Human chorionic gonadotropin (hCG) induces gonad reversal in a protogynous fish, the bluehead wrasse, Thalassoma bifasciatum (Teleostei, Labridae)

Female to male successive hermaphroditism (protogyny) is common in several groups of marine fish. Thalassoma bifasciatum, the bluehead wrasse (Labridae), found in the reefs of the Caribbean normally undergoes sex reversal after receiving behavioral cues. This report deals with the successful use of human chorionic gonadotropin (hCG) in inducing gonad reversal in this species. Eighty percent (n = 40) of the treated fish showed signs of reversal in 1-6 weeks; only 11% (n = 54) of the control (nontreated) group showed signs of reversal during the same period. The number of fish undergoing reversal increased with the length of the treatment period, 55% after 1 week, 100% after 6 weeks. A bluehead color pattern, typical of the terminal male phase, also appeared more frequently in the longer treated groups. To determine the efficacy of hCG in inducing gonad reversal, morphological criteria for reversal had to be established. For the majority of treated fish, the presence of both spermatogenic cysts and degenerating oocytes was sufficient to label these gonads unambiguously as undergoing reversal. However, at the extremes of this process, i.e., onset (early) and endpoint (late) stages, ambiguity could arise in identifying a gonad as undergoing reversal, and, therefore, key criteria were established for these stages. In our hands, the most consistent and reliable criterion for the early stage reversal was appearance of male germ line cells, clusters of "spermatogonial-like" cells. For the late stage, recognizable remnants of late-state oocytes had to be present. Some details of the histological changes that occur during early, middle, and late stages of reversal are also described. It is suggested that these results with hCG shed new light on the endocrine control of gonad reversal in T. bifasciatum.     

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.     

A multi-locus assessment of connectivity and historical demography in the bluehead wrasse (Thalassoma bifasciatum)

The pelagic larval stage of most coral reef fishes might allow extensive dispersal or, alternatively, some level of local recruitment might be important. Molecular markers can be used to obtain indirect estimates of dispersal to evaluate these alternatives, yet the extent of gene flow among populations is known for only a small number of species. The use of such markers must take into account the properties of the markers and the demographic history of the population when making inferences about current gene flow. In the Caribbean bluehead wrasse, Thalassoma bifasciatum, previous studies have found both substantial levels of local recruitment, in studies interpreting otolith microchemistry and, conversely, a lack of genetic differentiation inferred from mitochondrial DNA (mtDNA) restriction-fragment length polymorphism (RFLP) data and allozymes. However, if subtle differentiation exists, larger sample sizes and highly variable markers may be required to discern it. Here we present results from mitochondrial control region sequence and microsatellite data that indicate a lack of genetic differentiation at both small and large spatial scales. However, historical processes, such as changes in population size, may have affected the current distribution of genetic variation.     

Large mating aggregations and daily long-distance spawning migrations in the bluehead wrasse,Thalassoma bifasciatum

Synopsis The mating system of the labrid fishThalassoma bifasciatum has been extensively described in the literature, and the species has formed the basis for much work in ecology and behavior. However, most work in the past on this species has concentrated in small patch reef habitats. Here I describe a different mating pattern in bluehead wrasses occurring on a large, linear barrier reef in St. Croix, U.S. Virgin Islands. On this reef, very large group-mating aggregations (> 10000 fish) form daily at a single area on the forepart of the reef. Tagging studies showed that fishes are faithful to particular feeding schools arrayed along the forereef, and that they migrate to the spawning grounds along a specific path from distances more than 1.5 km away. No mating appears to occur in other upcurrent areas of the forereef. One-way migration times averaged 52 min from the farthest site studied, and feeding does not occur en route. Despite large differences in time spent in migration, no significant differences existed in fecundity or frequency of spawning among females living at different distances from the mating aggregation. The only apparent differences was a significantlyhigher growth rate in the fishes residing farthest from the aggregation. This higher growth rate corresponded to a higher overall feeding rate in the distant location, suggesting that food intake can more than compensate for the increased costs of migration.     

The feeding strategies of a facultative cleanerfish, Thalassoma bifasciatum (Pisces: Labridae)

The adult Bluehead wrasse (Thalassoma bifasciatum) is a facultative cleanerfish that can change its colouration into any one of three: patterns. These patterns correspond to several modes of feeding behaviour which, in turn, can be correlated with the movements of host fishes. The barred pattern occurred on individuals that are far ranging and also on those that formed unstable cleaning groups. These groups tended to form at locations having sharp drops in depth of several metres. Here these groups serviced large groups of host fishes. The striped pattern occurred on solitary individuals with a limited home range. They occurred in very shallow areas and serviced small groups of host fishes. The bright yellow pattern developed when a large food source was discovered and may be related to the attraction of conspecifics.     

Modulation of prey capture kinematics in the cheeklined wrasse Oxycheilinus digrammus (Teleostei: Labridae)

The ability to modulate prey capture behaviors is of interest to organismal biologists as it suggests that predators can perceive features of the prey and select suitable behaviors from an available repertoire to successfully capture the item. Thus, behavior may be as important a trait as morphology in determining an organism's diet. Using high-speed video, we measured prey capture kinematics in three cheeklined wrasse, Oxycheilinus digrammus. We studied the effects of three experimental prey treatments: live fish, dead prawn suspended in the water column, and dead prawn pieces anchored to the substrate in a clip. Live prey elicited significantly more rapid strikes than dead prey suspended in the water column, and the head of the predator was expanded to significantly larger maxima. These changes in prey capture kinematics suggest the generation of more inertial suction. With greater expansion of the head, more water can be accelerated into the buccal cavity. The attached prey treatment elicited strikes as rapid as those on live prey. We suggest that the kinematics of rapid strikes on attached prey are indicative of attempts to use suction to detach the prey item. More rapid expansion of the buccal or mouth cavity should lead to higher velocities of water entering the mouth and therefore to enhanced suction. Further modulation in response to the attached prey item, such as clipping or wrenching behaviors, was not observed. J. Exp. Zool. 290:88-100, 2001.     

Spatial Dispersion and Nonmigratory Spawning in the Bluehead Wrasse (Thalassoma bifasciatum)

Earlier studies of the behavior of the bluehead wrasse Thalassoma bifasciatum have shown it to be a migratory spawner with large terminal-phase males defending temporary spawning territories. We describe a variant social structure where fish occupy permanent home ranges, spawn within or near these home ranges, and are not territorial. Movements of identified terminal-phase and intermediate males and females were mapped in the backreef areas of three coral reefs in Puerto Rico. Locations of spawns were then compared with these home-range maps. All fish used most or all of their morning home range during the afternoon spawning period. Both terminal-phase males and females spawned within or near their home ranges. Males were relatively tolerant of other males during the afternoon spawning period: only one in five encounters between large males resulted in aggressive chases. The location of chases bore no relation to spawning sites, areas of morning foraging, or to home-range borders. A Monte Carlo computer simulation was used to measure home-range dispersion of terminal-phase and intermediate males at our main site. If males were territorial, we should expect their home ranges to be significantly overdispersed within the cumulative area they occupied. During the first year male home ranges were overdispersed but during the second year they were randomly dispersed, even though the same number of terminal-phase males occupied the same study site both years. Differences in social behavior between backreef areas and other areas reported in the literature, and found in other portions of our reefs, may be related to differences in feeding ecology between populations and between different portions of the same reef.     

sGnRH-A [(d-Arg6, Pro9, NEt-)LHRH] in combination with domperidone induces gonad reversal in a protogynous fish, the bluehead wrasse, Thalassoma bifasciatum

The effects of thrice weekly injections of Ovaprim (sGnRH-A in combination with domperidone) on the ovary of Thalassoma bifasciatum , a protogynous marine fish, after 4 and 6 weeks are described. At the end of 4 weeks of treatment, 72% ( n =13) of the original females showed signs of reversal. The majority of these were deemed early stage although mid- ( n =2) and late ( n =1) stage reversals were present. At 6 weeks, 92% of the females had ovaries that were undergoing transformation. None of these, however, progressed beyond the early stage. Only 3.8% of the control fish showed signs of reversal over the same periods. The blue head colour pattern, typical of the terminal phase, did not appear in any of the fish.
To our knowledge, this is the first report investigating the hypothalamic influence on sex reversal in T. bifasciatum . These observations shed new light on the neuroendocrine control of reversal of gender in this species and hermaphroditic fishes in general.     

Benefits of heterospecific foraging by the Caribbean wrasse,Halichoeres garnoti (Pisces: Labridae)

Synopsis Terminal-phase yellowhead wrasses, Halichoeres garnoti, foraged alone or in association with foraging goatfish, Pseudupeneus maculatus and Mulloides martinicus. Whereas H. garnoti did not dig for benthic infauna, the goatfish foraged almost exclusively on and in sand substrata. Wrasses in the company of goatfish made significantly more strikes on sand substrata than did solitary wrasses, although there were no significant differences in the frequency of strikes on sand and hard substrata combined. The frequencies of other behaviors (searches, scratches, flights, chases, rests, yawns, and cleanings) were not significantly different between solitary and associated wrasses. H. garnoti are attracted to foraging goatfish, which provide access to an otherwise unavailable food resource. To whom reprint requests should be sent     

The distribution, age and growth of wrasse (Labridae) in inshore waters of west Scotland

Wrasse were widely distributed in inshore waters of west Scotland in rocky and algal habitats. Ctenolabrus rupestris and Centrolabrus exoletus dominated commercial and experimental catches using creels, traps and fyke nets, but Crenilabrus melops, Labrus bergylta and L. mixtus were infrequent. Maximum age of sampled fish was 16yr in C. rupestris , 9 yr in C. exolefus , 5 yr in C. melops , 15 yr in L. bergylta , and 11 yr in L. mixtus . Growth was more rapid prior to maturation in all species. The von Bertalanffy growth model fitted data for C. rupestris and C. exoletus . Male C. rupestris and C. exoletus were significantly longer than females in most age groups and growth of all species captured in mid–Scottish areas was more rapid than in northern localities. Mortality (Z) of C. rupestris was 0–1861 per year and 0·4185 in C exoletus .     

Chromosomal analysis of repeated DNAs in the rainbow wrasse Coris julis (Pisces, Labridae)

Despite the interest of several authors, the karyotype of the labrid C. julis is still debated and in particular the presence of sex-chromosomes is still contradictory. In order to analyze the karyotype organization of C. julis we have performed an analysis with classical and molecular cytogenetic techniques. Our results after silver-, CMA₃- and DAPI-staining, C-banding and fluorescent in situ hybridization (FISH) using 28S rDNA, 5S rDNA, and telomeric repeat (TTAGGG) _n as probes allowed us to characterize the chromosomal location of several repetitive DNAs of C. julis. Finally, regardless of the technique used, no difference in the chromosome complement was found between males and females. This revised version was published online in August 2006 with corrections to the Cover Date.     

Hybridization in coral reef fishes: introgression and bi-directional gene exchange in Thalassoma (family Labridae)

Hybrids in coral reef fishes have traditionally been described based on external features using meristic characters and colouration to identify putative parental contributors. This study utilised molecular genetic techniques to verify hybrid status and identify putative parental species for five hybrid specimens (Labridae: Thalassoma) collected from Holmes Reef in the Coral Sea. Phylogenetic analyses support hybrid origins of the specimens. Mitochondrial COI gene, nuclear S7 (intron 1) and nuclear copy of mitochondrial (NUMT) D-loop region corroborate the identity of T. quinquevittatum as the maternal and T. jansenii as the paternal contributor. Backcrossing to parental species by hybrids and bi-directional gene exchange between the Holmes Reef populations of T. jansenii and T. quinquevittatum was detected, suggesting that hybrids are fertile and able to reproduce successfully. F(1) hybrids display a mixture of the colouration attributes of the two parental species, but subsequent backcrossed individuals were unrecognisable as hybrids and displayed colouration of either parental species. A large numerical imbalance exists between the putative parental species at Holmes Reef, with T. quinquevittatum outnumbering T. jansenii by approximately 25:1. In this case study, hybridization appears to be driven by ecological rather than evolutionary factors.     

Catching power of pots for wrasse Evaluation of the relative catching power of pots for north European wrasse (Labridae)

The relative catching power of three types of pot was examined in aquaria and field trials. The Sunart trap was more efficient in catching goldsinny ( Ctenolabrus rupestris (L.)) and rock cook wrasse ( Centrolabrus exoletus (L.)) than was the prawn creel or the Norwegian trap; all catches were enhanced by using fish bait. Although the catch rate is lower, the Norwegian pot may still be preferred by fishermen since damage to wrasse is minimised.     

The spawning, embryonic and early larval development of the green wrasse Labrus viridis (Linnaeus, 1758) (Labridae) in controlled conditions

Green wrasse, Labrus viridis (Linnaeus, 1758), is an endangered species in the southern Adriatic Sea, but it is also of interest for potential rearing in polyculture with other commercial species for the repopulation of areas where it is endangered or as a new aquaculture species. A parental stock of the green wrasse was kept in aquaria for six years. The spawning, embryonic and early larval development maintained under controlled laboratory conditions are described and illustrated. The average diameter of newly spawned eggs was 1.01±0.03 mm. Mature and fertilized eggs were attached to the tank bottom by mucus. Hatching started after 127 h at a mean temperature of 14.4±0.8°C. The average total length of newly hatched larvae was 4.80±0.22 mm. Absorption of the yolk-sac was completed after the 5th day when larvae reached 5.87±0.28 mm. Larvae were fed with the rotifers Brachionus plicatilis. The pigmentation of L. viridis larvae is similar to that of Labrus merula and Labrus bergylta, but the main differences between these species are in the size of larvae and the development time of the melanophores on the anal fin-fold (five days later than with L. merula) and on top of the head (nine days earlier than with L. merula).     

Social control of sex change in the Red Sea razorfish Xyrichtys pentadactylus (Teleostei,Labridae)

Synopsis The Red Sea razorfish, Xyrichtys pentadactylus, a territorial haremic labrid with dominance hierarchies within the harems. Theory predicts that primary males (fish developing initially as males) should be rare or nonexistent in haremic territorial species because the larger secondary males (males which have undergone sex and/or color change) limit access to females. Histological examination of gonads of 95 specimens showed that all males are derived from females by sex change (i.e. they are secondary males). During five months of field studies 100% of more than 200 observed matings were pair spawnings — the usual mating practice for monandric (having one type of male) species. Sex change in females was induced by male removal in nature. Isolation of four groups of females in aquaria showed that the largest female in the social group changes sex in the absence of a male, demonstrating that sex change is socially-controlled in this species.     

Dating the evolutionary origins of wrasse lineages (Labridae) and the rise of trophic novelty on coral reefs

We estimated ages of divergence between major labrid tribes and the timing of the evolution of trophic novelty. Sequence data for 101 labrid taxa and 14 outgroups consisting of two mitochondrial gene regions (12s, 16s), and two nuclear protein-coding genes (RAG2, TMO4c4), a combined 2567 bp of sequence, were examined using novel maximum likelihood, maximum parsimony and mixed model Bayesian inference methods. These analyses yielded well supported trees consistent with published phylogenies. Bayesian inference using five fossil calibration points estimated the minimum ages of lineages. With origins in the late Cretaceous to early tertiary, the family diversified quickly with both major lineages (hypsigenyine and julidine) present at approximately 62.7 Ma, shortly after the K/T boundary. All lineages leading to major tribes were in place by the beginning of the Miocene (23 Ma) with most diversification in extant lineages occurring within the Miocene. Optimisation of trophic information onto the chronogram revealed multiple origins of novel feeding modes with two distinct periods of innovation. The Palaeocene/Eocene saw the origins of feeding modes that are well represented in other families: gastropod feeders, piscivores and browsing herbivores. A wave of innovation in the Oligocene/Miocene resulted in specialized feeding modes, rarely seen in other groups: coral feeding, foraminifera feeding and fish cleaning. There is little evidence of a general relationship between trophic specialization and species diversity. The current trophic diversity of the Labridae is a result of the accumulation of feeding modes dating back to the K/T boundary at 65 Ma, with all major feeding modes on present day reefs already in place 7.5 million years ago.