juveniles from three taxa (Apogonidae, Gobiidae and Pinguipedidae) were collected from reefs with broadcast noise. The second experiment compared the movement of adult and juvenile fishes to reefs broadcasting high (>570 Hz), or low (<570 Hz) frequency reef noise, or to silent reefs. Of the 122 adults collected, the highest diversity was seen at the low frequency reefs; and adults from two families (Gobiidae and Blenniidae) preferred these reefs. A similar trend was observed in the 372 juveniles collected, with higher diversity at the reefs with low frequency noises. This preference was seen in the juvenile apogonids; however, juvenile gobiids were attracted to both high and low sound treatments equally, and juvenile stage Acanthuridae preferred the high frequency noises. This evidence that juvenile and adult reef fishes orientate with respect to the soundscape raises important issues for management, conservation and the protection of sound cues used in natural behaviour.     

Patterns of recruitment and spawning in Hawaiian reef fishes

Synopsis Recruitment of juvenile fishes to five 25 m² quadrats on an extensive natural reef in Kona, Hawaii was monitored over 51 months. Pronounced between-year variability in recruit abundance was evident for some species. Many exhibited strikingly low levels of recruitment. Overall recruitment was highly seasonal with a major peak in June and July, and a generally smaller, secondary peak in February and March. Recruitment was minimal during early winter (October–December) and a review of other studies similarly indicates minimal recruitment in Hawaii during this period. Spawning in Hawaiian fishes generally begins during the winter months of relatively low temperatures, increases during late winter and early summer and declines rapidly as maximum summer water temperatures are reached (September–October). Seasonal changes in food availability, ocean currents or salinity seem unlikely to be responsible for observed patterns of recruitment and spawning. Rather, the patterns appear to be most closely tied to changes in water temperature or photoperiod. Loss of propagules to offshore-moving eddies or other unfavorable currents may be responsible for the low levels of juvenile recruitment found in this and other Hawaiian studies. In Kona, at least 6 species of fishes recruited in pulses during quarter or new moon phases. Four other species have been reported elsewhere in Hawaii to recruit during either new or full moon phases. Lunar spawning periodicity was present in fewer than half of the species so far examined, and no single adaptive function for lunar periodicity was applicable to all species.     

Auditory sensitivity in settlement-stage larvae of coral reef fishes

The larval phase of most species of coral reef fishes is spent away from the reef in the pelagic environment. At the time of settlement, these larvae need to locate a reef, and recent research indicates that sound emanating from reefs may act as a cue to guide them. Here, the auditory abilities of settlement-stage larvae of four species of coral reef fishes (families Pomacentridae, Lutjanidae and Serranidae) and similar-sized individuals of two pelagic species (Carangidae) were tested using an electrophysiological technique, auditory brainstem response (ABR). Five of the six species heard frequencies in the 100–2,000 Hz range, whilst one carangid species did not detect frequencies higher than 800 Hz. The audiograms of the six species were of similar shape, with best hearing at lower frequencies between 100 and 300 Hz. Strong within-species differences were found in hearing sensitivity both among the coral reef species and among the pelagic species. Larvae of the coral reef species had significantly more sensitive hearing than the larvae of the pelagic species. The results suggest that settlement-stage larval reef fishes may be able to detect reef sounds at distances of a few 100 m. If true hearing thresholds are lower than ABR estimates, as indicated in some comparisons of ABR and behavioural methods, the detection distances would be much larger.     

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.     

Spawning ascent durations of pelagic spawning reef fishes

Predation risks have been hypothesized to influence spawning behaviors of coral reef fishes that broadcast gametes pelagically.The duration of spawning ascents of 13 species were measured from video footage at a single spawning site for multiple coral reef fishes to investigate if this behavior was influenced by varying risks of predation.Fishes that spawned in pairs had ascents of longer duration than group-spawning species.Duration of spawning ascents did not vary between fishes spawning at daytime and dusk,nor between group-spawning species with specific anti-predatory morphological adaptations.These results indicate that risk of predation may not significantly influence the duration of spawning ascents of pair spawning reef fishes at our study site,while group-spawning behaviors are influenced by predation.Avoidance of egg predation by benthic organisms and female mate choice are more likely to influence the pelagic spawning behaviors of all fishes observed [Current Zoology 58 ( 1 ):95-102,2012].     

Phylogeography and the conservation of coral reef fishes

Here we present a review of how the study of the geographic distribution of genetic lineages (phylogeography) has helped identify management units, evolutionary significant units, cryptic species, and areas of endemism, and how this information can help efforts to achieve effective conservation of coral reefs. These studies have confirmed the major biogeographic barriers that were originally identified by tropical species distributions. Ancient separations, identified primarily with mtDNA sequence comparisons, became apparent between populations on each side of the barriers. The general lack of correlation between pelagic larval duration and genetic connectivity across barriers indicates that life history and ecology can be as influential as oceanography and geography in shaping evolutionary partitions within ocean basins. Hence, conservation strategies require a recognition of ecological hotspots, those areas where habitat heterogeneity promotes speciation, in addition to more traditional approaches based on biogeography. Finally, the emerging field of genomics will add a new dimension to phylogeography, allowing the study of genes that are pertinent to recent and ongoing differentiation, and ultimately providing higher resolution to detect evolutionary significant units that have diverged in an ecological time scale.     

A behavioural reconnaissance of some Jamaican reef fishes

Three major social systems were observed: territorial, isolate, and gregarious. Territorials defend a particular area against intruders. Isolates neither chase conspecifics nor join other individuals, while gregarious species are found in groups. Some species of territorial pomacentrids were found in uni-specific colonies. As gregarious groups neared these colonies, they were attacked by several territorials. The number of chases directed against the various species of intruders was in proportion to the intruder's abundance in the study areas. Predators were uncommon and generally ignored. Isolates were either predators, or slow moving species. Gregarious species could be found in uni-specific or mixed-species groups. In mixed-species groups, there were three subunits: core, associate, and opportunist. Core species were always in the majority and led the group. Associates followed the core species for long periods of time and the opportunists were brief visitors. Quantitative comparisons between uni-specific and mixed-species groups revealed that associates tend to select the larger of the available core groups. Juveniles were also considered and compared to their adults. Juveniles did exhibit the general types of social behaviour found in adults. However, within a species, the adult's behaviour may be very different from their own juveniles. The coral, and the chases by pomacentrids, were related to the formation and dispersion of gregarious groups.     

Coral reef fishes which forage in the water column

Summary 1. Radiation of fishes representing many families in the tropical coral reef has repeatedly produced convergence in the specialization for water-column foragers.2. This life mode is depicted by a recognizable set of morphological and behavioral characteristics. Exploitation of this niche has parallel examples in fresh water, and non-tropical habitats.3. The distinguishing traits of this specialization, and the systematic, ecological and evolutionary features are described and discussed.

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«Fourrages» des poissons des récifs de coraux dans la colonne d'eau: Morphologie, comportement, écologie et évolution

Extrait Dans un biotope à récif de corail, des espèces étroitement liées peuvent servir d'exemple de différenciation sur le plan de l'évolution. La radiation évolutive de poissons représentant plusieurs familles du récif corallien tropical a conduit à plusieurs reprises à la formation de «fourragers dans la colonne d'eau». Ce mode de vie comporte une série de caractères morphologiques et éthologiques définis. On trouve des éxemples similaires dans l'eau douce et dans des habitats non tropicaux. Les traits distinctifs de cette spécialisation, la systématique, les caractéristiques écologiques et celles se rapportant à l'évolution sont décrits et discutés.