Sperm and milt characteristics and male v. female gametic investment in the Caribbean reef fish, Thalassoma bifasciatum

Individual sperm cells produced by two male morphs of the bluehead wrasse Thalassoma bifasciatum did not differ in size (i.e. cell volume). Initial phase (IP) males (high sperm competition) had a 60% higher sperm concentration in their milt than did terminal phase (TP) males (low sperm competition), which may reflect differences in how accurately the two male morphs need to allocate sperm to their spawns. The energy density of milt was about 16% lower than that of eggs. Estimates of gametic energy investment based on (a) the difference in testis weights between the beginning and the end of the spawning period and (b) the number of sperm released in natural spawns (determined in other studies), suggested that, on a daily basis, IP males invest about 65% of that of females. Estimates based on stripping milt from IP males at the beginning and the end of the spawning period, however, indicated that their daily energy investment in gamete production is about 10% of that of females Gametic investment by TP males is lower than that by both IP males and females.     

The role of Thalassoma lunare as a predator of juvenile fish on a sub-tropical coral reef

The process of predation causes significant mortality in coral reef fishes immediately following settlement. However, much of what we know of predator identity is based on a small number of detailed studies. This study aims to identify the key predator of early juvenile coral reef fishes on Ningaloo Reef, North-Western Australia. Video cameras were used to observe patch reefs stocked with newly settled reef fish in the back-reef area between 12:00 and 20:30 h. The cameras were fitted with >610 nm light sources to allow observation in low light conditions. All strikes (attempted and successful) on newly settled fish were recorded, along with the time spent in the vicinity of experimental patch reefs with or without juvenile fish. A total of 69 strikes were observed over the 199 h of recorded video footage, with the majority of strikes occurring mid-afternoon between 13:00 and 15:30 h. Only one strike was observed during the twilight period, an hour either side of sunset (~18:45 h), and no strikes were observed after this period. The moonwrasse, Thalassoma lunare, was responsible for the majority of strikes (75.4 %), with the sandperch (Parapercis clatharatha—10.1 %), spanish flag (Lutjanus carponotatus—5.8 %) and ring wrasse (Hologymnosus annulatus—2.9 %) the next highest contributors. T. lunare also spent significantly more time in the vicinity of reefs stocked with newly settled fish, than those without, during daylight hours. The results of the study are in contrast to the common perception that predation on newly settled fish is focused largely around crepuscular periods and suggests that diurnally active species, in particular T. lunare, are important predators of juvenile fish on the Ningaloo back-reef. The study also implies that generalist species can fulfil key functional roles and that the nature of these roles is not always apparent.     

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.     

Local extinction of a coral reef fish explained by inflexible prey choice

While global extinctions of marine species are infrequent, local extinctions are becoming common. However, the role of habitat degradation and resource specialisation in explaining local extinction is unknown. On coral reefs, coral bleaching is an increasingly frequent cause of coral mortality that can result in dramatic changes to coral community composition. Coral-associated fishes are often specialised on a limited suite of coral species and are therefore sensitive to these changes. This study documents the local extinction of a corallivorous reef fish, Oxymonacanthus longirostris, following a mass bleaching event that altered the species composition of associated coral communities. Local extinction only occurred on reefs that also completely lost a key prey species, Acropora millepora, even though coral cover remained high. In an experimental test, fish continued to select bleached A. millepora over the healthy, but less-preferred prey species that resisted bleaching. These results suggest that behavioural inflexibility may limit the ability of specialists to cope with even subtle changes to resource availability.     

Preliminary study on female choice of mates versus sites in a wrasse, Cirrhilabrus temminckii

Female choice of mates versus sites was studied in a wrasse, Cirrhilabrus temminckii. Males had territories within a restricted area on a rocky slope at which females visited and pair-spawned pelagic gametes. Females visited several males or territories before spawning, suggesting the opportunity of female choice. Of the four characteristics of territorial males examined—body size, ratio of pelvic fin length to body size, courtship, frequency and territory depth—only territory depth was significantly correlated with daily mating success of males. The former three male characters were not related to territory depth. These results suggest that female C. temminckii chooses deep sites rather than specific mates in mating.     

Organization of the male gonad in a protogynous fish,Thalassoma bifasciatum (Teleostei: Labridae)

Although the testis in teleosts has been investigated for many years, little attention has been paid to the structure of the outer layers that enclose the testis and to their possible contributions to its organization. The present study in a protogynous male labrid, Thalassoma bifasciatum (bluehead wrasse), describes the arrangement and cytology of these tissues (for convenience, referred to collectively as the outer wall, OW) which include: the outer peritoneal layer and subjacent collagen fibers, myoid cells and diverse other cells and tissues, e.g., fibrocytes, presumptive mesenchyme, macrophages, granulocytes, nerves, and blood vessels. Beneath the OW are two compartments; one is the gamete-laden spermatocysts, the other the interstitium, which is composed of cells and tissues that lie between the spermatocysts. Both OW and interstitium contain similar kinds of tissues and cells. Moreover, the layers of the OW immediately subjacent to the peritoneum are continuous with that in the interstitium. It is suggested that the continuity between these two areas provides opportunities for the exchange of cells that could aid in the maintenance and reorganization of the testis and with the myoid and neural tissue to establish an extensive, coordinated motile system that aids movement of sperm from spermatocysts to the ducts. A recent report on the reexamination of the germinal epithelium concept and its identification in the common snook, Centropomus undecimalis, stimulated us to examine the feasibility of applying this concept to gonad organization and gamete development in T. bifasciatum. In addition, the ultrastructure of the Sertoli cell and formation of spermatocysts are described. Spermatocysts increase in size during the development of gametes. Observations and discussion are presented suggesting how Sertoli cells may accommodate this growth and how new populations of these cells may arise in the mature adult. Finally, ultrastructural characteristics for each stage of spermatogenesis are presented and, using (3H)thymidine and autoradiography, data on the chronology of spermatogonia-sperm cycle are included.     

Global ecological success of Thalassoma fishes in extreme coral reef habitats

Phenotypic adaptations can allow organisms to relax abiotic selection and facilitate their ecological success in challenging habitats, yet we have relatively little data for the prevalence of this phenomenon at macroecological scales. Using data on the relative abundance of coral reef wrasses and parrotfishes (f. Labridae) spread across three ocean basins and the Red Sea, we reveal the consistent global dominance of extreme wave‐swept habitats by fishes in the genus Thalassoma, with abundances up to 15 times higher than any other labrid. A key locomotor modification—a winged pectoral fin that facilitates efficient underwater flight in high‐flow environments—is likely to have underpinned this global success, as numerical dominance by Thalassoma was contingent upon the presence of high‐intensity wave energy. The ecological success of the most abundant species also varied with species richness and the presence of congeneric competitors. While several fish taxa have independently evolved winged pectoral fins, Thalassoma appears to have combined efficient high‐speed swimming (to relax abiotic selection) with trophic versatility (to maximize exploitation of rich resources) to exploit and dominate extreme coral reef habitats around the world.     

Predation risk and the opportunity for female mate choice in a coral reef fish

The influence of predation risk on the opportunity for female mate choice was investigated in the tailspot wrasse Halichoeres melanurus at two sites on a coral reef in Okinawa, Japan. Females mated repeatedly with the nearest males, but they also changed mates frequently at both sites. Mate changes were seen not only in the context of spatiosocial changes (mate disappearance or shifts in male territories) but were also probably the result of actual mate choice by females. Females at one site (site A) changed mates more often and conducted longer spawning trips from their home ranges to male territories than at the other site (site B). Fish at site A were faced with a higher frequency of predators than that of site B. However, fish of site A suffered fewer attacks from predators because they had more shelter, suggesting lower predation risk in that site. These results suggest that females under higher predation risk had less opportunity to choose preferable mates and that they had to mate with the same, nearest males in most cases.     

A dynamic model of group-size choice in the coral reef fish Dascyllus albisella

We developed a dynamic programming model of group size choicefor settling coral reef fish to help understand variabilityin observed group sizes. Rather than calculating optimal groupsize, we modeled optimal choice and calculated the acceptablegroup sizes that arose from this choice. In the model, settling individualsweigh the fitness value of settling in a group against the expectedfitness of searching another day and encountering other groups, choosingthe option with the higher value. Model results showed that individualssettling on any given day in the settling season have several acceptablegroup sizes in which they can settle. The range of acceptablegroup sizes also changes across the season. Early in the season,when there is still adequate time to grow, large groups (withhigher survival) have the highest fitness. Late in the season,when the ability to grow fast becomes more important, smallgroups, which convey fast growth rates (although riskier), havehigher fitness. Thus, according to our model, even when fishall make the same, simple decisions, a variety of outcomes arepossible, depending on the specific options encountered andtemporally changing ecological pressures. Even when all fishbehave optimally, initial variability in group sizes will persist.     

Components of fertilization success in the bluehead wrasse, Thalassoma bifasciatum

In many species of marine organisms, males and females releasegametes directly into the water column. Although free-spawningmarine invertebrates appear to have highly variable fertilizationsuccess, in tropical reef fishes the average fertilizationsuccess is quite high, typically over 90%; nevertheless, substantialvariation has been reported, and fertilization has a directeffect on fitness. We investigated the factors affecting fertilization success in natural spawnings of the bluehead wrasse, Thalassoma bifasciatum. During a two-year study at a site in St. Croix,we found extensive and predictable variation in fertilizationsuccess in pair spawns of this reef fish. Fertilization successaveraged 95%, but was affected by the amount of sperm released,the water velocity at a site, the mating success of the male,and the size of the female. As sperm released in a spawn increases, and as water velocity at a site decreases, sperm concentrationsshould remain higher in the vicinity of eggs for a longer periodof time, and both of these factors are correlated with increasingfertilization success. The recent history of individuals withpartners or sites did not affect the fertilization successof their spawn. In an evolutionary context, the real and predictable variance in fertilization success in this species may influencethe mating choices of males and females. However, there iscurrently no evidence that females use differences in fertilizationsuccess among males or sites in their reproductive decisions.     

Competitive interactions and the availability of sleeping sites for a diurnal coral reef fish

A field study was made to test whether the population size of a diurnal reef fish, the wrasse Thalassoma bifasciatum (Bloch), was limited by inter- or intraspecific competition for sleeping shelter. T. bifasciatum is often attacked at dusk by two small territorial damselfishes, Eupomacentrus dorsopunicans (Poey) and E. planifrons (Cuvier). Although these three species sleep in the same general habitats, there are qualitative differences in the types of holes they use and how they use them. Wrasse holes are usually in these damselfishes' territories, but damselfish attacks do not prevent wrasses entering holes. Wrasses infrequently defend their holes intraspecifically. They regularly change their holes, with little intra- or interspecific aggressive interaction. When its hole is removed, a wrasse is late in retiring but finds a hole near its old one with little aggressive interaction, and does not have a higher mortality rate. Empty wrasse holes are rarely refilled, and then only by conspecifics. Wrasses added to reefs find unoccupied holes and do not usurp other fishes' holes. Damselfish defend their eggs and food against the wrasse, but not their sleeping shelter, nor living space per se. Sleeping sites are not limiting the wrasse, but are present in a surplus. Intraspecific hole defense by a wrasse prevents a delay in its retiring that would increase the risk of crepuscular predation on it.     

Live coral repels a common reef fish ectoparasite

Coral reefs are undergoing rapid changes as living corals give way to dead coral on which other benthic organisms grow. This decline in live coral could influence habitat availability for fish parasites with benthic life stages. Gnathiid isopod larvae live in the substratum and are common blood-feeding parasites of reef fishes. We examined substrate associations and preferences of a common Caribbean gnathiid, Gnathia marleyi. Emergence traps set over predominantly live coral substrata captured significantly fewer gnathiids than traps set over dead coral substrata. In laboratory experiments, gnathiids preferred dead coral and sponge and tended to avoid contact with live coral. When live gnathiids were added to containers with dead or live coral, significantly fewer were recovered from the latter after 24 h. Our data therefore suggest that live coral is not suitable microhabitat for parasitic gnathiid isopods and that a decrease in live coral cover increases available habitat for gnathiids.     

Positive indirect effects in a coral reef fish community

Indirect effects occur when two species interact through one or more intermediate species. Theoretical studies indicate that indirect interactions between two prey types that share common predators can be positive, neutral or negative. We document a positive indirect interaction between different types of prey fish on coral reefs in Australia. A high abundance of one type of prey fish (cardinalfishes: Apogonidae) resulted in higher recruitment, abundance and species richness of other prey fish. Our evidence indicates that these effects were not due to differential settlement but were instead due to differential post-settlement predation. We hypothesize that resident piscivores altered their foraging behaviour by concentrating on highly abundant cardinal-fish when they were present, leaving recruits of other species relatively unmolested. Indirect effects were evident within 48 h of settlement and persisted throughout the 42-day experiment, highlighting the importance of early post-settlement processes in these communities.     

Food availability affects growth in a coral reef fish

Summary Pomacentrus amboinensis is common on small patch reefs within One Tree Lagoon (Great Barrier Reef), where it preferentially settles onto deep reefs. A preliminary experiment, in which juveniles were transplanted to identical reef structures at two sites, within two depth strata, indicated that juvenile growth and survivorship were better in deeper water. The hypothesis that this difference was due to food availability was tested by a supplemental feeding experiment, carried out at another two randomly chosen sites, within the same two depth strata. Fish were fed each day over a one month period, during which no mortality was observed. The growth rates of juveniles were markedly higher on all food-supplemented reefs, when compared to controls. Growth differed between depth strata, but there was no interaction between the food x depth factors, which would have suggested a greater effect of food supplementation in either habitat. Thus, although the difference between depths cannot be attributed to food availability, the results have a more general significance. Food appears to be a limiting resource (in terms of growth) in both the marginal shallow habitat, and the more suitable deeper habitat.     

Long-term cleaner fish presence affects growth of a coral reef fish

Cleaning behaviour is considered to be a classical example of mutualism. However, no studies, to our knowledge, have measured the benefits to clients in terms of growth. In the longest experimental study of its kind, over an 8 year period, cleaner fish Labroides dimidiatus were consistently removed from seven patch reefs (61-285 m(2)) and left undisturbed on nine control reefs, and the growth and parasite load of the damselfish Pomacentrus moluccensis determined. After 8 years, growth was reduced and parasitic copepod abundance was higher on fish from removal reefs compared with controls, but only in larger individuals. Behavioural observations revealed that P. moluccensis cleaned by L. dimidiatus were 27 per cent larger than nearby conspecifics. The selective cleaning by L. dimidiatus probably explains why only larger P. moluccensis individuals benefited from cleaning. This is the first demonstration, to our knowledge, that cleaners affect the growth rate of client individuals; a greater size for a given age should result in increased fecundity at a given time. The effect of the removal of so few small fish on the size of another fish species is unprecedented on coral reefs.     

Social transmission of behavioural traditions in a coral reef fish

Traditional behaviours involve the non-genetic transmission of social information across age classes or generations. French grunts (Haemulon flavolineatum) exhibit social traditions of daytime schooling sites and twilight migration routes. Individuals transplanted to new schooling sites and allowed to follow residents at the new sites used the new migration routes and returned to the new sites in the absence of resident fish. Control fish with no opportunity to learn showed no such directionality or return. This is the first demonstration of apparent pre-cultural behaviour in free-living fish. Our observations suggest additional classes of behaviour and taxonomic groups in which pre-cultural activities are likely to have evolved.     

Eleven polymorphic microsatellite loci in a coral reef fish,Pterapogon kauderni

We describe the isolation and characterization of 11 polymorphic tetranucleotide microsatellite loci from a male mouthbrooding coral reef fish, the Banggai cardinalfish Pterapogon kauderni. In a sample of 37 fish from a natural population, polymorphism at these loci ranged from two to 15 alleles, with expected heterozygosities ranging from 0.107 to 0.928, enabling high‐resolution genetic studies of this coral reef fish.     

Diurnal variability in a fish assemblage of a Bahamian coral reef

Synopsis Variation in the diurnal composition of a fish assemblage of a Bahamian coral reef was investigated by comparing visual counts of fishes taken along a 100 × 4 m wide fixed transect at four times: 0900, 1200, 1500 and 1800 hours during the summer of 1979. One sample per day was obtained at each time period over a span of 20 consecutive days. Forty-two species were recorded in these samples, with 25 occurring frequently enough to permit statistical analysis of diurnal variations in abundance. Of the 25 species compared, nearly one fourth (6 species) displayed significant variation in abundance patterns among the four time periods tested. It is suggested, because of the strong possibility of bias which might otherwise be introduced because of these variations, that repetitive quantitative visual censusing of coral reef fishes be undertaken at about the same time each day.     

Island biogeography of Caribbean coral reef fish

Aim The goal of our study was to test fundamental predictions of biogeographical theories in tropical reef fish assemblages, in particular relationships between fish species richness and island area, isolation and oceanographic variables (temperature and productivity) in the insular Caribbean. These analyses complement an analogous and more voluminous body of work from the tropical Indo‐Pacific. The Caribbean is more limited in area with smaller inter‐island distances than the Indo‐Pacific, providing a unique context to consider fundamental processes likely to affect richness patterns of reef fish. Location Caribbean Sea. Methods We compiled a set of data describing reef‐associated fish assemblages from 24 island nations across the Caribbean Sea, representing a wide range of isolation and varying in land area from 53 to 110,860 km². Regression‐based analyses compared the univariate and combined effects of island‐specific physical predictors on fish species richness. Results We found that diversity of reef‐associated fishes increases strongly with increasing island area and with decreasing isolation. Richness also increases with increasing nearshore productivity. Analyses of various subsets of the entire data set reveal the robustness of the richness data and biogeographical patterns. Main conclusions Within the relatively small and densely packed Caribbean basin, fish species richness fits the classical species–area relationship. Richness also was related negatively to isolation, suggesting direct effects of dispersal limitation in community assembly. Because oceanic productivity was correlated with isolation, however, the related effects of system‐wide productivity on richness cannot be disentangled. These results highlight fundamental mechanisms that underlie spatial patterns of biodiversity among Caribbean coral reefs, and which are probably also are functioning in the more widespread and heterogeneous reefs of the Indo‐Pacific.