By Nayan Chouhan, Bhavesh Choudhary, Kunal Samadhan Tayde, Suvankar Rout and Rajalaxmi Dalabehera
The common name “tilapia” refers to cichlid species that are farmed all over the world and whose yearly production has been steadily increasing. The idea of sex reversal in fish species such as tilapia has become a major change in the complex field of aquaculture, revolutionizing breeding procedures and greatly increasing production. In addition to piquing experts’ interest, this phenomenon in which fish is switch from being female to male has created new opportunities for increasing the fish production.
Introduction
The common name “tilapia” refers to cichlid species that are farmed all over the world and whose yearly production has been steadily increasing. According to the FAO (2022), Nile tilapia is presently the third most widely cultivated finfish species globally. The main characteristics that this fish apart as an exceptional aquaculture species are its omnivorous feeding habit, quick growth, tolerance to a wide range of salinity, dissolved oxygen and temperatures, as well as its ease of reproduction (El-Sayed, 2019). Tilapia also accepts inert feed immediately after yolk sac absorption.
The idea of sex reversal in fish species such as tilapia has become a major change in the complex field of aquaculture, revolutionizing breeding procedures and greatly increasing production. In addition to piquing experts’ interest, this phenomenon in which fish is switch from being female to male has created new opportunities for increasing the fish production
Male tilapia develops larger and faster than female tilapia in the population. Therefore, manual sexualization, direct hormonal sex reversal hybridization, or genetic modification is the methods used to grow tilapia for monosex. Technically, it is also possible to produce only male hybrids; however, facilities for isolation are necessary to preserve the integrity of both parental lines, and partially incompatibility between two different species often results in lower seed yield.
In grow-out ponds on commercial fish farms, female tilapia is typically kept out of them to avoid overcrowding and stunting from unintended reproduction. Females can be physically removed by visually inspecting the juvenile fish’s urogenital papilla, although this method requires a lot of work. A more modern method of creating male fingerlings is called “sex reversal” or “sex inversion.” This can be accomplished by providing tilapia fry with diet that has been treated with a male hormone prior to the females’ primordial gonadal cells differentiating into ovarian tissue. Once the testes have grown enough to maintain normal levels of endogenous hormone, the dietary hormones can be stopped.
Applications in Aquaculture
Sex reversal has enormous potential to improve aquaculture methods, especially for tilapia production. Farmers are able to optimize production efficiency by controlling the sex ratio of their fish populations through the manipulation of environmental factors throughout critical phases of initial development. With this strategy, it is possible to produce all-male populations, which develop larger and faster than mixed-sex populations since they require less energy to reproduce. It seems that the YY male technology holds great promise for increasing tilapia yields and returns and embraces great potential for economic viability. Most likely, the largest potential for this technology’s widespread use is in stock enhancement.
Understanding Sex Reversal
During the induction process, an optimal dose of sex steroid is administered during a specific period to reverse the physical characteristics of a genetic female, transforming them into a male. However, the genetic male remains unchanged. Currently, there are established protocols for hormonal sex reversal in numerous species utilizing steroids (Pandian and Sheela, 1995). Understanding the genetic approach to sex manipulation for the production of all male, all female, or all sterile populations involves inducing ploidy (Lakra and Ayyappan, 2003). Two processes determine how sex is expressed:
(a) sex determination, which is typically set at fertilization time by combining sexdetermining genes from the paternal and maternal chromosomes in the developing zygote; and
(b) sex differentiation, which is the later development of the undifferentiated primordium into male or female gonads. Tilapia, a commonly farmed fish species, demonstrates notable adaptability in their sexual maturation. Tilapia, unlike mammals, can change their sex in response to environmental and social cues rather than being genetically determined.
The occurrence of sex reversal, also referred to as hermaphroditism, is most commonly observed in species such as Nile tilapia (Oreochromis niloticus) and Mozambique tilapia (O. mossambicus). This reversal is performed by adding steroids to the diet for a short length of time during the hormonal gender reversal phase. The method for generating fry for eventual sex reversal is founded on the following primary factors:
» Tilapia sex reversal must begin before the gonadal tissue of young genetic females has developed into ovaries.
» Fry production must be synchronized with the sex reversal surgery since the hormone treatment should be commenced immediately after fry harvest and the facilities will not be accessible to receive the subsequent batch of fry for another 25 to 30 days.
» It is often preferable to sex reverse uncommon batches of large numbers of fry rather than more frequent batches of fewer fry for efficient farm management.
» Based on the foregoing parameters, the suggested technique for fry production in ponds involves a 25-to-30-day cycle (including a 2–10-day turnaround time) with a single, full harvest of fry not exceeding 14 mm total length.
Techniques of Sex Reversal
Several approaches have been discovered to induce sex reversal in tilapia. Hormonal manipulation stands out as the most extensively employed approach. By injecting synthetic hormones such as 17α-methyltestosterone at early developmental stages, female tilapia can be efficiently turned into males.
This technique ensures a high degree of dependability and precision in sex control, permitting the generation of monosex populations with desirable features. Immersion treatment of androgen can also induce the reversal of sex in O. niloticus (Wassermann and Afonso, 2003). Non-steroidal compounds, such as aromatase inhibitors, have potential for production of monosex population in tilapia (Afonso et al., 2001).
Environmental Triggers also plays crucial role because the type and timing of therapies that are able to elicit sexual reversal differ substantially between species. Various environmental factors play a significant role in inducing sex reversal in tilapia. Treatments impacting sex determination and differentiation in fish include temperature, pH, density, exogenous hormone treatment, social variables, or a combination of these. Moreover, the gonad undergoes differentiation towards the male or female state within a certain time period known as the labile period (also known as the sensitive window or period) (Baroiller et al., 2009).
After the gonads have developed and sex has stabilized, temperature, hormones, and other treatments frequently lose their effectiveness (Ospina-Alvarez & Piferrer, 2008). Temperature, for instance, is a crucial influence, with specific thresholds producing sex difference. Short exposure to higher temperatures was sufficient to significantly skew the sex ratio toward males in O. niloticus (Nivelle et al., 2019). Additionally, social factors such as population density and social hierarchy can influence sex reversal, illustrating the complicated interplay between heredity and environmental cues.
Environmental sex reversal (ESR), results in a mismatch between genotypic and phenotypic sex, is well reported in various fish species and may be produced by chemical exposure. Historically, research with piscine ESR has been done out with a view to improve profitability in aquaculture or to understand the processes underlying sex determination and sexual differentiation.
Challenges and Considerations
Although sex reversal procedures have some encouraging effects, there are drawbacks and moral dilemmas with them as well. Hormonal treatments, for instance, raise concerns over their potential environmental impact and long-term implications on fish health. Furthermore, maintaining the ideal environment for sex reversal necessitates close attention to temperature, hormone dosages, and water quality, all of which need appropriate resources and experienced staff.
Despite effective use in various aquaculture species, sex reversal of fish by hormone injection modification should be conducted with caution to prevent any unwanted effects in the fish produced, to the farmers themselves and consumers, or to the environment. For example, hormone excesses or extended duration of therapy may create malformations, or even tilt sex ratios toward the non-target sex (Beardmore et al., 2001).
Conclusion
Sex reversal in tilapia represents a fascinating phenomenon that has transformed aquaculture methods globally. By leveraging the plasticity of tilapia’s sexual maturation, farmers may optimize production efficiency, enhance profitability, and contribute to global food security. However, the correct implementation of sex reversal technology demands careful consideration of environmental, ethical, and welfare problems. Sex-reversed tilapia exhibited superior growth rates compared to normal individuals due to the injection of androgens, which have both an androgenic and anabolic impact.
As scientists continue to uncover the secrets of sex determination in tilapia, the future holds enormous promise for sustainable and resilient aquaculture systems. Continued study into the mechanisms behind sex reversal in tilapia is vital for refining existing procedures. Advances in genetics and technologies show promise for finding genetic markers related with sex determination, enabling non-invasive alternatives to hormone manipulation. Moreover, interdisciplinary collaborations including biologists, geneticists, and aquaculture professionals can promote innovation and handle growing issues in tilapia production.
References and sources consulted by the author on the elaboration of this article are available under previous request to our editorial staff. Nayan Chouhan1, Bhavesh Choudhary2, Kunal Samadhan Tayde1, Suvankar Rout2 and Rajalaxmi Dalabehera2
1 Department of Aquatic Health & Environment, College of Fisheries, Central Agricultural University (Imphal), Lembucherra, Agartala 799210, Tripura, India.
2 Department of Aquaculture, College of Fisheries, Central Agricultural University (Imphal), Lembucherra, Agartala 799210, Tripura, India. Email: nayan101chouhan@gmail.com
