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A sobering view of a Two-toed Sloth as it makes its way along utility lines on our way to Monteverde Preserve. This is what can happen to animals faced with disappearing habitat.
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Fish Sex in the Coral Reef
ÒIn the lonely hearts club of coral reef fish, when the going gets tough, the tough change sex.Ó (Steinberg 1983)
Introduction
The discussion of sexuality among organisms is often limited to gonochorism, simple male and female reproduction that remains constant throughout a lifetime. However, hermaphroditism occurs in many organisms, including fish. The advantages, mechanisms and results of sex reversal are not fully understood but much research has given clues to this complex subject.
Types of sexuality
Sex determination occurs based on genetic and hormonal controls occurring at birth or even during the embryonic stage of development. It determines the original gonad function, female or male, as well as primary and secondary characteristics associated with the sexuality. Sex differentiation is the process of gonad development of complete sex reversal or maturation after original sex has been determined (Devlin 2002). Maturation is the activation of gonadal function from an immature to mature organism. Sex reversal is the process of a mature organism completely reversing the sex and becoming a mature member of the opposite sex.
There are two types of hermaphroditism, both of which are seen in fish. Synchronous hermaphrodites possess both male and female reproductive organs simultaneously but do not self fertilize and are rare among fish. Sequential hermaphrodites have both male and female reproductive organs but never at the same time and must undergo sex reversal to become the opposite sex (Ho 2002). Gonochorism occurs when all the individuals of a species function as one sex (male or female) throughout their lives and do not change. Most vertebrates have this type of sexuality.
Fishes can show different kinds of functional sequential hermaphroditism. Protogynous animals are a fully functioning female first then change into a functioning male. This type of sex change is most common. Protandrous animals are male first then become female. Both types of sex reversal are permanent. Bidirectional sex change also occurs among fish and is not permanent. In this case, organisms change fluently from male to female and back (Asoh 2004).
Sex determination
In most vertebrates, sex is genetically determined by the presence of sex chromosomes and the maturation schedule occurs on a relatively set time scale. However, research suggests that many fish often do not have sex chromosomes; instead sexuality is determined by multiple sites on autosomal chromosomes (Hobbs 2004). The Saddleback Wrasse, Thalassoma dupery, has no detectable sex chromosome but when it reaches sexual maturity it is exclusively produces either sperm or eggs. Sex is probably determined by multiple sites on different chromosomes (Steinberg 1983). This supports the theory that activation of specific genes on multiple chromosomes is easier and perhaps more energy efficient than activation of an entire chromosome when determining sex. Thus sexual development is more flexible and can be influenced by environmental conditions more efficiently.
Sex change due to size
Size is one social indication of dominance. One study showed that when three Thalassoma d. females are placed together, the largest of the three fish would change to a male. The other smaller two fish remained female. If the smaller two fish were male, the large female fish would change to male. As a result of size determination of sex, larger fish are usually male. According to Milton Diamond of the University of Hawaii in Honolulu, if the ratio of large males to small females changes, then a large portion of females will change sex to ensures more mating selection. Thus Òthe social situation of these animals determines their sexual physiology and behavior.Ó (Steinberg 1983) The presence of smaller individuals in a population promotes sex reversal in large females while inhibiting sex reversal in larger males (Larson2003).
Advantages of sex reversal
Coral reef fish sexuality determination and differentiation is often controlled based on changing social conditions. Changing social status can lead to opportunities to ascend in the social ladder. This requires change in behavior and physiology in both subordinate males and females which change into males (Hobbs 2004). Protogyny is favored over gonochorism when conditions are favorable for young, relativity small females and for few dominate males that are older and larger. This occurs most often in polygynous mating systems in which a few large males dominate mating opportunities and exclude smaller males from reproduction (Asoh 2004). Being born for successful reproduction means being born a female. Thus reproductive investment is status dependent with the more dominate members investing more energy into reproduction than subordinate members. Because decisions about when to mature are based on social conditions, individuals in isolation, do not show sexual maturation since there are a lack of social cues (Hobbs2004).
Endocrinology of sex reversal
Because sex reversal among fish is based largely on social events, the presence of relative size and sex of other fish is a visual cue and not pheromonal or tactile (Larson 2003). Visual stimulation must be transmitted into a chemical pathway to alter physiological changes. A study done by Larson et. al. (2003) studied Thalassoma d. to better understand how certain hormone levels changed in relation to visual stimulus. Many hormones are involved in the process of facilitating sex reversal.
Serotonin is a common hormone among most vertebrates influencing aggression in social interaction resulting in dominance hierarchies (Winberg et al., 1997). Research on damselfish indicated that social stress influences levels of serotonin. The subordinate individuals have lower serotonin levels than dominant, more aggressive males. High levels of serotonin is shown to inhibit sex reversal (Winberg and Nilsson 1993).
Androgen is a hormone secreted from the gonadotropins (GtH) in the pituitary glands. It controls development of primary masculine sex characteristics. Addition of Androgen to female fish such as the Bluehead Wrasse (Thalassoma bifasciatum) and the Stoplight Parrotfish (Sparisoma viride) resulted in sex reversal. One study by Tan-Fermin (1992) showed that the withdrawal of androgens in male Groupers, Epinephelus suillus, reverted back to female (Larson 2003).
Dopamine is another important hormone in sexuality as well as many other bodily functions including endocrine regulation. Dopamine has been found to regulate the levels of Gonadotropin-releasing hormone, GnRH and GtH in blood serum in Goldfish Carassium auratus (Chang and Peter, 1983a; Peter et al. 1986). Blocking dopamine seems to be the most effective way of initiating sex reversal because it indirectly controls the release of androgens through GtH. Norepinephrine (NE) influences sex reversal by also affecting the HPG axis which also stimulates GnRH, releasing other sex hormones (Larson 2003).
GnRH and GtH function together to control hormones that control sex reversal. GnRH is released from the hypothalamus and travels to the pituitary glands where it releases GtH. The presence of GtH controls the release of androgens, Follicle Stimulating Hormone, FSH, and Luteinizing Hormone, LH. GtH can induce sex reversal alone by the release of androgens while GnRH stimulation of sex reversal occurs only with presence of dopamine. The release of FSH affects some species of fish during different stages of life while in other species; there is no effect at all (Vacher 2000).
Research suggests that mRNA expression changes during sex reversal to express different genes. The control of mRNA is based on hormonal control stimulating a cascade pathway inside sex cells. Different expression of genes through translation of mRNA gives rise to different proteins leading to different gonad functions (Ruggeri 2007), thus sex reversal.
In the cummulation of hormones working together, scientists can understand how visual stimulus is transferred into endocrine signals. Under stable social conditions that do not induce sex reversal, stressful influences of the dominate male with high serotonergic activity prevent sex reversal by the largest females also with high serotonin levels (Larson et al., 1999). The removal of the dominate male decreases stress in the largest female resulting in decreased serotonin allowing for an increase in norepinephrine (Blandina et al., 1991; Larson et al., 1999; Larson et al., 2003). This stimulates increased of GnRH activity (Grober and Bass, 1991) in turn stimulating gonadotropin secretion and triggering androgen production. The androgen production brings on masculine characteristics and thus sex reversal from female to male (Koulish and Kramer, 1989).
What changes when sex changes?
Changes in dominance in a population can be abrupt. Changes a male goes through to become more dominant include increased dominate social behaviors, gonadal growth, alterations in ejaculate characteristics, increased number of sperm and elevated plasma androgen concentrations (Fitspatrick, 2008).
One obvious secondary characteristic in fish is color. Color of skin strongly communicates sexual activity, stage of life, camouflage or other social signaling. Many fish attain temporary color changes during courtship or to attract mates (Skšld 2008). Androgen has been shown to be effective in promoting color change during a female to male-type sex change (Cardwell and Liley, 1991; Godwin et al., 1997; Kramer et al., 1988). NE also causes color change by direct influence on skin chromatophores (Kasukawa et al., 1985).
Human influence
Endocrine disrupters are common in man-made compounds and mimic hormones found in biotic systems such as estrogen. They are chemically similar to the natural hormone and are found in many man-made products. One such endocrine disrupter, called Oxybenzone is typically found in sunscreen. It mimics estrogen, a female hormone. Southern California Coastal Water Research Project scientists found that when Oxybenzone was dumped into the ocean with sewage off the coast of Los Angeles, it settles into the ocean floor where bottom-dwelling fish such as sole and turbot consume it. Some of the male fish reacted by growing ovary tissue and the female egg protein, vitellogenin, in their testes. These individuals were infertile. However, the presence of estrogen in the water does not cause males to become transgendered (Bowles 2005). However, during the testing, the scientists found that males and females both had the same level of estrogen in their blood even though the levels are highest in contaminated areas. It is under debate as to whether or not the levels of estrogen in males are normal or due to contamination (Renner 2009).
Conclusion
In conclusion, reproduction in hermaphroditic fish defies typical ways of thinking about sexuality. The advantages of sex reversal are clear and the hormones used to control sex determination are found in most vertebrates. A better understanding of different sexualities in the animal kingdom not only provides science with a better appreciation of reproduction in fish, it benefits management of fish populations through human impacts. These human impacts range form sewage dumping, siltation, coral bleaching, fishing practices and chemical alteration of oceanic water. Further questions to ask include; is the fertility of individual fish that underwent sex reversal better or worse than a fish that did not under go sexual reversal? What impact does the abundance of multiple small females and few large males have on the sport fishing industry? Does removal of dominate males jeopardize the fish population? With current knowledge and future research fish conservation efforts can be more successful.
Cited works
Asoh, K (2004 July 8). Gonadal development in the coral reef damselfish Dascyllus flavicaudus from Moorea, French Polynesia. Marine Biology, 146, Retrieved May 12th, 2009
Bowles, J (2005, November 15). UCR scientist finds key to sex alterations in fish. The Press-Enterprise, Retrieved May 12th, 2009, from http://www.pe.com/breakingnews/local/stories/PE_News_Local_H_fish15.cf49b98.html
Chang, J.P., Peter, R.E., 1983a. Effects of dopamine on gonadotropin release in female goldfish, Carassius auratus. Neuroendocrinology From Larson (20003) 36, 351Ð357.
Chang, J.P., Peter, R.E., 1983b. Influences of norepinephrine and aadrenergic mechanisms on gonadotropin secretion in female goldfish, Carassius auratus. Gen. Comp. Endocrinol. 55, 89Ð95. From Larson 2003)
Devlin, Robert H.; Nagahama, Yoshitaka. ÒSex determination and sex differentiation in fish: an overview of genetic, physiological, and environmental influences.Ó Aquaculture, v. 208 issue 3/4, 2002, p. 191-364.
Flanagan CA, Millar RP, Illing N (1998). "Advances in understanding gonadotrophin-releasing hormone receptor structure and ligand interactions". Rev. Reprod. 2 (2): 113Ð20. retrieved May 13, 2009
Ho, L (2002). Hermaphroditism. Retrieved May 16, 2009, from Reefscapes.net Web site: http://www.reefscapes.net/articles/articles/2002/hermaphroditism.html
Jean-Paul A. Hobbs; Philip L. Munday; Geoffrey P. Jones. ÒSocial induction of maturation and sex determination in a coral reef fish.Ó Proceedings: Biological Sciences, v. 271 issue 1553, 2004, p. 2109-2114.
Larson, Earl T.; Norris, David O.; Gordon Grau, E.; Summers, Cliff H. ÒMonoamines stimulate sex reversal in the saddleback wrasse.Ó General & Comparative Endocrinology, v. 130 issue 3, 2003, p. 289-298.
Missale, C (1998, January). Dopamine Receptors: From Structure to Function. PHYSIOLOGICAL REVIEWS, 78, Retrieved May 13th, 2009, from http://physrev.physiology.org/cgi/reprint/78/1/189
Perry, Adam N.; Grober, Matthew S. ÒA model for social control of sex change: interactions of behavior, neuropeptides, glucocorticoids, and sex steroids.Ó Hormones & Behavior, v. 43 issue 1, 2003, p. 31-38.
Renner, R (2009, January 14). Sex-changing fish: caused by contamination or nature?. Environmental Science and Technology, 43(6), Retrieved May 12th, 2009, from http://pubs.acs.org/doi/full/10.1021/es8036912
Ruggeri, B., et al. ÒChanges of gonadal CB1 cannabinoid receptor mRNA in the gilthead seabream, Sparus aurata, during sex reversal.Ó General & Comparative Endocrinology, v. 150 issue 2, 2007, p. 263-269.
Skšld, Helen Nilsson, et al. ÒHormonal regulation of female nuptial coloration in a fish.Ó Hormones & Behavior, v. 54 issue 4, 2008, p. 549-556.
Steinberg, S. ÒSex switch stimulated by size.Ó Science News, v. 124 issue 6, 1983, p. 86-86.
Tan-Fermin, J.D., 1992. Withdrawal of exogenous 17-a methyltestosterone causes reversal of sex-inversed male grouper Epinephelus suillus Bloch and Schneider. Philippine Sci. 29, 33Ð39. From Larson(2003)
Vacher, C (2000, Dec). Modulation of Pituitary Dopamine D1 or D2 Receptors and Secretion of Follicle Stimulating Hormone and Luteinizing Hormone During the Annual Reproductive Cycle of Female Rainbow Trout.. Journal of Neuroendocrinology, 12, Retrieved May 13th, 2009, from http://web.ebscohost.com.proxy.lib.muohio.edu/ehost/pdf?vid=5&hid=103&sid=7a0a469d-572c-47de-a4c7-a1888ac5a127%40sessionmgr103
Winberg, S., Myrberg Jr., A.A., Nilsson, G.E., 1997. Agonisticinteractions affect brain serotonergic activity in an acanthopterygiian fish: the bicolor damselfish Pomacentrus partitus. Brain Behav. Evol. 48, 213Ð220.
Winberg, S., Nilsson, G.E., 1993. Roles of brain monoamine neurotransmitters in agonistic behaviour and stress reactions, with particular reference to fishes. Comp. Biochem. Physiol. 106C, 597Ð614.
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