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 fertilize 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.
Modeling sex change
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.
Large males that are in terminal color phase will defend breeding sites where females migrate to on a daily basis. A study was done to estimate the relative roles of each sex in choosing the location of such sites. All terminal-phase males or all females were replaced in local isolated populations, and the resulting site use was monitored. After males were replaced, the mating system was not affected. On the other hand, after the females were replaced, half of the old sites were lost and the same number of new sites came into use, and continued to be occupied for over a year after these manipulations. This occurred although large males originally continued to defend and display at the original sites. Therefore, this shows the importance of female choice in the feeding system of the blue-headed wrasse, and that males will respond to the females’ site preferences. 
Most of the literature on mating systems of the blue-headed wrasse was described in small patches of concentrated reef habitats. In a large, linear barrier reef in St. Croix, U.S. Virgin Islands, very large aggregations of group-matings form daily in a single area near the foreside of the reef. Tagging studies have shown that fish are generally faithful to particular feeding schools that are assorted throughout the forereef, and that they tend to migrate to spawning grounds over 1.5 kilometers away. There is no mating that appears to happen in other upcurrent areas of the forereef. Despite large differences in the times that are spent on the migration, there are no significant differences in the fecundity or frequency of spawning among females that live at different distances from the mating aggregation. The higher growth rate corresponded to a higher general feeding rate in the location, suggesting that food intake may outweigh the costs of the long migration. 
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