Seasonal temperature, the lunar cycle and diurnal rhythms interact in a combinatorial manner to modulate genomic responses to the environment in a reef-building coral

Evaluating the role of moonlight-darkness dynamics as proximate spawning cues in an Acropora coral

For sessile broadcast spawning marine invertebrates, such as corals, successful sexual reproduction depends on conspecifics spawning synchronously. The precise monthly, lunar, and diel timing and the extent of synchrony, i.e., proportion of population reproducing at the same time, are likely to play a key role in coral population recovery, persistence, and adaptation. Despite its importance, the mechanisms by which different environmental factors trigger corals to spawn on specific dates within the lunar cycle remain poorly understood. Periods of darkness post-sunset around full moon of the spawning month have been shown to induce spawning in merulinid corals, whereas for Acropora, moonlight is considered the main determinant driver of night of spawning. Here, we conducted two manipulative field experiments around full moon in Palau using the common table coral Acropora aff. hyacinthus to disentangle the role of moonlight and darkness post-sunset as proximate cues. Coral fragments were assigned to three treatments providing different post-sunset darkness conditions, versus control and procedural control fragments exposed to natural conditions. In contrast to previous studies on Acropora, we found that Acropora aff. hyacinthus can spawn synchronously in the absence of moonlight during the nights leading to spawning. Corals exposed to darkness post-sunset for at least two to three consecutive nights advanced their spawning compared to controls. This finding indicates that periods of darkness post-sunset can act as an inducer for spawning in Acropora as well as in merulinid corals, suggesting that this mechanism may be more widespread than previously thought.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00338-025-02618-9.

Rhythms and Clocks in Marine Organisms

The regular movements of waves and tides are obvious representations of the oceans' rhythmicity. But the rhythms of marine life span across ecological niches and timescales, including short (in the range of hours) and long (in the range of days and months) periods. These rhythms regulate the physiology and behavior of individuals, as well as their interactions with each other and with the environment. This review highlights examples of rhythmicity in marine animals and algae that represent important groups of marine life across different habitats. The examples cover ecologically highly relevant species and a growing number of laboratory model systems that are used to disentangle key mechanistic principles. The review introduces fundamental concepts of chronobiology, such as the distinction between rhythmic and endogenous oscillator-driven processes. It also addresses the relevance of studying diverse rhythms and oscillators, as well as their interconnection, for making better predictions of how species will respond to environmental perturbations, including climate change. As the review aims to address scientists from the diverse fields of marine biology, ecology, and molecular chronobiology, all of which have their own scientific terms, we provide definitions of key terms throughout the article.

Moonrise timing is key for synchronized spawning in coral Dipsastraea speciosa

Synchronized mass coral spawning typically occurs several days after a full moon once a year. It is expected that spawning day is determined by corals sensing environmental change regulated by the lunar cycle (i.e., tide or moonlight); however, the exact regulatory mechanism remains unknown. Here, we demonstrate how moonlight influences the spawning process of coral, Dipsastraea speciosa When corals in the field were shaded 1 and 3 d before the full moon or 1 d after the full moon, spawning always occurred 5 d after shading commenced. These results suggest moonlight suppresses spawning: a hypothesis supported by laboratory experiments in which we monitored the effects of experimental moonlight (night-light) on spawning day. Different night-light treatments in the laboratory showed that the presence of a dark period between day-light and night-light conditions eliminates the suppressive effect of night-light on spawning. In nature, moonrise gets progressively later during the course of the lunar cycle, shifting to after sunset following the day of the full moon. Our results indicate that this period of darkness between sunset and moonrise triggers synchronized mass spawning of D. speciosa in nature.

Seasonal influence on the bathymetric distribution of an endangered fish within a marine protected area

The spatio-temporal variability of fish distribution is important to better manage and protect the populations of endangered species. In this sense, the vertical movements of a vulnerable and protected species, Sciaena umbra, were assessed in a marine protected area (the Réserve Naturelle Marine de Cerbère-Banyuls, south of France) to study the variability of their bathymetric distribution at different time scales. Twenty adults were marked with acoustic transmitters and acoustically monitored over 2.5 years. This revealed that some individuals remained at shallow waters (< 8 m) all year round, while others presented vertical segregation at deeper waters during the cold months (mean depth of 22.5 ± 0.04 m) and all aggregated in shallow waters during the warm months. The brown meagre was more active during the night, except in June and July when peaks of activity were observed at dusk. These patterns are likely associated with foraging and reproductive behavior during the cold and warm periods, respectively, and likely regulated by water temperature and the depth of the thermocline. Here, we provide valuable information on when and where in the water column critical periods of S. umbra life cycle are expected to occur, which should be considered in management and protection plans.

Thyroid hormone regulation of thermal acclimation in ectotherms: Physiological mechanisms and ecoevolutionary implications

The pathways that regulate adaptive thermal plasticity in ectothermic vertebrates have received little attention relative to those in birds and mammals. However, there is increasing evidence that thyroid hormone represents a critical regulator of thermal plasticity in both ectothermic and endothermic vertebrates. In this review, I summarize the evidence for thyroid hormone-mediated thermal compensation responses in ectothermic vertebrates, with specific focus on effects on the whole animal, skeletal muscle, and cardiac muscle. Interestingly, these effects can differ wildly between focal tissues and species. I move on to discuss what the role of thyroid hormone in ectotherm thermal plasticity can reveal about stressor interactions and central vs. peripheral levels of thyroid hormone regulation. Lastly, I focus on the conserved nature of thyroid hormone signaling in animal thermal responses, with specific reference to the ectotherm → endotherm spectrum. I use this framework to highlight research avenues that will further resolve the evolutionary trajectory of thyroid hormone actions across animals. I hope to emphasize what thyroid hormone-mediated cold acclimation in a 3 cm fish can contribute to ongoing debates surrounding the impacts of stressor interactions, the potential costs of plasticity, the evolution of endothermy, and the impacts of global change.

Circalunar clocks-Old experiments for a new era

Circalunar clocks, which allow organisms to time reproduction to lunar phase, have been experimentally proven but are still not understood at the molecular level. Currently, a new generation of researchers with new tools is setting out to fill this gap. Our essay provides an overview of classic experiments on circalunar clocks. From the unpublished work of the late D. Neumann we also present a novel phase response curve for a circalunar clock. These experiments highlight avenues for molecular work and call for rigor in setting up and analyzing the logistically complex experiments on circalunar clocks. Re-evaluating classic experiments, we propose that (1) circalunar clocks in different organisms will have divergent mechanisms and physiological bases, (2) they may have properties very different from the well-studied circadian clocks and (3) they may have close mechanistic and molecular relations to seasonal rhythms and diapause.

A framework for understanding gene expression plasticity and its influence on stress tolerance

Phenotypic plasticity can serve as a stepping stone towards adaptation. Recently, studies have shown that gene expression contributes to emergent stress responses such as thermal tolerance, with tolerant and susceptible populations showing distinct transcriptional profiles. However, given the dynamic nature of gene expression, interpreting transcriptomic results in a way that elucidates the functional connection between gene expression and the observed stress response is challenging. Here, we present a conceptual framework to guide interpretation of gene expression reaction norms in the context of stress tolerance. We consider the evolutionary and adaptive potential of gene expression reaction norms and discuss the influence of sampling timing, transcriptomic resilience, as well as complexities related to life history when interpreting gene expression dynamics and how these patterns relate to host tolerance. We highlight corals as a case study to demonstrate the value of this framework for non-model systems. As species face rapidly changing environmental conditions, modulating gene expression can serve as a mechanistic link from genetic and cellular processes to the physiological responses that allow organisms to thrive under novel conditions. Interpreting how or whether a species can employ gene expression plasticity to ensure short-term survival will be critical for understanding the global impacts of climate change across diverse taxa.

The Still Dark Side of the Moon: Molecular Mechanisms of Lunar-Controlled Rhythms and Clocks

Starting with the beginning of the last century, a multitude of scientific studies has documented that the lunar cycle times behaviors and physiology in many organisms. It is plausible that even the first life forms adapted to the different rhythms controlled by the moon. Consistently, many marine species exhibit lunar rhythms, and also the number of documented "lunar-rhythmic" terrestrial species is increasing. Organisms follow diverse lunar geophysical/astronomical rhythms, which differ significantly in terms of period length: from hours (circalunidian and circatidal rhythms) to days (circasemilunar and circalunar cycles). Evidence for internal circatital and circalunar oscillators exists for a range of species based on past behavioral studies, but those species with well-documented behaviorally free-running lunar rhythms are not typically used for molecular studies. Thus, the underlying molecular mechanisms are largely obscure: the dark side of the moon. Here we review findings that start to connect molecular pathways with moon-controlled physiology and behaviors. The present data indicate connections between metabolic/endocrine pathways and moon-controlled rhythms, as well as interactions between circadian and circatidal/circalunar rhythms. Moreover, recent high-throughput analyses provide useful leads toward pathways, as well as molecular markers. However, for each interpretation, it is important to carefully consider the, partly substantially differing, conditions used in each experimental paradigm. In the future, it will be important to use lab experiments to delineate the specific mechanisms of the different solar- and lunar-controlled rhythms, but to also start integrating them together, as life has evolved equally long under rhythms of both sun and moon.

Demystifying Circalunar and Diel Rhythmicity in Acropora digitifera under Constant Dim Light

Life on earth has evolved under constant environmental changes; in response to these changes, most organisms have developed an endogenous clock that allows them to anticipate daily and seasonal changes and adapt their biology accordingly. Light cycles synchronize biological rhythms and are controlled by an endogenous clock that is entrained by environmental cues. Light is known to play a key role in the biology of symbiotic corals as they exhibit many biological processes entrained by daily light patterns. In this study, we aimed at determining the effect of constant dim light on coral's perception of diel and monthly cycles. Our results show that under constant dim light corals display a loss of rhythmic processes and constant stimuli by light, which initiates signal transduction that results in an abnormal cell cycle, cell proliferation, and protein synthesis. The results emphasize how constant dim light can mask the biological clock of Acropora digitifera.

•

Light entrains many biological processes governed by the endogenous clock

•

Constant dim light overrides the biological clock of A. digitifera corals

•

Artificial light impacts the processes that allow corals to thrive in our oceans

•

The increase of artificial light in coastal areas is a growing threat to coral reefs

Seasonal temperature, the lunar cycle and diurnal rhythms interact in a combinatorial manner to modulate genomic responses to the environment in a reef-building coral

Rhythms of various periodicities drive cyclical processes in organisms ranging from single cells to the largest mammals on earth, and on scales from cellular physiology to global migrations. The molecular mechanisms that generate circadian behaviours in model organisms have been well studied, but longer phase cycles and interactions between cycles with different periodicities remain poorly understood. Broadcast spawning corals are one of the best examples of an organism integrating inputs from multiple environmental parameters, including seasonal temperature, the lunar phase and hour of the day, to calibrate their annual reproductive event. We present a deep RNA-sequencing experiment utilizing multiple analyses to differentiate transcriptomic responses modulated by the interactions between the three aforementioned environmental parameters. Acropora millepora was sampled over multiple 24-hr periods throughout a full lunar month and at two seasonal temperatures. Temperature, lunar and diurnal cycles produce distinct transcriptomic responses, with interactions between all three variables identifying a core set of genes. These core genes include mef2, a developmental master regulator, and two heterogeneous nuclear ribonucleoproteins, one of which is known to post-transcriptionally interact with mef2 and with biological clock-regulating mRNAs. Interactions between diurnal and temperature differences impacted a range of core processes ranging from biological clocks to stress responses. Genes involved with developmental processes and transcriptional regulation were impacted by the lunar phase and seasonal temperature differences. Lastly, there was a diurnal and lunar phase interaction in which genes involved with RNA-processing and translational regulation were differentially regulated. These data illustrate the extraordinary levels of transcriptional variation across time in a simple radial cnidarian in response to the environment under normal conditions.