Thalassoma bifasciatum is a species of saltwater fish in the wrasse family (family Labridae) of order Perciformes native to the coral reefs of the Caribbean Sea. Like its relative T. amblycephalum it is commonly called "blue-headed wrasse", "blunt-headed wrasse" or "bluehead". Blueheads are small (less than 110 mm standard length) and rarely live longer than 2 years. They form large schools over the reef and feed primarily on planktonic copepods as well as small benthic crustaceans.
Like many other wrasse species, the bluehead is a protogynous sequential hermaphrodite: individuals may begin life either as males or females, but females can change sex later in life and function as males. Young/small females and males are yellow and white in color, often with black lateral stripes and occasionally dark vertical bars. This coloration is known as the Initial Phase. These individuals can rapidly alter the presence or intensity of their yellow color, stripes, and bars, and these color changes appear to correspond to behavioral changes. Large females and some males can permanently change coloration and/or sex and enter the Terminal Phase coloration, which has a blue head, black and white bars behind the head, and a green body. It is this color phase that gives the species its name.
The initial phase males ('IP males') have comparatively larger testes than the larger, terminal phase males. This enables the initial phase males to produce lots of sperm for the snatched opportunities they must take when trying to fertilise the eggs of the females in the guarded harem. Initial phase males also achieve fertilizations through participating in group spawns. These groups consist of 20–50 or more IP males that congregate at specific sites during the daily spawning period on medium and larger sized reefs. Females visit these groups to spawn and release eggs in a 'spawning rush'. IP males attempt to position themselves next to a female when she releases her eggs as this maximizes their probability of fertilizing these eggs. Releasing large numbers of sperm also increases this probability and this is thought to also help explain the large testis size observed in IP males. This type of mating competition is referred to as 'sperm competition' and is seen in many species.
The bluehead wrasse and its congener, the Hawaiian saddleback wrasse (Thalassoma duperrey), have become important models for understanding the physiological and neurobiological bases of sex change. Sex change can be induced socially in both species by making large females the largest members of social groups. Sex change in experimental pens by saddleback wrasses involves complete gonadal transformation with associated decreases in a key steroid hormones (estradiol and 11-ketotestosterone) and steroid hormone synthesizing enzymes in the gonads. Sex changing saddleback wrasses also show substantial changes in brain levels of the monoamine neurotransmitters serotonin, dopamine, and norepinephrine.
Sex change has been studied in bluehead wrasses primarily using field manipulations, where it can be induced in large females by removing dominant TP males from small reefs. Gonadotropin-releasing hormone (GnRH) neurons differ across sexual phenotypes in the hypothalamus of bluehead wrasses and also with androgen implants that induce sex change. Behavioral sex change is very rapid in bluehead wrasses under field conditions, with male-typical behaviors being observed within minutes to hours after dominant terminal phase males are removed. Interestingly, behavioral sex change occurs even in females whose gonads (ovaries) have been surgically removed prior to becoming socially dominant. Behavioral sex change is associated with increases in expression of a neuropeptide hormone termed arginine vasotocin or AVT and these increases occur regardless of whether sex changing females have gonads or not. Injections of AVT can induce sexual and aggressive behaviors in terminal phase male bluehead wrasses while injections of fluoxetine (tradename: Prozac) can reduce aggressive behaviors by terminal phase males.
- "Thalassoma bifasciatum". Integrated Taxonomic Information System. http://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=170568. Retrieved 30 January 2006.
- Froese, Rainer, and Daniel Pauly, eds. (2005). "Thalassoma bifasciatum" in FishBase. 10 2005 version.
- Godwin J., Neuroendocrinology of sexual plasticity in teleost fishes, Frontiers in Neuroendocrinology, 2010. 31(2): p. 203–216.
- Godwin, J., D. Crews, and R.R. Warner, Behavioural sex change in the absence of gonads in a coral reef fish. Proceedings of the Royal Society of London B-Biological Sciiences, 1996. 263: p. 1683–1688.
- Godwin, J., R. Sawby, R.R. Warner, D. Crews, and M.S. Grober, Hypothalamic arginine vasotocin mRNA abundance variation across sexes and with sex change in a coral reef fish. Brain Behavior and Evolution, 2000. 55: p. 77–84.
- Grober, M.S. and A.H. Bass, Neuronal correlates of sex/role change in labrid fishes: LHRH-like immunoreactivity. Brain Behavior and Evolution, 1991. 38: p. 302–312.
- Grober, M.S., I.M.D. Jackson, and A.H. Bass, Gonadal steroids affect LHRH preoptic cell number in sex/role changing fish. Journal of Neurobiology, 1991. 22: p. 734–741.
- Hourigan, T.F., M. Nakamura, Y. Nagahama, K. Yamauchi, and E.G. Grau, Histology, ultrastructure, and in vitro steroidogenesis of the testes of two male phenotypes of the protogynous fish, Thalassoma duperrey (Labridae). General and Comparative Endocrinology, 1991. 83: p. 193–217.
- Munday, P.L., P.M. Buston, and R.R. Warner, Diversity and flexibility of sex-change strategies in animals. Trends in Ecology and Evolution, 2006. 21(1): p. 89–95.
- Nakamura, M., T.F. Hourigan, K. Yamauchi, Y. Nagahama, and E.G. Grau, Histological and ultrastructural evidence for the role of gonadal steroid hormones in sex change in the protogynous wrasse Thalassoma duperrey. Environmental Biology of Fishes, 1989. 24: p. 117–136.
- Perreault, H.A., K. Semsar, and J. Godwin, Fluoxetine treatment decreases territorial aggression in a coral reef fish. Physiology & Behavior, 2003. 79: p. 719–724.
- Semsar, K., F.L.M. Kandel, and J. Godwin, Manipulations of the AVT system shift social status and related courtship and aggressive behavior in the bluehead wrasse. Hormones and Behavior, 2001. 40: p. 21–31.
- Semsar, K. and J. Godwin, Social influences on the arginine vasotocin system are independent of gonads in a sex-changing fish. Journal of Neurosciience, 2003. 23: p. 4386–4393.
- Semsar, K., H.A. Perreault, and J. Godwin, Fluoxetine-treated male wrasses exhibit low AVT expression. Brain Research, 2004. 1029: p. 141–147.
- Warner, R.R., Large mating aggregation and daily long-distance spawning migrations in the bluehead wrasse, Thalassoma bifasciatum. Environmental Biology of Fishes, 1995. 44: p. 337–345.
- Warner, R.R. and L.M. Dill, Courtship displays and coloration as indicators of safety rather than of male quality: the safety assurance hypothesis. Behavioral Ecology, 2000. 11(4): p. 444–451.
- Warner, R.R. and Robertson, D.R. 1978. Sexual patterns of the labroid fishes of the western Caribbean: 1. The wrasses (Labridae). Smithsonian Contributions to Zoology, 254, 1–27.
|This Labridae article is a stub. You can help Wikipedia by expanding it.|