| Summary: | The behavioural endocrinology of a protogynous coral reef fish, the stoplight parrotfish (Sparisoma viride, Scaridae) was investigated at Glover's Reef, Belize. Detailed behavioural observations in the field were combined with radioimmunoassay of steroids circulating in plasma (11-ketotestosterone, testosterone and 17[sub β]-estradiol) and histological examination of gonads to obtain precise correlations of behaviour, colouration and gonadal condition with endocrine status.
The size, sex and colour phase distribution, together with histological analysis suggests that some individuals of this species undergo sex change as mature adult females, while others change sex as immature individuals, becoming functional males without passing through a female phase. Furthermore, some individuals change sex and colour phase simultaneously while others retain female-like 'initial phase' (Iph) colouration and function as Iph males before acquiring 'terminal phase' (Tph) colouration.
Large Tph males defend permanent, all-purpose territories on which they pair-spawn daily with the females of a harem group. Smaller Tph males (bachelors) neither defend territories, nor spawn, but feed in groups and inhabit overlapping home-ranges.
Females also inhabit overlapping home-ranges within the confines of a Tph male's territory. They spawn with the same male every day at high tide.
Iph males are rare in this population. They spawn by releasing milt into the gamete cloud left after a pair-spawning event. Iph males also pair-spawn with females in the absence of Tph males.
Sex change is correlated with the onset of 11-ketotestosterone production, and a dramatic decrease in plasma levels of estradiol. This is the first report to show that a naturally-occurring androgen increases in plasma concentration during sex change in a protogynous marine species. Administration of 11-ketotestosterone promotes sex and colour change in adult females. Thus, 11-ketotestosterone appears to play a key role in sex and colour phase change in this species. Males that retain Iph colouration after sex change have lower levels of 11-ketotestosterone (undetectable) and higher levels of estradiol than Tph males or males with transitional colouration. This suggests that estradiol may suppress colour phase change in Iph males.
Bachelor Tph males have lower levels of testosterone and 11-ketotestosterone than territorial males. Bachelors rapidly take over experimentally vacated territories, confinning the hypothesis that they are normally excluded from suitable habitat by territorial males. One week after territory acquisition, 11-ketotestosterone and testosterone increase to levels over and above those in undisturbed territorial males, but by three weeks, androgen levels are not significantly different from those in undisturbed territorial males. Simulated territorial intrusion promoted increased androgen production in Tph males, while access to territories without neighbours did not. Thus, the pattern of androgen production seen after territory acquisition is due to interactions with neighbouring males during territory boundary re-establishment. Increased levels of androgen during territorial challenges may promote increased aggressiveness and territorial vigilance, thereby increasing the chances of successfully defending against the challenge.
These findings are discussed in light of recent theory in behavioural endocrinology. === Science, Faculty of === Zoology, Department of === Graduate
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