Most organisms reproduce in a dynamic environment, and life-history theory predicts that this can favor the evolution of strategies that capitalize on good times and avoid bad times. When offspring experience these environmental changes, fitness can depend strongly upon environmental conditions at birth and at later life stages. Consequently, fitness will be influenced by the reproductive decisions of parents (i.e., birth date effects) and developmental decisions (e.g., adaptive plasticity) of their offspring. We explored the consequences of these decisions using a highly iteroparous coral reef fish (the sixbar wrasse, Thalassoma hardwicke) and in a system where both parental and offspring environments vary with the lunar cycle. We tested the hypotheses that (1) reproductive patterns and offspring survival vary across the lunar cycle and (2) offspring exhibit adaptive plasticity in development time. We evaluated temporal variation in egg production from February to June 2017, and corresponding larval developmental histories (inferred from otolith microstructure) of successful settlers and surviving juveniles that were spawned during that same period. We documented lunar-cyclic variation in egg production (most eggs were spawned at the new moon). This pattern was at odds with the distribution of birth dates of settlers and surviving juveniles-most individuals that successfully survived to settlement and older stages were born during the full moon. Consequently, the probability of survival across the larval stage was greatest for offspring born close to the full moon, when egg production was at its lowest. Offspring also exhibited plasticity in developmental duration, adjusting their age at settlement to settle during darker portions of the lunar cycle than expected given their birth date. Offspring born near the new moon tended to be older and larger at settlement, and these traits conveyed a strong fitness advantage (i.e., a carryover effect) through to adulthood. We speculate that these effects (1) are shaped by a dynamic landscape of risk and reward determined by moonlight, which differentially influences adults and offspring, and (2) can explain the evolution of extreme iteroparity in sixbars.

Pelagic eggs and larvae of many coral reef fishes will encounter a dynamic and risky environment as they disperse between the reef and offshore habitats. Life-history theory predicts that spawning adults should synchronize their reproductive effort with specific environmental conditions that facilitate offspring survival. Favourable conditions for reproduction may be determined by local environmental conditions at the spawning site, or signalled by larger-scale environmental cues, such as the lunar cycle. Multiple cues may interact in complex ways to cause additional variation in spawning intensity. We evaluated a set of environmental variables that potentially determine temporal and spatial variation in spawning patterns of a highly iteroparous fish, the sixbar wrasse (Thalassoma hardwicke). Specifically, we monitored focal territories of terminal-phase males over a 5-month period, quantified spawning activities, and evaluated a hierarchical set of predictive models using a model selection approach (AICc). Temporal variation in spawning (and population densities at the spawning site) was most strongly associated with the lunar cycle and maximal around the new moon. Local hydrodynamic conditions and other environmental variables observable at the spawning site were less strongly correlated with temporal variation in spawning. Territory proximity to the reef edge was a strong predictor of spatial variation in spawning intensity; territories closest to the reef edge experienced more spawning. These observations suggest that females make predictable decisions about where and when they spawn. Females appear to recognize strong, persistent spatial gradients in spawning habitat quality, and primarily vary their spawning effort in accordance with a large-scale environmental cue (the lunar cycle).

<p>Life-history theory suggests that an organism must balance its available energy between two competing physiological processes to maximize fitness: reproduction and somatic growth. Energetic trade-offs are a fundamental concept of life history theory and form the basis of intra- and inter-specific variation in life-history strategies. In fishes, reproduction-growth trade-offs are an essential component of life-history optimization. This is particularly true for species with protogynous sex- change (the most common reproductive mode among coral reef fish species), where reproductive success rapidly and disproportionally increases with body size/ corresponding social status. In such systems, lifetime fitness is inherently linked to patterns of growth and energy allocation strategies determined by an individual’s size-specific rank within the dominance hierarchy. However, energy allocation strategies in a protogynous species may not only be a function of body size. Coral reef fish species are exposed to extremely variable environmental conditions and this can favour the evolution of strategies that utilize good times and avoid disadvantageous times for reproduction. Consequently, size- specific parental investment decisions may vary greatly in time and space according to environmental cues. My thesis focuses on the protogynous reef fish, Thalassoma hardwicke (the sixbar wrasse), which is extremely abundant on shallow coral reefs throughout the Indo-Pacific region. Specifically, I evaluate patterns of spawning and reproductive investment as a function of body size, social status, lunar phase and other environmental parameters. I address the question of whether females/males of differing size make different fitness-related decisions when away from spawning sites, and I evaluate context-dependency in these decisions. Finally, I will attempt to reconstruct the developmental histories (e.g., larval growth rates) of individuals from otoliths to evaluate potential relationships between developmental histories and fitness attributes.</p>

Coral reef fish species experience variable environments and may employ a range of reproductive strategies to help them maximize fitness. Here, we explore patterns of variation in size- and sex-dependent spawning strategies (pair spawning, group spawning, and streaking tactics) of a sex-changing, highly iteroparous fish (the sixbar wrasse Thalassoma hardwicke) across the lunar month, and in relation to proximity to the reef edge. We test the hypothesis that adults vary their investment strategies as a function of body size. We evaluated reproductive behaviour patterns at 2–3 days intervals over 5 consecutive months, and found that frequencies of pair spawning, group spawning, and streaking attempts all significantly increased around the new moon and were greatest in the most exposed territories (i.e., those located closest to the reef edge). Smaller females and IP males largely curtailed reproductive activities during the full moon and third quarter moon, while larger individuals reproduced more consistently across the lunar month. Size-dependent reproductive decisions appeared to mediate frequencies of specific reproductive tactics (e.g., prevalence of pair spawning, group spawning, and streaking strategies). We speculate that (1) optimal tactics are shaped by size- and sex-dependent trade-offs between reproduction and growth in relation to environmental conditions, and that (2) individual investment decisions determine the prevailing reproductive mode at a spawning site. Variation in female choice might thereby act as a mechanism for maintaining these two reproductive modes (pair spawning and group spawning) in a population.

<p>Life-history theory suggests that an organism must balance its available energy between two competing physiological processes to maximize fitness: reproduction and somatic growth. Energetic trade-offs are a fundamental concept of life history theory and form the basis of intra- and inter-specific variation in life-history strategies. In fishes, reproduction-growth trade-offs are an essential component of life-history optimization. This is particularly true for species with protogynous sex- change (the most common reproductive mode among coral reef fish species), where reproductive success rapidly and disproportionally increases with body size/ corresponding social status. In such systems, lifetime fitness is inherently linked to patterns of growth and energy allocation strategies determined by an individual’s size-specific rank within the dominance hierarchy. However, energy allocation strategies in a protogynous species may not only be a function of body size. Coral reef fish species are exposed to extremely variable environmental conditions and this can favour the evolution of strategies that utilize good times and avoid disadvantageous times for reproduction. Consequently, size- specific parental investment decisions may vary greatly in time and space according to environmental cues. My thesis focuses on the protogynous reef fish, Thalassoma hardwicke (the sixbar wrasse), which is extremely abundant on shallow coral reefs throughout the Indo-Pacific region. Specifically, I evaluate patterns of spawning and reproductive investment as a function of body size, social status, lunar phase and other environmental parameters. I address the question of whether females/males of differing size make different fitness-related decisions when away from spawning sites, and I evaluate context-dependency in these decisions. Finally, I will attempt to reconstruct the developmental histories (e.g., larval growth rates) of individuals from otoliths to evaluate potential relationships between developmental histories and fitness attributes.</p>