Protandrous Research Articles

Distinctive morphological variation of saccular otoliths in relation to hermaphroditism type in Sarpa salpa and Serranus scriba (Teleostei: Perciformes) from the Mediterranean Sea in Bizerte, Tunisia

This study aimed to describe and compare the morphological characteristics of saccular otoliths (sagittae) in the protandrous hermaphrodite Sarpa salpa and the synchronous hermaphrodite Serranus scriba collected from the Bizerte site located in northwestern Tunisia, and to examine the effect of hermaphroditism type on the otolith shape and size at the inter- and intraspecific levels, using elliptical Fourier analysis. At the interspecific level, a significant bilateral asymmetry in shape was observed between the left and right otoliths. At the intraspecific level, a significant bilateral asymmetry in shape was detected between the left and right otoliths only in S. scriba. Discriminant function analysis (DFA) produced two separate main groups corresponding to the two species, and intrasexually differentiated between left and right otoliths in S. salpa. In addition, a significant bilateral asymmetry was found in Lo, Wo, Ao, and Po in S. salpa and in Po only in synchronous S. scriba. Thus, protandrous hermaphroditism did not affect otolith shape in S. salpa, but showed an apparent bilateral asymmetry in otolith size. However, synchronous hermaphroditism exhibited a significant bilateral asymmetry in otolith shape and Po only in S. scriba. The causes of this significant inter- and intraspecific bilateral asymmetry observed in otolith shape and size were discussed.

Genomic structural variation in Barramundi Perch Lates calcarifer and potential roles in speciation and adaptation.

Advancements in genome sequencing and assembly techniques have increased the documentation of structural variants in wild organisms. Of these variants, chromosomal inversions are especially prominent due to their large size and active recombination suppression between alternative homokaryotypes. This suppression enables the 2 forms of the inversion to be maintained and allows the preservation of locally adapted alleles. The Barramundi Perch (BP; Lates calcarifer) is a widespread species complex with 3 main genetic lineages located in the biogeographic regions of Australia and New Guinea (AUS + NG), Southeast Asia (SEA), and the Indian Subcontinent (IND). BP are typically considered to be a protandrous sequential hermaphrodite species that exhibits catadromy. Freshwater occupancy and intraspecific variation in life history (e.g. partially migratory populations) exist and provide opportunities for strongly divergent selection associated with, for example, salinity tolerance, swimming ability, and marine dispersal. Herein, we utilize genomic data generated from all 3 genetic lineages to identify and describe 3 polymorphic candidate chromosomal inversions. These candidate chromosomal inversions appear to be fixed for ancestral variants in the IND lineage and for inverted versions in the AUS + NG lineage and exhibit variation in all 3 inversions in the SEA lineage. BP have a diverse portfolio of life history options that includes migratory strategy as well as sexual system (i.e. hermaphroditism and gonochorism). We propose that the some of the life history variabilities observed in BP may be linked to inversions and, in doing so, we present genetic data that might be useful in enhancing aquaculture production and population management.

Cell-Death Metabolites from Cocconeis scutellum var. parva Identified by Integrating Bioactivity-Based Fractionation and Non-Targeted Metabolomic Approaches.

Epiphytic diatoms growing in Mediterranean seagrass meadows, particularly those of the genus Cocconeis, are abundant and ecologically significant, even in naturally acidified environments. One intriguing aspect of some benthic diatoms is their production of an unidentified cell-death-promoting compound, which induces destruction of the androgenic gland in Hippolyte inermis Leach, 1816, a shrimp exhibiting protandric hermaphroditism, principally under normal environmental pH levels. The consumption of Cocconeis spp. by this shrimp is vital for maintaining the stability of its natural populations. Although many attempts have been made to reveal the identity of the apoptotic compound, it is still unknown. In this study, we strategically integrated a bioactivity-based fractionation, a metabolomic approach, and two different experimental avenues to identify potential apoptotic metabolites from Cocconeis scutellum var. parva responsible for the sex reversal in H. inermis. Our integrated analysis uncovered two potential candidate metabolites, one putatively identified as a lysophosphatidylglycerol (LPG) (16:1) and the other classified as a fatty acid ester. This is the first time LPG (16:1) has been reported in C. scutellum var. parva and associated with cell-death processes. These candidate metabolites mark substantial progress in elucidating the factors responsible for triggering the removal of the androgenic gland in the early post-larval phases of H. inermis.

First Report of Glauconome chinensis (Gray, 1828) from West Bengal-Odisha Coast, India

The intertidal marine bivalve Glauconome chinensis (Chinese glauconome), Gray 1828 is a burrowing benthic fauna inhabiting in soft muddy to sandy sediments of Western pacific mangrove areas. The specimen is characterised by long siphon (inhalant and exhalent) for feeding and a prominent foot for locomotion. They are mostly gonochoric, few are sometimes protandric hermaphrodites. According to Gray’s description their embryonic development starts through free swimming trochophore larvae and succeeded by the veliger. Glauconome chinensis has been rediscovered for the first time from the intertidal coastal mangrove areas of Talsari, confluence of Subarnarekha estuary of West Bengal-Odisha coast, India. Quadrates having areas of 1m² were placed randomly on study sites and the studied specimens were unearthed, counted their size, biomas, population density and distribution pattern. Physico-chemical parameters were also evaluated. For anatomical study tissues were fixed in 4% formalin solution and processed through alcoholic grades. Body length, siphon, gill, mantle, foot, adductor muscles, digestive gland and gonads were examined. The studied specimens were stayed in 10-40cm depth from sediment surface and showed clumped distribution pattern. The mean length, width and biomass of the specimen were 39.2(mm) ±0.393, 14.5(mm) ±0.351and 3.89(gm) ±0.382 respectively. The report on the occurrence of such intertidal bivalve adds relevant data for the good assessment of food chain and food web of marine coastal mangrove ecosystem. The present study also aims to gather knowledge of valuable ecological services of Glauconome chinensis and to maintain the biological integrity of West Bengal-Odisha coastal region with Bay of Bengal.

The complete genome sequence of the protogynous hermaphrodite North American annual killifish Millerichthys robustus (Rivulidae, Cyprinodontiformes).

The Mexican Rivulus, Millerichthys robustus, is an enigmatic species of seasonal killifish endemic of the Southeast Mexico that has changed paradigms on the evolution of annualism in killifishes. This species survives in ephemeral environments that experience a period of seasonal drought that causes the death of all adult fish. However, populations persist due to their drought-resistant embryos capable of arresting their development in diapause until the next the rainy season. Millerichthys evolved unique characteristics within annual killifishes as a functional sequential hermaphroditism, in which females change to males (protogynous) under perceived conditions of mate competition. Also, M. robustus express different color phenotypes in both sexes: five-color phenotypes continuously distributed in various perceptual units between yellow and red in males, and different number of ocelli disposed in caudal peduncle in females. The phylogenetic relationships of M. robustus revealed that it is a sister clade to two non-annual species found exclusively in Cuba (Rivulus cylindraceus and Rivulus berovidesi), indicating that the annual life cycle, through the acquisition of embryonic diapause, has evolved independently in this species. Here, we present the complete genome sequences for the North American annual killifish Millerichthys robustus. The raw data and assembled genome are available in GeneBank.

Sex‐specific gene expression in eastern oyster, Crassostrea virginica, gonad and mantle tissues

AbstractThe eastern oyster (Crassostrea virginica) is a protandrous hermaphrodite of commercial importance. As with many marine invertebrates, little is known about sex determination and differentiation systems in this species. Such knowledge has important implications not only for understanding the evolution of sex but also for applied questions in aquaculture. In order to examine mechanistic differences in reproductive development between the sexes, we compared the transcriptomes of gonad and mantle tissues from six male and six female oysters. A total of 7675 transcripts were differentially expressed between male and female gonads (3936 and 3739 were upregulated in males and females, respectively). Transcripts identified include those associated with sex in other invertebrate and vertebrate species such as Dmrt1, Sox‐30, Bindin, Dpy‐30, and Histone H4 in males and Foxl2, Vitellogenin, and Bystin in females. GO terms associated with transcripts upregulated in male gonads include protein modification, reproductive process, and cell projection organization, whereas RNA metabolic process and amino acid metabolic process were associated with transcripts upregulated in females. Far fewer transcripts were differentially expressed between male and female mantle tissues, with 87 transcripts upregulated in females and 16 upregulated in males. However, 41% of transcripts identified as differentially expressed between mantle tissues were also differentially expressed between male and female gonads including Histone H4 and Bystin. This study represents the first characterization of eastern oyster male and female gonad transcriptomes. We further identify differing expression profiles between male and female mantle tissues, which provides evidence for sex‐specific functions of the mantle and suggests that this tissue could harbor biomarkers for identifying oyster sex non‐destructively.

On reproduction in the Pacific boreal limpet Erginus (Problacmaea) puniceus Lindberg, 1988 (Patellogastropoda: Rhodopetalidae)

ABSTRACT Fifty years ago, it was reported that individuals of the Pacific boreal limpet Problacmaea (= Erginus) bear a penis for copulation and brood their young in the pallial cavity. These observations were based on light microscopy, but now we reveal new details of the reproductive biology of Erginus (Problacmaea) puniceus with electron microscopy. Gametogenesis is fundamentally similar to other Patellogastropoda, but there are some key differences. Oocytes develop in the ovary, dorsal to the foot in the posterior half of the body, before passing through the oviduct to the pallial cavity, where they are fertilized and brooded. Development is direct with embryos being brooded to the crawl-away juvenile stage. However, sometimes fertilization occurs internally with embryos beginning development in or near the gonad. Free sperm, which were likely autosperm (self) but could have been allosperm (cross), were found near developing oocytes in the gonad. Spermiogenesis results in the formation of an enta-quasperm with a cap-like acrosome, a bullet-shaped nucleus and spherical mitochondria in the midpiece. Copulation was not observed, but we confirm the presence of a penis with a dorsal penial groove that lies below the right cephalic tentacle in males, which is typical of functional penises of some molluscs. Males were always smaller, but intermediate-sized individuals were hermaphroditic to different extents. The largest individuals were entirely female, and their penises were smaller and often lacked the penial groove. Thus, E. (Problacmaea) puniceus appears to be a protandrous hermaphrodite, passing from male to female phase. Remnants of the testis persist into the female phase, suggesting that they are protandrous hermaphrodites with overlap, possibly allowing for self-fertilization to occur on occasion. However, the largest females lacked any testis remnants, suggesting instead that the species may be protandrous sequential hermaphrodites.

Evidence for selfing in a vertebrate from whole-genome sequencing.

A growing number of recent genomic studies report asexual parthenogenetic reproduction in a wide range of taxa, including vertebrate species from the reptile, bird, and fish lineages. Yet, self-fertilization (selfing) has been recorded only in a single vertebrate, the mangrove killifish Kryptolebias marmoratus In cichlid fishes, sex determination is notably diverse and can be influenced by the environment, and sequential hermaphroditism has been reported for some species. Here, we present evidence for a case of facultative selfing in the cichlid fish Benitochromis nigrodorsalis, which is otherwise known as biparentally reproducing ovophilic mouthbrooder from Western Africa. Our laboratory observations revealed that a wild-caught individual produced repeatedly viable offspring in absence of a mating partner. By analyzing genome-wide single-nucleotide polymorphism (SNP) data, we compare that individual and two of its offspring to shed light on its reproductive mode. First, our results confirm uniparental reproduction. Second, overall heterozygosity is reduced in the offspring compared with outbred individuals. Retained maternal heterozygosity in the offspring is ∼51%, which is close to the theoretically expected value of a heterozygosity reduction of 50% by selfing. Heterozygosity patterns along individual chromosomes do not point to alternative parthenogenetic reproductive mechanisms like automixis by terminal or central fusion. Facultative selfing may represent an adaptive strategy ensuring reproduction when mating partners are absent and, hence, contribute to the cichlids' enormous evolutionary success.

Correction to: Comparative demography of three black seabreams found in the Ryukyu Archipelago: implication for the definition of protandrous hermaphrodites

Correction to: Comparative demography of three black seabreams found in the Ryukyu Archipelago: implication for the definition of protandrous hermaphrodites

Transcriptomic signatures associated with underlying rapid changes in the early phase brain of bi-directional sex change in Trimma okinawae.

Teleost fish exhibit remarkable sexual plasticity and divergent developmental systems, including sequential hermaphroditism. One of the more fascinating models of sexual plasticity is socially controlled sex change, which is often observed in coral reef fish. The Okinawa rubble goby, Trimma okinawae, is a bi-directional sex-changing fish. It can rapidly change sex in either direction based on social circumstances. Although behavioural and neuroendocrine sex change occurs immediately and is believed to trigger gonadal changes, the underlying mechanisms remain poorly understood. In this study, we conducted a de novo transcriptome analysis of the T. okinawae brain and identified genes that are differentially expressed between the sexes and genes that were immediately controlled by social stimulation implicating sex change. Several genes showed concordant expression shifts regardless of the sex change direction and were associated with histone modification in nerve cells. These genes are known to function in the neuroendocrine control of reproduction in nerve cells. Overall, we identified genes associated with the initiation of sex change, which provides insight into the regulation of sex change and sexual plasticity.

Sexual antagonism in sequential hermaphrodites.

Females and males may have distinct phenotypic optima, but share essentially the same complement of genes, potentially leading to trade-offs between attaining high fitness through female versus male reproductive success. Such sexual antagonism may be particularly acute in hermaphrodites, where both reproductive strategies are housed within a single individual. While previous models have focused on simultaneous hermaphroditism, we lack theory for how sexual antagonism may play out under sequential hermaphroditism, which has the additional complexities of age-structure. Here, we develop a formal theory of sexual antagonism in sequential hermaphrodites. First, we construct a general theoretical overview of the problem, then consider different types of sexually antagonistic and life-history trade-offs, under different modes of genetic inheritance (autosomal or cytoplasmic), and different forms of sequential hermaphroditism (protogynous, protoandrous or bidirectional). Finally, we provide a concrete illustration of these general patterns by developing a two-stage two-sex model, which yields conditions for both invasion of sexually antagonistic alleles and maintenance of sexually antagonistic polymorphisms.

Reproductive characteristics of the hermaphroditic four-finger threadfin, Eleutheronema tetradactylum (Shaw, 1804), in tropical coastal waters

This study investigated the reproductive traits of the hermaphroditic four-finger threadfin, Eleutheronema tetradactylum, along the coasts of Thailand during January to December 2021. Fish samples were collected from Pattani Bay, Thailand to assess the sex ratio, gonadosomatic index (GSI), maturity stage and fecundity. Additional fish samples were also collected from other areas to evaluate the length and weight at first sex change (Ls50 and Ws50) and length at first maturity (Lm50). The overall sex ratio for male and female was 1:0.69 with male being predominant throughout the year. Threadfin fish spawn the whole year round with peaks during moderate rainy and heavy rainy seasons. Histological examination confirmed its protandrous hermaphrodite posing multiple spawning habits. The average fecundity was 1.85 × 105 ± 1.05 × 105 eggs and positively related with standard length, body weight, gonad weight, and egg diameter (p < 0.05). The Ls50 and Ws50 were 27.58 cm and 419.39 g, and 29.71 cm and 457.28 g, for fish from Pattani Bay and Samut Prakan province, respectively. The Lm50 of male from Pattani Bay and Samut Prakan province were 25.78 cm and 25.56 cm, respectively, which were larger than those from Satun and Nakhon Sri Thammarat provinces. The Lm50 of females from Pattani Bay was smaller than that from Samut Prakan province. This study provided fundamental information on the reproductive characteristics of E. tetradactylum, which can be implemented to support management of natural fish stock and aquaculture development.

Induced Sex Reversal in Adult Males of the Protandric Hermaphrodite Centropomus undecimalis Using 17 β-Estradiol: Enhancing Management Strategies for Captive Broodstock

The common snook (Centropomus undecimalis) is a protandric hermaphrodite fish that undergoes a sex change during its life cycle. In nature, common snook females develop directly from males shortly after spawning. However, the factors triggering this process remain unknown. This knowledge gap poses challenges for managing the species in captivity. To address this, we conducted a study on sex change induction in three-year-old males using estradiol and evaluated the potential effects of photoperiod manipulation on early maturation. Four treatment groups were employed: (1) fish with estradiol + natural photoperiod; (2) fish without estradiol + natural photoperiod; (3) fish without estradiol + controlled photoperiod; and (4) fish with estradiol + controlled photoperiod. The effectiveness of these treatments was assessed through histological procedures, which allowed for the examination of the fishes’ gonads. Furthermore, the concentration of alkali labile phosphorus in fish plasma was measured and correlated with the histological results. Our findings revealed that administering 2 mg/kg estradiol implants resulted in a remarkable 100% female population within the estradiol-treated groups. No significant effect on fish maturation was observed due to the manipulated photoperiod conditions. This protocol offers improved management strategies for captive broodstock. Firstly, the concentration of estradiol used in this study proved sufficient to induce sex change in this hermaphroditic species, enabling the production of viable females at an early age and smaller size and facilitating easier broodstock manipulation. Secondly, the implementation of the alkali labile phosphorus technique allows for sex identification without the need to sacrifice the fish. In conclusion, our study provides valuable insights into sex change induction and photoperiod manipulation in common snook. The findings contribute to enhanced management practices for captive broodstock. However, further research is needed to explore the underlying mechanisms triggering sex change and to optimize protocols for long-term maintenance and successful reproduction in captivity.

Genome-wide identification and expression analysis of Dmrt genes in bivalves

In recent years, some common themes in the development of sex-specific traits in different animal lineages have started to emerge since the discovery of the Dmrt (doublesex-mab3-related transcription factor gene) genes. Bivalves are characterized by a diversity of sexual systems, including simultaneous hermaphroditism, sequential hermaphroditism, and strict gonochorism. However, to date, no research has focused on the genome-wide characterization and analysis of Dmrt genes in bivalves. In this study, the identification and analysis of Dmrt genes in 15 bivalves were performed using bioinformatics methods. A total of 55 Dmrt genes were retrieved in the studied bivalve genomes. The number of Dmrt genes in different species ranged from 3 to 5. The phylogenetic tree showed that Dmrt genes in bivalves can be subdivided into 5 classes: the Dmrt2-like class, Dmrt3-like class, Dmrt4/5-like class, Dsx-like class, and scallop-specific Dmrt class. The Ka/Ks ratios suggested that all Dmrt classes underwent purifying selection pressure. Furthermore, the spatiotemporal expression of Dmrt genes in four bivalve species suggested that different Dmrt genes may have different functions, and scallop-specific Dmrt genes may play a key role in sex determination/differentiation. In general, this study provides a molecular basis for in-depth examination of the functions of Dmrt genes and phylogenomic analyses in bivalves.

Sexual and mating system inThor hainanensis(Decapoda: Natantia: Thoridae) from the Lakshadweep Archipelago, India

AbstractWe tested the hypothesis that the Hainan’s squat shrimp Thor hainanensisXu &amp; Li, 2014 undergoes a sex change and displays protandric hermaphroditism, a well-known feature from other congeners (T. dicaprioAnker &amp; Baeza, 2021 and T. amboinensis (De Man, 1888)). We used morphological traits indicative of male function to categorize shrimp as prehensile males, non-prehensile males, and females. The linear relationship among the different morphological characters and the sexual morphs revealed that T. hainanensis has a sexual system of partial protandric hermaphroditism, where 50% of the population stays as active males and the rest of the population changes to female. Observations on the social and mating behaviour suggest that T. hainanensis exhibits a ‘search and defend’ mating system unlike other tropical caridean shrimps featuring protandrous hermaphroditism.

Sexual Biology and Mating Behaviour in Decapod Crustaceans: A Case Study with Coral Reef Dwelling Caridean Shrimps

Crustaceans have diversified sexual systems, incorporating various reproductive modalities such as gonochorism and a wide variety of hermaphroditism. In particular, the coral reef- dwelling caridean shrimps, living in symbiosis with other marine invertebrates, have developed unusual sexual systems that have given origin to a range of social systems, including the highly advanced eusociality. Protandric hermaphroditism, common among the caridean shrimps, has also reached a unique sexual condition, known as protandric simultaneous hermaphroditism, in the genus, Lysmata. Similarly, mating systems in the caridean shrimps are highly diverse and divergent. In addition, their occurrence in the coral reef shrimps is correlated to their life style adaptations such as their commensal association with other invertebrates. Other ecological factors like the predation risk from outside of the host animals and differences in the host characteristics may also be the driving forces in determining mating associations. Furthermore, the transition from sexual monogamy to social monogamy under conditions of their symbiotic life led to complex community living in several synalpheid shrimps. Understandably, the existence of various sexual and mating systems in these caridean shrimps has a dependence on the eco- social conditions prevalent in the coral reef biota.

Integrative study of the reproductive biology and growth of Acestrorhynchus pantaneiro Menezes, 1992 (Characiformes, Acestrorhynchidae).

We studied the reproductive strategy, sexual system and growth of dientudo paraguayo Acestrorhynchus pantaneiro. After 2 years of monitoring in shallow areas of a floodplain lake from the lower Paraná basin (Argentina), it was evidenced that water temperature modulated gonadal maturation, but it wasthe river water level the synchronising stimulus that triggered spawning. This species exhibited a single annual breeding period from October to January, with most spawning activity in November. According to the von Bertalanffy growth curve, fish would reach autumn to winter months with LS of ~120 mm, already mature males. The first mature females were found at LS of 210 mm, becoming sexually mature between the second and third breeding seasons. This is the first integrative study that includes the body-length frequency distribution and sex differential size at first maturity and growth, and reports the presence of intersex gonads questioning its sexualpattern from gonochoristic to sequential hermaphrodite species. The sexual pattern, the multiple spawning behaviour and a medium to high absolute fecundity support the opportunistic andinvasive behaviorobserved in previous contributions for this characiform species.

Cyp19 expression and sex change timing in captive-bred false clownfish.

The false clown anemonefish (Amphiprion ocellaris) is a protandrous hermaphrodite with a distinctive reproductive behavior. This study elucidates the genetic mechanisms and timing of sex changes in captive-bred A. ocellaris by examining the expression of key genes involved in this process, specifically cyp19a1a and cyp19a1b. Gonadal histological analyses and gene expression studies were conducted on subadult fish paired for 0, 1, 2, 3, 4, 5, and 16 months. Our findings reveal that alterations in cyp19 gene expression coincide with a pairing period starting after 3 months. Both cyp19a1a and cyp19a1b expression levels were significantly elevated in paired females compared with their male counterparts and unpaired controls. Histological investigations demonstrated that sex conversion to females occurred during the 3-month pairing period. This study highlights the crucial role of cyp19a1a and cyp19a1b in the sex change process of A. ocellaris and indicates that a minimum of 5 months of pairing is necessary for completing the sex change.

Aseasonal and short life cycles of the protandrous hermaphrodite blue threadfin (Eleutheronema tetradactylum) in a near-equatorial tropical region

Context Eleutheronema tetradactylum is exploited throughout its distribution in the Indo–West Pacific region. However, there is a lack of data on its life cycle in near-equatorial regions and the northern hemisphere. Aim To investigate the age, growth, sex change and reproduction of Eleutheronema tetradactylum in southern Thailand. Methods Analysis of length, otolith and gonad data of 449 fish. Key results Contrary to findings from regions at higher latitudes, otoliths did not show seasonal increment formation. Instead, with the exception of large individuals, daily increments were distinguishable. Within the age range of 69–341 days, growth was linear, with results indicating a total length of 430 mm (69% of maximum length) at 1 year. The youngest mature male was 137 days old. Moreover, the length at which half the individuals changed from male to female was 376 mm, corresponding to 282 days. The gonadosomatic index indicated no specific spawning season, and the hatch-date calculations indicated year-round hatching. Conclusions Eleutheronema tetradactylum inhabiting near-equatorial regions is characterised by rapid population turnover compared with populations of higher latitudes. Implications Our findings indicated life-history variability, including seasonality loss in near-equatorial habitats, of an important coastal fish. This information is important for species conservation.

Divulging the social sex change mechanism in a unique model system for studying the sexual plasticity of protogynous hermaphrodite fish, three bamboo leaf wrasse (Pseudolabrus sieboldi)

The gonadal sexual fate of vertebrates is either defined by genetics or environment, or a combination of both factors. Interestingly, in sequential hermaphroditism, the animal can undergo natural sex changes from female-to-male, male-to-female, and bidirectional way throughout their lives. This change exhibits the process which shifts between oogenesis and spermatogenesis and is regarded as an ideal instance of sexual plasticity. To develop the experimental model for studying the sexual plasticity of protogynous fish, the social conditions that induce sex changes were defined in wrasse, Pseudolabrus sieboldi. When six females were kept together in a tank, the largest female became a male, whereas a similar conversion did not occur when only two females were present in a tank. A semi-gonadectomy analysis developed in the present study verified the direct relationship between gonadal sex and body coloration. In P. sieboldi, the sex change is controlled by the relative body size of an individual within a group, rather than by absolute body size. When six females were kept in smaller sized tanks, delayed sex change or unchanged individuals was observed. Overall, more than 90% of the largest females demonstrated sex change after being housed with five smaller females in different sizes of tanks ranging from 80 to 500 L. Furthermore, the experiment using a transparent barrier suggested that visual stimuli are one of the major cues to initiate sex change. Our findings on the laboratory conditions leading to the initiation of sex change in wrasse suggest the usefulness of this species as a model organism for comparative studies in molecular, cellular, and physiological mechanisms of sexual plasticity, as well as on social and reproductive behaviors.