Sexually dichromatic protogynous angelfish Centropyge ferrugata (Pomacanthidae) males can change back to females

Protogynous hermaphroditism, female-to-male sex change, is well known among reef fishes where large males monopolize harems of females. When the dominant male disappears from a harem, the largest female may change sex within a few weeks. Recently, from experiments with some protogynous haremic fishes in which two males' cohabitated, it was confirmed that sexual behavior and gonads were completely reversible according to individual social status. However, the ability to reverse secondary-developed sexual body coloration has never been examined in any protogynous fish. We conducted two male cohabitation experiments with the protogynous haremic angelfish, Centropyge ferrugata, which has conspicuous sexual dichromatism on the dorsal fin. Smaller males of C. ferrugata soon performed female-specific mating behaviors when they became subordinated after losing a contest. They then completed gonadal sex change to females 47 or 89 d (n=2) after beginning cohabitation. In the course of the reversed gonadal sex change, male-specific coloration on the dorsal fin changed to that of a female. Thus, the sex of C. ferrugata, including secondary developed sexually dichromatic characteristics, can be completely reversible in accord with their social status.     

Alternative Contexts of Sex Change with Social Control in the Bucktooth Parrotfish, Sparisoma radians

Social control of sex change occurs in a variety of hermaphroditic fishes; upon removal of the dominant individual, the largest individual of the opposite sex typically changes sex and acquires mating priority with the remaining members of the social group. Social control may allow a phenotypically plastic response to social situations that convey cues about the relative advantages of functioning as one sex or the other, and should be advantageous in highly heterogeneous habitats such as coral reefs. Parrotfishes (family Scaridae) are dominant members of herbivorous coral reef fish assemblages, and numerous histological examinations of gonads have demonstrated the hermaphroditic life history of many species in the family. However, social control of sex change has never been conclusively demonstrated in the parrotfishes. To test a new version of the size-advantage model for sex change, we conducted removal experiments of dominant male bucktooth parrotfish, Sparisoma radians, in St. Croix, U.S. Virgin Islands. A total of seven females from five different reefs changed sex following removals, clearly demonstrating social control of sex change. In addition, all but one of those individuals changing sex were smaller than the largest females remaining in the harems, and this contrasts with nearly all previous studies of sex change in fishes. Sex change proceeds via a novel sequence of events when compared with previous studies. Rather than behavioral sex change preceding morphological sex change, the appearance of male coloration is followed by the development of male behavior that is fully expressed approximately 20 days after removal. We show how differing arrival rates of bachelor males at our study sites may facilitate alternative contexts of sex change, with sex change occurring within social groups in some locations and with bachelor males filling harem vacancies in other locations. Alternative contexts of sex change further illustrate the astonishing phenotypic plasticity in the social and mating behavior of parrotfishes.     

Physical interactions facilitate sex change in the protogynous orange-spotted grouper,Epinephelus coioides

Sex change in teleost fishes is commonly regulated by social factors. In species that exhibit protogynous sex change, such as the orange-spotted grouper Epinephelus coioides, when the dominant males are removed from the social group, the most dominant female initiates sex change. The aim of this study was to determine the regulatory mechanisms of socially controlled sex change in E. coioides. We investigated the seasonal variation in social behaviours and sex change throughout the reproductive cycle of E. coioides, and defined the behaviour pattern of this fish during the establishment of a dominance hierarchy. The social behaviours and sex change in this fish were affected by season, and only occurred during the prebreeding season and breeding season. Therefore, a series of sensory isolation experiments was conducted during the breeding season to determine the role of physical, visual and olfactory cues in mediating socially controlled sex change. The results demonstrated that physical interactions between individuals in the social groups were crucial for the initiation and completion of sex change, whereas visual and olfactory cues alone were insufficient in stimulating sex change in dominant females. In addition, we propose that the steroid hormones 11-ketotestosterone and cortisol are involved in regulating the initiation of socially controlled sex change.     

Reversed Sex Change in The Haremic Protogynous Hawkfish Cirrhitichthys falco in Natural Conditions

Bi‐directional sex change has recently been reported in a range of reef fishes, including haremic species that were earlier thought to be protogynous (female to male). However, the occurrence of this phenomenon and the social conditions driving the reversion of males to females (reversed sex change) have been poorly documented under natural conditions. Reversed sex change is predicted to occur in low‐density populations where facultative monogamy is common. However, few studies have evaluated this over a long period in such populations. We documented the occurrence of bi‐directional sex change during a 3‐yr demographic survey of a population characterised by small harem sizes in haremic hawkfish Cirrhitichthys falco. New males were derived following a change in sex of functional females (secondary males; n = 3) and juveniles always matured first as females (n = 3). Thus, C. falco exhibited a typical protogynous sexual pattern, consistent with a range of haremic fish species. We observed reversed sex change in two males. In both cases, all the females disappeared from their harems and the neighbouring males expanded their territories to encompass the territories of the sex changers. However, bachelor males did not always revert to females. A dominant male experienced bachelor status twice but regained mating opportunities following the immigration of a female into his territory or by taking a female from a neighbouring harem. Thus, we conclude that bachelor males use reversed sex change as a facultative tactic to regain reproductive status in a haremic mating system. In addition, we discuss the influence of harem size upon occurrence of reversed sex change.     

Territorial behaviour reflects sexual status in groups of false clown anemonefish (Amphiprion ocellaris) under laboratory conditions

Anemonefishes (genus Amphiprion) are well known as protandrous sex-changing fish. They live symbiotically with sea anemones and form a social unit that consists of a breeding pair and several sexually immature individuals with ambisexual gonads. In general, the hierarchy of the social rank is strictly maintained in a group, and this hierarchy is considered to be one of the critical factors that induce sex change or sex differentiation in anemonefishes. Field observations have shown that behavioural characteristics of anemonefishes vary depending on the environment. In this study we examined the differences in territorial aggression directed at conspecific intruders according to sexual status in the false clown anemonefish, Amphiprion ocellaris, under laboratory conditions. Resident–intruder tests were conducted on groups consisting of three anemonefish: a dominant female, a second-ranked male and a subordinate ambisexual individual. Females and males showed more aggressive behaviour than did ambisexual individuals, and behaviours were specifically directed at intruders of the same sexual status, not at those of the opposite sex. Notably, the female residents displayed fierce aggressive behaviours particularly toward female intruders, whereas male residents engaged more often in side-by-side swimming with male intruders than with female or ambisexual intruders. Ambisexual intruders were threatened by all residents, rarely attacked. We observed only few aggressive acts towards tank mates. These results suggest that sexually mature resident anemonefish in a confined space, such as under laboratory conditions, perceive intruders of the same sexual status as competitors for reproductive status and thus, dominant social rank.     

Reversed Sex-Change in the Protogynous Reef Fish Labroides dimidiatus

Protogynous hermaphroditism, or female-to-male sex change, is known for many reef fishes including wrasses (family Labridae) in which large males monopolize mating. When the dominant male disappears from a polygynous group, the largest female may change sex within a few weeks. Such social control of sex change was first documented in harems of the cleaner wrasse Labroides dimidiatus almost 30 yr ago. To examine whether change of social status would induce males of L. dimidiatus to perform reversed sex-change, we conducted experiments: (i) releasing single males near lone males whose mates have been removed in the field; and (ii) keeping two males in a tank. Smaller males changed back to females when they became subordinate: it took 53–77 d (n=3) for them to complete gonadal sex change and release eggs in the aquarium. The male–male pairs performed spawning behavior, with the smaller male in the female role already 5–58 d before completion of gonadal sex change. This is the first report of reversed sex-change among protogynous wrasses. Moreover, we conducted another experiment, keeping a pair of a male and a larger female in a tank (n=1). We found sex change by both mates, which has not been reported from any fishes. Thus, the sex of L. dimidiatus is strictly determined by social status whenever it changes after mate loss.     

Gonad development in Midas cichlids and the evolution of sex change in fishes

Some fishes mature and function as one sex and later transform to the other sex in response to social interactions. Previous evidence suggested that a change in developmental timing may be involved in the evolution of adult sex change in fishes. The most recent support for this idea came from reports that sex in the Midas cichlid, Amphilophus citrinellus, was determined by social conditions experienced at the juvenile stage. Differentiation as a male was reported to be dependent on large body size relative to group-mates, and thought to be mediated through aggressive interactions. Here I demonstrate that socially controlled sex determination does not occur as was originally reported. Previously, I found that sex was not associated with body size in juveniles either in nature or in captivity. Similarly, I found no association between aggressive behavior and sex in juveniles. I later demonstrated that socially controlled sex determination does not typically occur in the Midas cichlid and closely related species and supported an alternative mechanism to explain large body size in adult males. Finally, in the current study I analyze gonad histology of fish from the same population used by the original authors and lay to rest the idea of socially controlled sex determination in this species. Recent observations of socially controlled sex determination in juveniles of species that typically change sex at the adult stage are examples of phenotypic plasticity, not genetic variation. Therefore, juvenile socially controlled sex determination does not support a theory that a change in developmental timing is involved in the evolution of adult sex change in fishes.     

Size,sex and social experience: Experimental tests of multiple factors mediating contest behaviour in a rockpool fish

Aggressive contests amongst conspecifics are important to understand from an ecological and evolutionary perspective as contest dynamics can directly influence individual fitness. For some species, individual attributes such as relative body size closely predict the outcome, intensity and duration of contests, whereas for others, prior social experiences play a key role. However, disentangling the relative effects of individual attributes and social experiences is challenging from an experimental perspective, and because of this, few studies have investigated relative effects of multiple factors. Rockpool fishes have been well studied in terms of factors governing abundance, distribution and community structure, but much less so in terms of contest behaviour. This is surprising because contest dynamics are likely to directly affect the distribution of fishes along the rocky shore, and hence indirectly govern population and community composition. Here, we explored multiple factors potentially influencing contest behaviour in a numerically dominant, resident intertidal fish species, Bathygobius cocosensis (Gobiidae). Using a series of staged pairwise contest trials, we investigated the effect of size, sex and social experiences (namely prior residency and winner–loser experiences) on contest dynamics. We found no evidence that prior residency influenced contest outcome, suggesting social experiences play a minor role in contest dynamics. Previous winner/loser experience also did not influence contest outcome, although this is likely a product of low sample size. In addition, the likelihood of winning was unrelated to contestant sex, and the combination of sexes in paired contests did not influence contest intensity or duration. Instead, body size was related to contest outcome, intensity and duration in the majority of experimental trials. These results suggest that body size rather than sex or social experiences is the key determinant of contest dynamics in this species. We suggest that the dynamic biotic and abiotic environment inhabited by intertidal fish may reduce the influence of prior social experiences in modulating contest dynamics.     

Multiple mechanisms of phenotype development in the bluehead wrasse

Despite having detailed information on mechanisms mediating sex-typical behavior in many species, we have little understanding of whether the same mechanisms regulate these behaviors when they are performed in the same species under different social contexts. In the five field experiments of this study of bluehead wrasses (Thalassoma bifasciatum), a sex-changing fish, we examined the roles of arginine vasotocin (AVT) and the potent teleost androgen 11-ketotestosterone (11KT) in mediating sexual and aggressive behaviors typical of dominant males. We demonstrated that AVT appears necessary for the assumption of dominant territorial status in males and females, but is sufficient only in the socially dominant terminal phase (TP) male phenotype. Specifically, an AVP V(1) receptor antagonist prevented both TP males and females from gaining dominance over recently vacated territories. However, unlike TP males in a previous study, neither females nor initial phase males responded to AVT treatment with increases in display of TP male typical behaviors when under social conditions that inhibit sex change. Treating females with 11KT did not alter responsiveness to AVT, but did induce male coloration and courtship behavior that was not observed in oil-treated females. Combined with the results of a previous study, these results indicate that the ability of AVT to induce male-typical behavior differs among sexual phenotypes and that this differential responsiveness appears to be dependent on social context and not directly on exposure to 11KT. Furthermore, since 11KT can induce courtship behavior in females that is not affected by AVT, there may be different hormonal mechanisms mediating courtship behavior under different social contexts.     

Growth acceleration,behaviour and otolith check marks associated with sex change in the wrasse <Emphasis Type="Italic">Halichoeres miniatus</Emphasis>

In protogynous sex-changing fishes, females are expected to compete for the opportunity to change sex following the loss of a dominant male and may exhibit growth and behavioural traits that help them maintain their dominant status after sex change. A male removal experiment was used to examine changes in female growth and behaviour associated with sex change in the haremic wrasse Halichoeres miniatus and to test whether any changes in growth associated with sex change were recorded in otolith microstructure. Dominant females began displaying male-characteristic behaviour almost immediately after the harem male was removed. The frequency of interactions between females increased following male removal. In contrast, feeding frequency of females decreased. The largest one to three females in each social group changed sex following male removal and exhibited an increase in growth associated with sex change. Sex changers grew more than twice as fast as non-sex changers during the experimental period. This growth acceleration may enable new sex-changed males to rapidly reach a size where they can defend the remaining harem from other males. An optical discontinuity (check mark) was present in the otoliths of sex-changed fish, and otolith accretion rate increased significantly after the check mark, corresponding with the increased growth rate of sex-changing females. Wild caught males, but not females, exhibited an analogous check mark in their otoliths and similar increases in otolith increment widths after the check. This indicates that an increase in growth rate is a regular feature of sex-change dynamics of H. miniatus. Communicated by Environment Editor Prof. Rob van Woesik     

A model for environmental sex reversal in fish

A mathematical model is presented which combines genetic XX-female/XY-male sex determination with environmental pressure for phenotypic sex reversal. This may occur when fishes are exposed to endocrine disrupters, specifically masculinization by exposure to androgens and feminization by exposure to estrogens. A generic model is derived for the sex ratio in successive generations and three special cases, with chronic and constant pressure to sex reverse, are discussed in detail. These show that, with extreme environmental pressure to masculinize, the male genotype is at risk of dying out but with less extreme pressure, masculinization will not be detectable since the proportion of phenotypic males becomes one-half. With feminization at any pressure to sex reverse, the male and female genotypes will be maintained in a stable sex ratio in which the proportion of genotypic males exceeds one-half and is close to one-half if YY offspring (eggs) are not viable. In converse, the model is also applicable to the genetic ZZ-male/ZW-female system of sex determination in fish. At present suitable data are not available with which to validate the model, but proposals are made for relevant experimental studies.     

Environmental habituation and sexual composition affect juveniles' shoaling activity in a cichlid fish (Pelvicachromis taeniatus)

Group living is widespread in animals, and many fishes form shoals. Examining within-group interactions in fishes may contribute to the general understanding of dynamic social structures in animals. The sex ratio of a group has been shown to influence grouping decisions of fishes and can be expected to affect behaviour at group level. Behavioural experiments usually involve relatively short acclimatisation times, although the establishment of environmental habituation in fishes is understudied. This study tests whether the sex ratio and long-term habituation to experimental conditions influence general shoal performance (activity parameters, density) and responses of shoals to an acoustic-mechanical disturbance cue in juveniles of the cichlid fish Pelvicachromis taeniatus via individual tracking. The disturbance consisted of a defined hit against the experimental tank, which caused sudden noise and water movement. We found that a higher proportion of females increases shoal activity (swimming speed and distance covered), suggesting that female P. taeniatus are more active than males. Furthermore, shoal activity declined when shoals habituated to the experimental settings and with the time that the shoals were grouped together, which may reflect intensified group member familiarity. Moreover, behavioural changes after disturbance were weaker when individuals were kept with their group longer and more familiar to the experimental conditions. For prey species, lower activity might be beneficial under natural conditions due to lower conspicuousness of the group. We did not find any significant effects of the investigated factors on shoal density (mean interindividual distance) and speed synchronisation. The results indicate that sexual composition, familiarity between shoal members and habituation to the experimental environment affect shoal performance in a cichlid fish.     

Sex Change in Both Directions by Alteration of Social Dominance in Trimma okinawae (Pisces: Gobiidae)

The mating system of the gobiid fish Trimma okinawae is one of polygynous hermaphroditism, in which the largest female of a social unit changes sex following the removal of the dominant male. Histological observations of the gonads however, revealed that males have an ovarian tissue within a functional testis. The occurrence of ovarian tissue in the functional male suggests that T. okinawae males should be able to revert back into being functional females. To test this prediction, we placed females in an aquarium and allowed them to change sex. After confirming sex change from female to male, we individually placed new males into another aquarium and added a larger male to each. Our experiments revealed that females change sex and become males upon the removal of dominant males, and that those males changed sex again and became females in the presence of larger males. Sex change in both directions may be advantageous when a male is forced to become subordinate following the take over of the social unit by a larger male.     

Size‐selective fishing affects sex ratios and the opportunity for sexual selection in Alaskan sockeye salmon Oncorhynchus nerka

Selective exploitation can cause adverse ecological and evolutionary changes in wild populations and also affect sex ratios but few studies have empirically documented skewed sex ratios in exploited fishes (other than species with extreme sexual size dimorphism, SSD). To investigate the possibility of sex‐selective fishing on Alaskan sockeye salmon Oncorhynchus nerka, we assessed sex ratios in fish at two spatial scales: within each of five fishing districts and among 13 breeding populations in one of these districts. We predicted that populations’ sex ratios would vary based on the average size of fish and SSD because size affects vulnerability to fishing. At the larger scale, we found a small but significant bias in fish returning to four of the five fishing districts (average = 52% females), and in four of the five districts males were caught at significantly higher rates than females. At the finer scale there was marked variation in sex ratio on the breeding grounds, ranging from 36% to 47% males. Populations with fish of intermediate sizes experienced the greatest sex ratio biases; the greater vulnerability of males than females to fishing resulted from a combination of larger SSD and different harvest rates between the sexes associated with the fishery size‐selectivity curve shape. Skewed sex ratios may change competition and behavior on the breeding grounds, relaxing selection on male traits associated with mate choice by females or intra‐sexual competition and altering demographic and evolutionary pressures on the fish. Assessment of the size selectivity of fishing gear and the population's SSD can help to illuminate if and how exploitation can affect sex ratios. Future studies examining size‐selective fishing should also evaluate the consequences for sex ratios, as this might help explain changes in harvested population structure and sustainability.     

Group sex ratio and sex reversal

In hermaphroditic fishes, the initiation of sex reversal by male removal explains the replacement of lost males but does not explain how the number of males in a group may increase. Since numerous species apparently cannot produce primary males, a second means of initiating sex reversal must exist. In the present study we formulate a model which suggests the existence of an additional mechanism governing sex change: as soon as the ratio of adult females to males within a group exceeds a certain threshold value, a female changes sex even though no male has been removed. This process is inferred from comparison of data collected in the Red Sea and the western Indian Ocean with the model's predictions concerning size at sex reversal and the sex ratio of groups. The results suggest how several ecological factors may influence the occurrence rate of sex reversal and the development and growth of social groups.     

Why do anemonefishes inhabit only some host actinians?

Synopsis The 25 species ofAmphiprion and one ofPremnas (family Pomacentridae) are obligate symbionts of 10 species of facultatively symbiotic sea anemones. Throughout the tropical Indo-West Pacific range of the relationship, a fish species inhabits only certain of the hosts potentially available to it. This specificity is due to the fishes. Five fishes occupy six sea anemone species at Lizard Island, Great Barrier Reef, Australia.Entacmaea quadricolor harborsP. biaculeatus, A. melanopus andA. akindynos. Adults ofPremnas occur deeper than about 3 m in large, primarily solitary actinians; juveniles may occupy peripheral members ofEntacmaea clones in shallow water. Specimens ofA. melanopus live exclusively in clonal anemones, which are found no deeper than 3 m. Most individuals ofA. akindynos inEntacmaea are juveniles, occurring shallow and deep, in solitary anemones or at the margins of clones. Interspecific as well as intraspecific social control of growth may be responsible for keeping fish small at clone fringes. Conspicuous specimens ofE. quadricolor depend upon their anemonefish to survive. Actinians cleared of symbionts disappeared within 24 h, probably having been eaten by reef fishes.Entacmaea, the most abundant and widespread host actinian at Lizard Island and throughout the range of the association, is also arguably the most attractive to anemonefishes. I believe its vulnerability to predation was a factor in its evolving whatever makes it desirable to fishes. Experimental transfers pitted fish of one species against those of another, controlling for ecophenotype of host, and sex, size and number of fish. Competitive superiority was in the same order as abundance and over-all host specificity:P. biaculeatus, A. melanopus, A. akindynos. At least three factors are necessary to explain patterns of species specificity - innate or learned host preference, competition, and stochastic processes.     

Patterns of multiple paternity and maternity in fishes

The characterization of patterns of multiple mating is a major facet of molecular ecology and is paramount to understanding the evolution of behaviours associated with parental care and mate choice. Over the last 15 years, fishes have been particularly well studied with respect to multiple maternity and paternity thanks to the widespread application of microsatellite markers. The present review focusses on the impressive literature on genetic parentage in fishes. In studies of natural populations, we find that multiple paternity is extremely common across fish species, whereas rates of multiple maternity are much more variable. In species with nest defence, for example, rates of multiple maternity are strongly bimodal, and the occurrence of multiple dams per brood is either rare or the rule. The sex of the care‐giving parent is correlated with the rate of multiple parentage: when males provide uniparental care, rates of multiple paternity are low compared to rates of multiple maternity; when females provide parental care, either alone or assisted by males, rates of multiple paternity are highly variable, whereas rates of multiple maternity are quite low. These patterns may reflect conflicts between the reproductive interests of males and females. We also find that fishes in which females brood the offspring internally display much higher rates of multiple paternity compared to mammals or birds, whereas reptiles are intermediate. Male‐nesting fish species, however, show rates of multiple paternity more similar to those found in other vertebrates. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 103 , 735–760.     

Differential Responses of Brain,Gonad and Muscle Steroid Levels to Changes in Social Status and Sex in a Sequential and Bidirectional Hermaphroditic Fish

Sex steroids can both modulate and be modulated by behavior, and their actions are mediated by complex interactions among multiple hormone sources and targets. While gonadal steroids delivered via circulation can affect behavior, changes in local brain steroid synthesis also can modulate behavior. The relative steroid load across different tissues and the association of these levels with rates of behavior have not been well studied. The bluebanded goby (Lythrypnus dalli) is a sex changing fish in which social status determines sexual phenotype. We examined changes in steroid levels in brain, gonad and body muscle at either 24 hours or 6 days after social induction of protogynous sex change, and from individuals in stable social groups not undergoing sex change. For each tissue, we measured levels of estradiol (E₂), testosterone (T) and 11-ketotestosterone (KT). Females had more T than males in the gonads, and more E₂ in all tissues but there was no sex difference in KT. For both sexes, E₂ was higher in the gonad than in other tissues while androgens were higher in the brain. During sex change, brain T levels dropped while brain KT increased, and brain E₂ levels did not change. We found a positive relationship between androgens and aggression in the most dominant females but only when the male was removed from the social group. The results demonstrate that steroid levels are responsive to changes in the social environment, and that their concentrations vary in different tissues. Also, we suggest that rapid changes in brain androgen levels might be important in inducing behavioral and/or morphological changes associated with protogynous sex change.     

Social environment and sex differentiation in the false clown anemonefish, Amphiprion ocellaris

Plasticity in sex differentiation is known to be common in teleost fishes. Anemonefishes are protandrous; females are the largest and dominant members of social groups, displaying frequent aggressive behavior towards other members of groups. The second-ranked individuals become males and others remain as non-reproductive individuals. Here we examine the influence of social interaction in-group on sex differentiation in the false clown anemonefish, Amphiprion ocellaris, under laboratory conditions. Three juvenile anemonefish were kept in a tank for 180 days and their behaviors observed once a month. The social rank of individuals was distinguishable by their interactions in a group, with rank order clearly correlated with aggressive and appeasing behaviors. The dominant individuals occupied the shelter in the tank from the start to completion of the observation period. The body mass of dominant individuals increased compared to group-housed control fish, while third-ranked individuals showed growth suppression. The ratio of testicular tissue in gonads increased in dominant and second-ranked individuals but decreased in the third-ranked individuals. Differences in the plasma concentrations of estradiol, testosterone, and cortisol were not significant, but the concentration of 11-ketotestosterone was significantly higher in dominant individuals. These results suggest that, in false clown anemonefish, reproductive suppression of lower-ranked individuals becomes apparent in the first stage of group formation, and sex differentiation of upper-ranked individuals is gradually determined by long-term social interactions.     

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.