A Secret under the sea
Sexual identity of individual is determined early in life, during embryonic stages. In case of higher vertebrates like human, sex determination is strictly under genetic combination and hormonal influence during early embryonic stage. Not only sex determination is earlier event of life, once determined it is not changeable under natural circumstances. But animal world has all surprises for us. Sex reversal is a common phenomenon in some of the teleost fishes. Among them the most studied one is Thalassoma Sp. or blue headed wrasse. This is a coral reef fish living in the seas of Kaneohe Bay, Oahu, Hawaii.
These fishes can live with sexual flexibility for their entire lifetime. Typically a shoal of fish contains one terminal phase (TP) male and few initial phase (IP) male and female. TP male is the guide of the shoal. It enjoys reproductive priority, bright coloration and it is responsible for defense and safety of the shoal. IP males and females are look alike, though they have gonadal and other molecular (like hormone, gene regulation) differences. When a shoal is devoid of TP male, an IP male or female can take over its role by going through all possible biological and behavioral changes. IP male gets a brighter color pattern, reduced testes size and changes in genetic function. The female changes are more dramatic. An IP female does not have any testicular tissue before sex reversal but, during sex reversal they completely degenerate female gonad and develop functional testes. According to a school of scientists this sex reversal of female is initiated by visual cues. They can ‘sense’ that a shoal is devoid of TP male and is full of under sized IP male or females. Under such circumstances the largest female changes its sexual identity. Some scientists say cues could be hormone (estrogen) also. Once converted in to terminal male no more change is allowed!!
Reference: 1. Terminal Phase Males Stimulate Ovarian Function and Inhibit Sex Change in the Protogynous Wrasse Thalassoma duperrey; Morrey et al; ZOOLOGICAL SCIENCE 103–109 (2002)
2. Female Mimicry by Sneaker Males Has a Transcriptomic Signature in Both the Brain and the Gonad in a Sex-Changing Fish; Erica V. Todd et al; Molecular Biology and Evolution; doi:10.1093/molbev/msx293
3. Estrogenic Control of Behavioral Sex Change in the Bluehead Wrasse, Thalassoma bifasciatum; K. Erica Marsh-Hunkin et al; Integrative and Comparative Biology; September 14, 2013