Differing Mechanisms Underlie Sexual Size-Dimorphism in Two Populations of a Sex-Changing Fish

Abstract

Variability in the density of groups within a patchy environment lead to differences in interaction rates, growth dynamics and social organization. In protogynous hermaphrodites there are hypothesised trade-offs among sex-specific growth, reproductive output and mortality. When differences in density lead to changes to social organization the link between growth and the timing of sex-change is predicted to change. The present study explores this prediction by comparing the social organisation and sex-specific growth of two populations of a protogynous tropical wrasse, Halichoeres miniatus, which differ in density. At a low density population a strict harem structure was found, where males maintained a tight monopoly of access and spawning rights to females. In contrast, at a high density population a loosely organised system prevailed, where females could move throughout multiple male territories. Otolith microstructure revealed the species to be annual and deposit an otolith check associated with sex-change. Growth trajectories suggested that individuals that later became males in both populations underwent a growth acceleration at sex-change. Moreover, in the high density population, individuals that later became males were those individuals that had the largest otolith size at hatching and consistently deposited larger increments throughout early larval, juvenile and female life. This study demonstrates that previous growth history and growth rate changes associated with sex change can be responsible for the sexual dimorphism typically found in sex-changing species, and that the relative importance of these may be socially constrained.

Phenotypic plasticity in sex allocation for a simultaneously hermaphroditic coral reef fish

“…Density can influence sexual selection and the social environment and lead to variation in growth and reproductive investment patterns as has been illustrated in several protogynous coral reef fishes (Warner 1984b;Munday et al 2006;Walker and McCormick 2009;McCormick et al 2010). For instance, in the bluehead wrasse Thallasoma bifasciatum, reproductive success among competing males is more evenly spread on large, high-density reefs than on small reefs, where only a few large, dominant males gained most of the matings with females (Warner 1984b).…”

Gender-specific benefits of eating eggs at resident reef fish spawning aggregation sites

“…Given the demonstrated beneficial effects of advantages in early development to subsequent developmental trajectories, growth, performance, fitness and survival (e.g. Gagliano et al 2007, Walker et al 2007, McCormick et al 2010, it is likely that egg predators feeding at FSASs will make a greater contribution to the next generation for their species at the cost of the species aggregating to spawn. If this is the case, then the periodicity in spawning intensity at spawning sites (and hence egg predation) may influence the periodicity of recruitment of egg predator juveniles through enhanced larval survival.…”

Mass spawning aggregation of the giant bumphead parrotfish Bolbometopon muricatum

“…() describe B. muricatum at Wake Island as a ‘lek polygyny mating system’, where males defend small territories equidistantly 4–6 m apart and females pursue males in pair‐spawning. The authors note that at higher population densities such as in Palau, group spawning may be the dominant mode of spawning in B. muricatum , as is common in labroids (McCormick et al, ). Such dual spawning strategies have been observed in epinephelines, where isolated pair‐spawning has been hypothesized to become increasingly common following heavy fishing pressure on spawning aggregations (Johannes et al, ).…”