Influence of the lunar cycle and spatial gradients on size-dependent male and female reproductive investment decisions of a protogynous reef fish

Abstract

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.

Lunar rhythms and their carry-over effects may shape environmental sex determination in a coral reef fish

“…Parents spawn pelagic eggs, and on a per egg basis, these are relatively energetically inexpensive. Their lunar spawning phenology is evident at the scale of the local population because smaller individuals largely curtail reproduction during the full and waning moons, while larger individuals reproduce more consistently across the lunar month [25]. Spawning at the new moon may be a reproductive tactic for many sixbars, and may be included within their spawning portfolio as part of a diversified bet-hedging approach.…”

“…Maturing as a primary male may be advantageous in some circumstances, as it affords opportunities to engage in surreptitious matings (via 'sneaking' or 'streaking' strategies), as well as group spawning events, with many females [25,26]. Additionally, evidence from other Thalassoma suggests that primary males have a higher likelihood of attaining a terminal male status (relative to females that change sex) [40], which can further increase an individual's fitness when territorial defence and female monopolization are achievable [27,28].…”

“…We found that fish that initially matured as females were disproportionately born on full moons, suggesting that spawning at this time tends to make females (figure 4c). Interestingly, most spawning activity at the full moon is from larger females, whereas smaller females abstain until closer to the new moon [25]. Thus, older females (which have survived longest as females) produce a larger proportion of female offspring, while younger females invest more heavily in a 'high-risk, high-reward' strategy that biases maturation of their offspring to primary males.…”

Effects of nocturnal illumination on fitness-related traits of the New Zealand common triplefin (Forsterygion lapillum)

“…Day and Taylor (1997)'s model for growth asserts that small fish preferentially allocate energy towards growth, and large fish tend to invest energy in reproduction. The modified female growth rates across body size in this treatment could therefore be a function of shifting investment choice from growth to reproduction (Mitterwallner and Shima, 2022) as body size increased. Males in this treatment showed the opposite relationship.…”

“…Individual investment choices between these factors can determine the timing, mode, and outcome of reproduction (Mitterwallner and Shima, 2022). The outcome of the energetic tradeoff contributes to the timing and success of reproduction, which dictates facets of offspring fitness, and in turn affects patterns of community recruitment, population size, structure, composition, and genetic variation (Cargnelli and Gross, 1996).…”