The relative influence of environmental cues on reproductive allocation of a highly iteroparous coral reef fish

Abstract

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).

Fish reproduction in a warming world: vulnerable points in hormone regulation from sex determination to spawning

“…In such species, however, warming temperatures can promote gametogenesis if temperatures remain within a normative range. In tropical species, lunar cycles or water flow changes across wet and dry seasons of precipitation may be foremost factors synchronizing gametogenesis and spawning [50,52]. Even so, temperature still affects reproduction in many tropical fishes, and some tropical species may actually live closer to their upper thermal limits for reproduction, given the narrower temperature ranges these species typically experience [53,54].…”

“…Gametogenesis in fishes is influenced by environmental conditions, including photoperiod, lunar cycle, tides, water flow rate and temperature, with the importance of different factors varying among taxa (e.g. [50,51]). For species in temperate and polar latitudes, changes in photoperiod may be the primary cue entraining gametogenesis, with temperature acting secondarily [52].…”

The natural of coastal light pollution and its relationship with morphological variation in a larval fish around Wellington, New Zealand

“…I used a gamma distribution with log link to improve model fit, as the response variable was non-integer, left-skewed, positive data. Following data analysis methods in Mitterwallner & Shima (2022), I started with the fully saturated model including all explanatory variables and their interactions; cloud cover (CC) and lunar face (LF), along with location (L) for spatial differences, and time (T) for temporal differences. To account for the moon being above or below the horizon during each observation, LF was encoded as 0 when moon visibility was 0.…”

“…Prior to industrialisation, variation in nocturnal brightness was driven largely by the lunar cycle, with predictable changes in nightly levels and timing of illumination acting as a cue for biological activity across taxa and habitats (J. S. Shima et al, 2022). Lunar periodicity has been observed in spawning of corals (Foster et al, 2018), sponges (Neely & Butler, 2020), algae (Mooney & Van Staden, 1984), and fish (Shima et al, 2020;Mitterwallner & Shima, 2022), with timing of reproduction hypothesized to correspond to advantageous conditions for larval dispersal, growth, and eventual settlement. The lunar cycle also plays an important role in community structure by moderating nocturnal predator-prey interactions, with many prey species (e.g.…”

“…By increasing the number of bright hours per night, ALAN also alters predator-prey interactions in nearshore and reef-dwelling species, leading to an increase in time spent foraging by predators and associated changes in community structure (Bolton et al, 2017), as well as fitness consequences for developing fish through metabolic effects (Hillyer et al, 2021). Development of larvae exposed to ALAN may also be affected, since optimal spawning times determined by the lunar cycle would not necessarily correlate with minimized risk of predation at key life history stages if DVM and pelagic predator activity is altered by increased nocturnal illumination (O'Connor et al, 2019;Mitterwallner & Shima, 2022;Shima et al, 2022).…”