Sequential Hermaphroditism in Animals: Species That Change Sex

Sequential hermaphroditism is a reproductive strategy in which an individual animal changes sex during its lifetime, functioning first as one sex and later as the other. This article explains the two main forms, protandry (male to female) and protogyny (female to male), and provides a practical species table with sex-change direction and the social cues that trigger transformation. The content is written for students, researchers, life-science professionals, and informed general readers who need a reliable reference on which species change sex and how the process works.

What Is Sequential Hermaphroditism

Sequential hermaphroditism describes animals that begin life as one sex and later transition to the other sex. This differs from simultaneous hermaphroditism, where an individual holds functional male and female reproductive tissue at the same time, and from gonochorism, where sex is fixed for life. Among vertebrates, fishes show the most striking examples of this reproductive plasticity, with some species capable of changing sex from male to female, female to male, or serially in both directions across their life cycle. Social factors such as the disappearance of a dominant male or female from a group often cue the change, as documented in research on fish sexual diversity and plasticity.

The term sequential hermaphroditism covers three distinct patterns. Protandry describes individuals that mature first as males and later become females. Protogyny describes individuals that mature first as females and later become males. Bidirectional sex change describes species that can switch back and forth between male and female function, sometimes multiple times. Each pattern carries different ecological and evolutionary implications, and each is represented across different animal groups.

Protandry: Male to Female Sex Change

Protandrous species begin reproductive life as males and transition to female function later in life. This pattern is less common than protogyny among fishes but appears in several commercially and ecologically important species.

Barramundi Perch

The barramundi perch, Lates calcarifer, is typically considered a protandrous sequential hermaphrodite. This species also exhibits catadromy, meaning it migrates from freshwater to marine environments to spawn. Research on genomic structural variation in barramundi perch has identified three genetic lineages distributed across Australia and New Guinea, Southeast Asia, and the Indian Subcontinent. The species shows a diverse portfolio of life history options, including variation in migratory strategy and sexual system, with both hermaphroditism and gonochorism documented. Candidate chromosomal inversions appear linked to some of this life history variability, information that may prove useful for aquaculture production and population management.

For aquaculture operators working with barramundi, the practical implication is that sex ratios in captive populations can shift as fish age. Males that transition to female function later in life may alter the reproductive output of a broodstock tank. Records of individual fish age, size, and sex should be maintained so that transitions are detected early and broodstock composition can be adjusted.

Anemonefish and Clownfish

Clownfish and anemonefish are classic examples of protandrous sex change. These fish live in social groups associated with sea anemones, typically with a dominant breeding female, a breeding male, and several nonbreeding subordinates. When the dominant female dies or is removed, the breeding male changes sex to become female, and the largest subordinate matures to become the breeding male. This social control of sex change ensures that the group maintains a breeding pair.

The mangrove killifish, another sequential hermaphrodite, has been studied for behavioral differences between hermaphrodites and secondary males. Research on this species found that both secondary males and hermaphrodites show repeatable individual behavior, with no average difference between the sexes in exploration, boldness, or aggression. Aggression scores differed between genotypes, suggesting strong genetic control, and male boldness was more repeatable than hermaphrodite boldness, potentially reflecting sexual selection pressures.

Protogyny: Female to Male Sex Change

Protogynous species begin reproductive life as females and transition to male function later. This is the most common form of sequential hermaphroditism among fishes, and it predominates over protandry across teleost lineages. Evolutionary analyses of 4614 fish species suggest that the non-duality of the embryological origin of teleost gonads may explain why protogyny is more common than protandry.

Wrasses

Wrasses, family Labridae, provide some of the best-studied examples of protogynous sex change. The bluehead wrasse, Thalassoma bifasciatum, is an iconic tropical species where female-to-male sex change is socially cued and pervasive. Research comparing three wrasse species, including the temperate spotty wrasse, Notolabrus celidotus, and the kyusen wrasse, found that gonadal sex change was preceded by downregulation of cyp19a1a, the gene encoding gonadal aromatase that converts androgens to estrogens, and accompanied by upregulation of amh, encoding anti-Müllerian hormone that primarily regulates male germ cell development. These genes may act together to orchestrate ovary-to-testis transformation.

The spotty wrasse has been established as a temperate model for experimental investigation of sex change. Captive fish were induced to change sex using aromatase inhibition or manipulation of social groups, with complete female-to-male transition occurring over 60 days in both cases. Early-stage decreases in plasma estradiol concentrations or gonadal aromatase expression were not detected in spotty wrasse, despite these being commonly associated with sex change onset in subtropical and tropical protogynous hermaphrodites. Instead, expression of the masculinizing factor amh increased during early sex change, suggesting a potential role as a proximate trigger for masculinization.

California Sheephead

The California sheephead, Semicossyphus pulcher, is a temperate protogynous labrid that inhabits nearshore rocky environments from central California to southern Baja California, Mexico. Research examining the effects of exploitation on this species found that fishing practices can alter life history characteristics beyond simple size and growth changes. Where recreational and commercial fishing intensified and annual survivorship declined, males and females shifted to significantly smaller sizes, and the timing of maturation and sex change shifted as well. At a location where fishing remained light and survivorship stayed high, no changes in size structure or timing of maturation or sex change were observed.

This research carries direct implications for fisheries management. Selective harvesting of larger individuals, which in protogynous species are often males, can skew sex ratios and alter the timing of sex change within the population. Managers should monitor size structure and sex ratios over time, and fishing regulations should account for the possibility that harvest pressure can induce evolutionary changes in life history traits.

Groupers and Other Reef Fish

Many grouper species are protogynous hermaphrodites, with females transitioning to male function at larger sizes and older ages. The loss of large males through fishing can trigger premature sex change in smaller females, potentially reducing reproductive output and genetic diversity. Social cues, particularly the removal of dominant males, can accelerate the timing of sex change in these species.

Bidirectional Sex Change

Some fish species can change sex in both directions, functioning as males, then females, then males again, or switching based on social conditions. This serial sex change is less common than unidirectional protandry or protogyny but has been documented in several reef fish families. The mechanisms underlying bidirectional sex change are less well understood than those for unidirectional change, but research suggests that similar hormonal and genetic pathways are involved, with social cues playing a central role.

Social Control of Sex Change

Social factors are the primary triggers for sex change in many sequential hermaphrodites. The disappearance of a dominant male or female from a group can cue the largest individual of the appropriate sex to transition. This social control ensures that groups maintain reproductive function even when key individuals are lost.

Research on the neural mechanisms underlying socially controlled sex change has revealed that the process begins in the brain. Shifts in dominance status alter cortisol release and neuromodulator signaling, including dopamine and arginine vasotocin. Radial glia detect these changes and modulate local estradiol synthesis via brain aromatase production, which in turn influences neurogenesis and gene expression in multiple types of glia and neurons involved in pituitary-gonadotroph regulation. The altered gonadotroph control directs the gonadal transformation. Protogyny typically occurs faster than protandry, with differences in the order of behavioral, gonadal, and morphological changes.

For aquarium keepers and aquaculture operators maintaining social species, this means that removing or losing a dominant individual can trigger rapid sex change in remaining group members. Records of group composition and individual sex should be maintained, and any removal of dominant fish should be planned with awareness that sex change may follow.

Genetic and Hormonal Mechanisms

The genetic regulation of sex change in hermaphroditic fish involves a complex interplay of genes and signaling pathways. Research has identified several candidate genes that orchestrate the transformation of gonadal tissue. The enzyme aromatase, encoded by cyp19a1a, converts androgens to estrogens and plays a central role in maintaining ovarian function. Downregulation of this gene precedes gonadal sex change in protogynous species, while upregulation of amh accompanies the transformation.

Hormonal control of sex change involves shifts in the balance of estrogens and androgens. In protogynous species, declining estrogen production and increasing androgen signaling drive the transformation of ovarian tissue to testicular tissue. In protandrous species, the reverse pattern occurs. The speed and completeness of sex change can vary by species and by the method of induction, as demonstrated by the spotty wrasse model where complete transition occurred over 60 days.

At a Glance: Species Table

Evolutionary Patterns and Stability

Evolutionary analyses of sexual systems in teleost fish have revealed important patterns in how hermaphroditism evolves and persists. Gonochorism, where sex is fixed for life, is the likely ancestral condition in teleost fish. While all hermaphroditic forms revert quickly to gonochorism, protogyny and simultaneous hermaphroditism are evolutionarily more stable than protandry. Simultaneous hermaphroditism does not evolve directly from gonochorism but can evolve slowly from sequential hermaphroditism, particularly protandry.

Life history theory predictions are supported by these evolutionary analyses. Protogynous species live longer than gonochoristic species and invest the least in male gonad mass. The distribution of sexual systems across the fish tree of life does not reflect adaptive predictions alone, suggesting that adaptations may not fully explain why some sexual forms evolve in some taxa but not others. Future research should incorporate mating systems, spawning behaviors, and the diversity of sex-determining mechanisms, some of which might constrain the evolution of hermaphroditism.

Protandry and Protogyny Beyond Fish

While sequential hermaphroditism is most common and best studied in fishes, the terms protandry and protogyny also apply to other contexts in animal biology. In some invertebrates, including certain snails and polychaete worms, individuals change sex during their lifetimes. Among insects, the fruit fly Drosophila melanogaster shows a protogyny phenotype in which females eclose, or emerge from pupae, on average four hours faster than males due to sexual differences in the pupal period. Research on this phenotype found that the master sex switch gene Sxl establishes protogyny through a noncanonical pathway, independent of the traditional sex determination cascade.

In amphibians, the Japanese headwater frog Rana sakuraii exhibits a pattern described as autumn protogyny and spring protandry. Females migrate to breeding sites before males in autumn, while males arrive before females in spring. This pattern depends on differences in threshold temperature for hibernation, which is higher for females than males. Pairings begin during autumn migrations and mostly end during aquatic hibernation. This research proposes the surefire pairing hypothesis, suggesting that two-stage pairing occurs more securely when autumn and spring migrations are considered together.

Practical Assessment Steps

For researchers, aquaculturists, and fisheries managers working with sequential hermaphrodites, the following assessment steps can help track and manage sex change in populations.

First, establish baseline records of population structure. Record the size, age, and sex of all individuals in the population or broodstock. For species with external sexual dimorphism, visual inspection may suffice. For species without obvious external differences, consider gonadal examination or hormonal assays.

Second, monitor social group composition. Because social cues trigger sex change, track the presence and status of dominant individuals. Note any removals, deaths, or introductions that could alter social dynamics.

Third, track individual growth and development. Sex change in many species correlates with size and age. Maintain growth records and compare them against known size thresholds for sex change in the species.

Fourth, document any observed sex changes. Record the date, the individual involved, the social context, and any behavioral or morphological changes observed. This documentation supports both research and management decisions.

Fifth, evaluate the impact of any sex changes on population or broodstock function. Consider whether the sex ratio remains balanced and whether reproductive output is maintained.

Records and Measurements

Accurate record keeping is essential for managing sequential hermaphrodites in captivity or in wild populations. The following measurements and records are recommended.

Individual identification is the foundation of tracking sex change. Use tags, fin clips, or photographic identification to distinguish individuals. Record species, source, and any genetic lineage information where relevant.

Size measurements should include length and weight, taken at regular intervals. Growth rate calculations can help predict when sex change may occur. For species with known size thresholds for sex change, these measurements allow proactive management.

Sex determination records should note the method used, whether visual inspection, gonadal biopsy, hormonal assay, or genetic analysis. The date and confidence level of each determination should be recorded.

Social group records should document group composition, dominance hierarchies, and any changes to group membership. Note the removal or addition of individuals and the dates of these events.

Behavioral observations can provide early indicators of sex change. In many species, behavioral changes precede gonadal transformation. Record courtship behavior, aggression, and territorial displays.

Common Failure Patterns

Several common problems arise when working with sequential hermaphrodites in captivity or managing wild populations.

Unexpected sex change following social disruption is the most common issue. Removing a dominant individual without planning for the consequences can trigger rapid sex change in remaining group members, potentially leading to unbalanced sex ratios or loss of breeding function.

Failure to detect sex change early can complicate management. In species without obvious external sexual dimorphism, sex change may go unnoticed until reproductive failure occurs. Regular monitoring and hormonal or genetic analysis can help detect transitions earlier.

Fishery-induced shifts in sex change timing represent a significant management challenge. Selective harvesting of larger individuals, which are often male in protogynous species, can skew sex ratios and trigger earlier sex change in smaller females. This can reduce reproductive output and alter population dynamics over time.

Genetic and environmental interactions can produce unexpected outcomes. Temperature can modulate sex differentiation in fish to a limited degree, though genetic factors remain the primary determinants of sexual traits. Operators should be aware that environmental conditions may influence sex change timing even when genetic factors are controlled.

Limitations and Knowledge Gaps

Current understanding of sequential hermaphroditism is dominated by studies on a relatively small number of subtropical and tropical teleost species, often in wild settings. Temperate species and non-fish taxa are less well studied, limiting the generalizability of findings across groups.

The genetic mechanisms underlying sex change are incompletely understood. While several candidate genes have been identified, the degree to which a conserved genetic machinery orchestrates sex change across species has not been fully addressed. Comparative studies across multiple species are needed to determine which mechanisms are universal and which are species-specific.

The evolutionary history of sexual systems remains partially unresolved. While gonochorism appears ancestral in teleosts, the factors that favor the evolution and maintenance of different hermaphroditic forms are not fully explained by adaptive predictions alone. The Williams paradox, where sexual forms do not always match adaptive expectations, highlights the need for research incorporating mating systems, spawning behaviors, and sex-determining mechanisms.

Welfare and Safety Context

For those maintaining sequential hermaphrodites in captivity, welfare considerations should guide management decisions. Sex change is a natural process for these species, but the social disruption that triggers it can cause stress. When manipulating social groups to induce or prevent sex change, minimize handling and provide appropriate environmental enrichment.

Hormonal manipulation to induce sex change, as demonstrated in the spotty wrasse model, requires careful attention to dosages and welfare outcomes. Any experimental manipulation should follow institutional animal care guidelines and be reviewed by appropriate ethics committees.

For fisheries managers, the welfare of wild populations depends on maintaining balanced sex ratios and protecting the social structures that support natural reproduction. Harvest regulations should account for the size-selective nature of most fishing methods and the potential for fishery-induced shifts in life history traits.

Professional Escalation Criteria

When working with sequential hermaphrodites, certain situations warrant consultation with specialists or escalation to higher authorities.

If unexplained reproductive failure occurs in a captive population despite apparently appropriate conditions, consult a fish reproductive physiologist or veterinary specialist with experience in the species.

If fishery data show unexpected shifts in size structure, sex ratios, or timing of sex change, escalate to fisheries management authorities. These changes may indicate overfishing or other population-level stressors requiring regulatory action.

If genetic analysis reveals unexpected lineage composition or structural variants in a population, consult a population geneticist. Such findings may have implications for aquaculture breeding programs or conservation management.

If behavioral or physiological abnormalities are observed during sex change, particularly in experimental settings, consult the institutional animal care committee or equivalent oversight body.

Frequently Asked Questions

What is the difference between protandry and protogyny?

Protandry is a form of sequential hermaphroditism where an individual functions first as a male and later changes to female. Protogyny is the reverse, where an individual functions first as a female and later changes to male. Protandry is less common than protogyny among fishes, and evolutionary analyses suggest the embryological origin of teleost gonads may explain why protogyny predominates.

Which fish species change sex from male to female?

Barramundi perch and clownfish are well-known protandrous species. Barramundi perch are typically considered protandrous sequential hermaphrodites, while clownfish live in social groups where the dominant female is replaced by the breeding male when she dies. The mangrove killifish also shows sequential hermaphroditism with both hermaphrodite and secondary male forms.

Which fish species change sex from female to male?

Many wrasse species, including the bluehead wrasse and spotty wrasse, are protogynous. California sheephead are also protogynous, and research has shown that fishing pressure can alter the timing of sex change in this species. Groupers and many other reef fish species also change from female to male.

What triggers sex change in sequential hermaphrodites?

Social factors are the primary triggers. The disappearance of a dominant male or female from a group can cue the largest individual of the appropriate sex to change. Research on the neural mechanisms shows that shifts in dominance status alter cortisol release and neuromodulator signaling, beginning in the brain and directing gonadal transformation.

How long does sex change take?

The duration varies by species and by the method of induction. In the spotty wrasse model, complete female-to-male transition occurred over 60 days when induced by aromatase inhibition or social group manipulation. Protogyny typically occurs faster than protandry, with differences in the order of behavioral, gonadal, and morphological changes.

Can fish change sex more than once?

Yes, some species exhibit bidirectional sex change, switching between male and female function serially in their life cycle. This is less common than unidirectional protandry or protogyny but has been documented in several reef fish families.

How does fishing affect sex-changing fish populations?

Selective fishing of larger individuals can alter population size structure, growth rates, and the timing of maturation and sex change. Research on California sheephead found that intensified fishing led to smaller sizes and shifts in the timing of sex change, while lightly fished populations showed no such changes.

Are there examples of protandry and protogyny outside of fish?

Yes, the terms also apply to other contexts. Some invertebrates, including certain snails, change sex during their lifetimes. The fruit fly Drosophila melanogaster shows a protogyny phenotype where females emerge from pupae faster than males. The Japanese headwater frog exhibits autumn protogyny and spring protandry in migration timing.

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References and Further Reading

• NCBI Literature Resources. National Center for Biotechnology Information.
• PubMed. National Library of Medicine.
• A new experimental model for the investigation of sequential hermaphroditism.. Scientific reports, 2021.
• Sequential hermaphroditism and personality in a clonal vertebrate: the mangrove killifish.. Behavioural processes, 2012.
• Diversity and plasticity of sex determination and differentiation in fishes.. Sexual development : genetics, molecular biology, evolution, endocrinology, embryology, and pathology of sex determination and differentiation, 2013.
• Conservation and diversity in expression of candidate genes regulating socially-induced female-male sex change in wrasses.. PeerJ, 2019.
• Genomic structural variation in Barramundi Perch Lates calcarifer and potential roles in speciation and adaptation.. G3 (Bethesda, Md.), 2024.
• [Sperm competition in fish: `bourgeois' males and parasitic spawning.](https://pubmed.ncbi.nlm.nih.gov/21238275). Trends in ecology & evolution, 1998.
• Size-selective harvesting alters life histories of a temperate sex-changing fish.. Ecological applications : a publication of the Ecological Society of America, 2007.
• Autumn protogyny and spring protandry: Mechanisms and adaptive significance in a Japanese headwater frog, Rana sakuraii.. 2025.
• Plasticity of brain sexual dimorphism as revealed by sex changing fish.. 2025.
• The Concept of Fertility in the Field of Fruit Growing and Its Evolution from Ancient Times to Present Day.. 2025.
• Switches, stability and reversals in the evolutionary history of sexual systems in fish.. 2022.
• Understanding Genetic Regulation of Sex Differentiation in Hermaphroditic Fish.. 2025.
• Noncanonical function of the Sex lethal gene controls the protogyny phenotype in Drosophila melanogaster.. 2022.
• Sex-biased phenotypic plasticity affects sexual dimorphism patterns under changing environmental conditions.. 2024.
• Evolution and Speciation: Essays in Honor of M. J. D. White. 2011.
• Sex differences in cardiac recovery and ventricular gene expression in a rat model of donation after circulatory death. Biology of Sex Differences, 2026.
• One Health and sex and gender-related perspective in the ecosystem: Interactions among drivers involved in the risk of leptospirosis in Europe. A scoping review. One Health, 2024.
• Pharmacokinetic parameters of caffeine in laboratory animals in the context of assessing the functional state of the liver.. International Journal of Veterinary Medicine, 2023.
• Sex change in fish. Encyclopedia of Reproduction, 2018.

This article is educational and does not replace institutional policy, professional advice, or applicable safety and regulatory requirements.