Marine biorhythms: bridging chronobiology and ecology

Adaptation and Survival of Marine-Associated Spiders (Araneae)

Aquatic environments are an unusual habitat for most arthropods. Nevertheless, many arthropod species that were once terrestrial dwelling have transitioned back to marine and freshwater environments, either as semiaquatic or, more rarely, as fully aquatic inhabitants. Transition to water from land is exceptional, and without respiratory modifications to allow for extended submergence and the associated hypoxic conditions, survival is limited. In this article, we review marine-associated species that have made this rare transition in a generally terrestrial group, spiders. We include several freshwater spider species for comparative purposes. Marine-associated spiders comprise less than 0.3% of spider species worldwide but are found in over 14% of all spider families. As we discuss, these spiders live in environments that, with tidal action, hydraulic forces, and saltwater, are more extreme than freshwater habitats, often requiring physiological and behavioral adaptations to survive. Spiders employ many methods to survive inundation from encroaching tides, such as air bubble respiration, airtight nests, hypoxic comas, and fleeing incoming tides. While airway protection is the primary survival strategy, further survival adaptations include saltwater-induced osmotic regulation, dietary composition, predator avoidance, reproduction, locomotory responses, and adaptation to extreme temperatures and hydrostatic pressures that challenge existence in marine environments.

Multiscale oscillations of the annual course of temperature affect the spawning events of rudd (Scardinus erythrophthalmus)

Identifying climate impacts on ecosystems and their components requires observing time series of sufficient length to ensure adequate statistical power and reasonable coverage of the historical range of variability inherent in the system. The complexity of the hierarchy of climate effects reflected in temporal patterns in time series creates a need to be accurately modeled. The life cycle phenomena of living organisms, including fish spawning, have the character of one-time or time-limited events in time. An approach to finding the relationship between continuous components of time dynamics of environment properties and life cycle events of living organisms was proposed. This approach allowed us to evaluate the role of temperature patterns in the phenology of spawning rudd (Scardinus erythrophthalmus Linnaeus, 1758) in the Dnipro River basin water bodies. The atmospheric temperature time series may be decomposed into the following components: trend, annual cycle, episodic component, harmonic component, extreme events, and noise. Systematically low water temperatures at the beginning of the spawning period were observed in the Protoka River system and the Obukhov floodplain, and systematically elevated temperatures were recorded in the Dnipro River. The annual temperature dynamics was shown to be presented as a composition of oscillatory processes of different scale levels. The sinusoidal trend was previously extracted from the temperature series data. The average annual temperature, amplitude, and phase shift were calculated on the basis of the sinusoidal regression model. The residuals of the sinusoidal trend were processed by means of redundancy analysis with variables derived from symmetric distance-based Moran’s eigenvector maps as explanatory predictors. A set of 104 orthogonal dbMEM variables was extracted from the annual time series. These temporal variables were divided into the broad-, medium-, and fine-scale components. The parameters of temperature dynamics and biotope type are able to explain 51–72% of variability of spawning event. The time of spawning in water bodies corresponds to the time of spawning start: the earlier spawning starts, the earlier it ends. The duration of the spawning season is influenced by the patterns of different scale levels, as well as the amplitude and shift of phases. In this case, the duration of spawning in all water bodies does not differ. Spawning temperature depends on medium- and fine-scale temperature patterns, but does not depend on the characteristics of the sinusoidal annual trend. The annual temperature variation has been shown to be such that it can be decomposed into a sinusoidal trend, patterns of a multiscale nature, and a random fraction. Over the time range studied, the trend of increasing mean annual temperature was not statistically significant for spawning events. The sinusoidal trend explains 78.3–87.6% of the temperature variations and depends on the mean annual temperature, the amplitude of temperature variations during the year, and the earlier or later seasons of the year. Amplitude and phase shift play a role in describing spawning phenology. The residuals of the sinusoidal trend have been explained using dbMEM variables. This variation was decomposed into large-scale, medium-scale, and small-scale components. Winter and spring temperature fluctuations prior to spawning initiation had the greatest effect on spawning. Water temperature determines the lower possible limit for the start of spawning, but the actual start of spawning is determined by the preceding temperature dynamics. The results of the study have implications for understanding the dynamics of fish populations and assessing the influence of environmental conditions on the harmonization of the various components of ecosystems.

Seascape genomics identify adaptive barriers correlated to tidal amplitude in the shore crab Carcinus maenas

Most marine invertebrates disperse during a planktonic larval stage that may drift for weeks with ocean currents. A challenge for larvae of coastal species is to return to coastal nursery habitats. Shore crab (Carcinus maenas L.) larvae are known to show tidal rhythmicity in vertical migration in tidal areas and circadian rhythmicity in microtidal areas, which seems to increase successful coastal settlement. We studied genome-wide differentiation based on 24,000 single nucleotide polymorphisms of 12 native populations of shore crab sampled from a large tidal amplitude gradient from macrotidal (~8 m) to microtidal (~0.2 m). Dispersal and recruitment success of larvae was assessed with a Lagrangian biophysical model, which showed a strong effect of larval behaviour on long-term connectivity, and dispersal barriers that partly coincided with different tidal environments. The genetic population structure showed a subdivision of the samples into three clusters, which represent micro-, meso- and macrotidal areas. The genetic differentiation was mostly driven by 0.5% outlier loci, which showed strong allelic clines located at the limits between the three tidal areas. Demographic modelling suggested that the two genetic barriers have different origins. Differential gene expression of two clock genes (cyc and pdp1) further highlighted phenotypic differences among genetic clusters that are potentially linked to the differences in larval behaviour. Taken together, our seascape genomic study suggests that tidal regime acts as a strong selection force on shore crab population structure, consistent with larval behaviour affecting dispersal and recruitment success.

Are cyclic plant and animal behaviours driven by gravimetric mechanical forces?

The celestial mechanics of the Sun, Moon, and Earth dominate the variations in gravitational force that all matter, live or inert, experiences on Earth. Expressed as gravimetric tides, these variations are pervasive and have forever been part of the physical ecology with which organisms evolved. Here, we first offer a brief review of previously proposed explanations that gravimetric tides constitute a tangible and potent force shaping the rhythmic activities of organisms. Through meta-analysis, we then interrogate data from three study cases and show the close association between the omnipresent gravimetric tides and cyclic activity. As exemplified by free-running cyclic locomotor activity in isopods, reproductive effort in coral, and modulation of growth in seedlings, biological rhythms coincide with temporal patterns of the local gravimetric tide. These data reveal that, in the presumed absence of rhythmic cues such as light and temperature, local gravimetric tide is sufficient to entrain cyclic behaviour. The present evidence thus questions the phenomenological significance of so-called free-run experiments.

Drought at a coastal wetland affects refuelling and migration strategies of shorebirds

Droughts can affect invertebrate communities in wetlands, which can have bottom-up effects on the condition and survival of top predators. Shorebirds, key predators at coastal wetlands, have experienced widespread population declines and could be negatively affected by droughts. We explored, in detail, the effects of drought on multiple aspects of shorebird stopover and migration ecology by contrasting a year with average wet/dry conditions (2016) with a year with moderate drought (2017) at a major subarctic stopover site on southbound migration. We also examined the effects of drought on shorebird body mass during stopover across 14 years (historical: 1974–1982 and present-day: 2014–2018). For the detailed comparison of two years, in the year with moderate drought we documented lower invertebrate abundance at some sites, higher prey family richness in shorebird faecal samples, lower shorebird refuelling rates, shorter stopover durations for juveniles, and, for most species, a higher probability of making a subsequent stopover in North America after departing the subarctic, compared to the year with average wet/dry conditions. In the 14-year dataset, shorebird body mass tended to be lower in drier years. We show that even short-term, moderate drought conditions can negatively affect shorebird refuelling performance at coastal wetlands, which may carry-over to affect subsequent stopover decisions. Given shorebird population declines and predicted changes in the severity and duration of droughts with climate change, researchers should prioritize a better understanding of how droughts affect shorebird refuelling performance and survival.

De novo assembly and annotation of the mangrove cricket genome

Objectives

The mangrove cricket, Apteronemobius asahinai, shows endogenous activity rhythms that synchronize with the tidal cycle (i.e., a free-running rhythm with a period of ~ 12.4 h [the circatidal rhythm]). Little is known about the molecular mechanisms underlying the circatidal rhythm. We present the draft genome of the mangrove cricket to facilitate future molecular studies of the molecular mechanisms behind this rhythm.

Data description

The draft genome contains 151,060 scaffolds with a total length of 1.68 Gb (N50: 27 kb) and 92% BUSCO completeness. We obtained 28,831 predicted genes, of which 19,896 (69%) were successfully annotated using at least one of two databases (UniProtKB/SwissProt database and Pfam database).

Circadian regulation of diel vertical migration (DVM) and metabolism in Antarctic krill Euphausia superba

Antarctic krill (Euphausia superba) are high latitude pelagic organisms which play a key ecological role in the ecosystem of the Southern Ocean. To synchronize their daily and seasonal life-traits with their highly rhythmic environment, krill rely on the implementation of rhythmic strategies which might be regulated by a circadian clock. A recent analysis of krill circadian transcriptome revealed that their clock might be characterized by an endogenous free-running period of about 12-15 h. Using krill exposed to simulated light/dark cycles (LD) and constant darkness (DD), we investigated the circadian regulation of krill diel vertical migration (DVM) and oxygen consumption, together with daily patterns of clock gene expression in brain and eyestalk tissue. In LD, we found clear 24 h rhythms of DVM and oxygen consumption, suggesting a synchronization with photoperiod. In DD, the DVM rhythm shifted to a 12 h period, while the peak of oxygen consumption displayed a temporal advance during the subjective light phase. This suggested that in free-running conditions the periodicity of these clock-regulated output functions might reflect the shortening of the endogenous period observed at the transcriptional level. Moreover, differences in the expression patterns of clock gene in brain and eyestalk, in LD and DD, suggested the presence in krill of a multiple oscillator system. Evidence of short periodicities in krill behavior and physiology further supports the hypothesis that a short endogenous period might represent a circadian adaption to cope with extreme seasonal photoperiodic variability at high latitude.

The brown clock: circadian rhythms in stramenopiles

Circadian clocks allow organisms to anticipate environmental changes associated with the diurnal light/dark cycle. Circadian oscillators have been described in plants and green algae, cyanobacteria, animals and fungi, however, little is known about the circadian clocks of photosynthetic eukaryotes outside the green lineage. Stramenopiles are a diverse group of secondary endosymbionts whose plastid originated from a red alga. Photosynthetic stramenopiles, which include diatoms and brown algae, play key roles in biogeochemical cycles and are important components of marine ecosystems. Genome annotation efforts indicated the presence of a novel type of oscillator in these organisms and the first circadian clock component in a stramenopile has been recently discovered. This review summarizes the phenotypic characterization of circadian rhythms in stramenopiles and current efforts to determine the mechanisms of this 'brown clock'. The elucidation of this brown clock will enable a deeper understanding of the role of self-sustained oscillations in the adaptation to life in marine environments.

Phenotypic Variation in Growth and Gene Expression Under Different Photoperiods in Allopatric Populations of the Copepod Tigriopus californicus

Daylength is a major environmental condition that varies seasonally and predictably along a latitudinal cline, where higher latitudes exhibit greater ranges in total daylengths. Generally, the circadian clock acts as a network of genes whose expression dynamics are known to control daily rhythms in response to daylength, and it enables the control of many physiological processes such as growth and development. While well studied in many model animals, the influence of daylength variation on phenotypic evolution is poorly examined in marine species. In this study we demonstrate that two allopatric populations of the intertidal crustacean Tigriopus californicus exhibit plastic and divergent phenotypic responses to changes in daylength. Using common-garden experiments, we discovered that shorter daylengths promoted decreased adult body size and faster growth rates in the two divergent populations, suggesting a plastic response to shortened days. In addition, the higher-latitude population exhibited a faster growth rate at any daylength condition, indicating a fixed response, possibly as a result of adaptation to respective natural light regimes. Gene expression profiles of several circadian clock genes, monitored throughout the day by quantitative polymerase chain reaction, revealed that the key core clock genes reach higher daily transcription maxima in the southern population compared to the northern population, pointing to divergent strategies used to respond to changes in daylength. Many modifier genes to the circadian clock showed similar plastic responses to the different daylengths, supporting the existence of at least some conserved gene expression across both populations. Ultimately, our results suggest that photoperiod and daylength exert a potent selective pressure underexplored in marine systems and warranting further future research.

Bivalve mollusc circadian clock genes can run at tidal frequency

Marine coastal habitats are complex cyclic environments as a result of sun and moon interactions. In contrast with the well-known circadian orchestration of the terrestrial animal rhythmicity (approx. 24 h), the mechanism responsible for the circatidal rhythm (approx. 12.4 h) remains largely elusive in marine organisms. We revealed in subtidal field conditions that the oyster Crassostrea gigas exhibits tidal rhythmicity of circadian clock genes and clock-associated genes. A free-running (FR) experiment showed an endogenous circatidal rhythm. In parallel, we showed in the field that oysters' valve behaviour exhibited a strong tidal rhythm combined with a daily rhythm. In the FR experiment, all behavioural rhythms were circatidal, and half of them were also circadian. Our results fuel the debate on endogenous circatidal mechanisms. In contrast with the current hypothesis on the existence of an independent tidal clock, we suggest that a single 'circadian/circatidal' clock in bivalves is sufficient to entrain behavioural patterns at tidal and daily frequencies.

Environmental entrainment demonstrates natural circadian rhythmicity in the cnidarian Nematostella vectensis

Considerable advances in chronobiology have been made through controlled laboratory studies, but distinct temporal rhythms can emerge under natural environmental conditions. Lab-reared Nematostella vectensis sea anemones exhibit circadian behavioral and physiological rhythms. Given that these anemones inhabit shallow estuarine environments subject to tidal inputs, it was unclear whether circadian rhythmicity would persist following entrainment in natural conditions, or whether circatidal periodicity would predominate. Nematostella were conditioned within a marsh environment, where they experienced strong daily temperature cycles as well as brief tidal flooding around the full and new moons. Upon retrieval, anemones exhibited strong circadian (∼24 h) activity rhythms under a light-dark cycle or continuous darkness, but reduced circadian rhythmicity under continuous light. However, some individuals in each light condition showed circadian rhythmicity, and a few individuals showed circatidal rhythmicity. Consistent with the behavioral studies, a large number of transcripts (1640) exhibited diurnal rhythmicity compared with very few (64) with semidiurnal rhythmicity. Diurnal transcripts included core circadian regulators, and 101 of 434 (23%) genes that were previously found to be upregulated by exposure to ultraviolet radiation. Together, these behavioral and transcriptional studies show that circadian rhythmicity predominates and suggest that solar radiation drives physiological cycles in this sediment-dwelling subtidal animal.

Differential Impacts of the Head on Platynereis dumerilii Peripheral Circadian Rhythms

The marine bristle worm Platynereis dumerilii is a useful functional model system for the study of the circadian clock and its interplay with others, e.g., circalunar clocks. The focus has so far been on the worm's head. However, behavioral and physiological cycles in other animals typically arise from the coordination of circadian clocks located in the brain and in peripheral tissues. Here, we focus on peripheral circadian rhythms and clocks, revisit and expand classical circadian work on the worm's chromatophores, investigate locomotion as read-out and include molecular analyses. We establish that different pieces of the trunk exhibit synchronized, robust oscillations of core circadian clock genes. These circadian core clock transcripts are under strong control of the light-dark cycle, quickly losing synchronized oscillation under constant darkness, irrespective of the absence or presence of heads. Different wavelengths are differently effective in controlling the peripheral molecular synchronization. We have previously shown that locomotor activity is under circadian clock control. Here, we show that upon decapitation worms exhibit strongly reduced activity levels. While still following the light-dark cycle, locomotor rhythmicity under constant darkness is less clear. We also observe the rhythmicity of pigments in the worm's individual chromatophores, confirming their circadian pattern. These size changes continue under constant darkness, but cannot be re-entrained by light upon decapitation. Our works thus provides the first basic characterization of the peripheral circadian clock of P. dumerilii. In the absence of the head, light is essential as a major synchronization cue for peripheral molecular and locomotor circadian rhythms, while circadian changes in chromatophore size can continue for several days in the absence of light/dark changes and the head. Thus, in Platynereis the dependence on the head depends on the type of peripheral rhythm studied. These data show that peripheral circadian rhythms and clocks should also be considered in "non-conventional" molecular model systems, i.e., outside Drosophila melanogaster, Danio rerio, and Mus musculus, and build a basic foundation for future investigations of interactions of clocks with different period lengths in marine organisms.

Nest initiation and flooding in response to season and semi-lunar spring tides in a ground-nesting shorebird

BACKGROUND

Marine and intertidal organisms face the rhythmic environmental changes induced by tides. The large amplitude of spring tides that occur around full and new moon may threaten nests of ground-nesting birds. These birds face a trade-off between ensuring nest safety from tidal flooding and nesting near the waterline to provide their newly hatched offspring with suitable foraging opportunities. The semi-lunar periodicity of spring tides may enable birds to schedule nest initiation adaptively, for example, by initiating nests around tidal peaks when the water line reaches the farthest into the intertidal habitat. We examined the impact of semi-lunar tidal changes on the phenology of nest flooding and nest initiation in Snowy Plovers (Charadrius nivosus) breeding at Bahía de Ceuta, a coastal wetland in Northwest Mexico.

RESULTS

Using nest initiations and fates of 752 nests monitored over ten years we found that the laying season coincides with the lowest spring tides of the year and only 6% of all nests were flooded by tides. Tidal nest flooding varied substantially over time. First, flooding was the primary cause of nest failures in two of the ten seasons indicating high between-season stochasticity. Second, nests were flooded almost exclusively during the second half of the laying season. Third, nest flooding was associated with the semi-lunar spring tide cycle as nests initiated around spring tide had a lower risk of being flooded than nests initiated at other times. Following the spring tide rhythm, plovers appeared to adapt to this risk of flooding with nest initiation rates highest around spring tides and lowest around neap tides.

CONCLUSIONS

Snowy Plovers appear generally well adapted to the risk of nest flooding by spring tides. Our results are in line with other studies showing that intertidal organisms have evolved adaptive responses to predictable rhythmic tidal changes but these adaptations do not prevent occasional catastrophic losses caused by stochastic events.

Circatidal gene expression in the mangrove cricket Apteronemobius asahinai

The mangrove cricket Apteronemobius asahinai is endemic to mangrove forest floors. It shows circatidal rhythmicity, with a 12.6-h period of locomotor activity under constant conditions. Its free-running activity also has a circadian component; i.e. it is more active during the subjective night than during the day. In this study, we investigated rhythmic gene expression under constant darkness by RNA sequencing to identify genes controlled by the biological clock. Samples collected every 3 h for 48 h were analysed (one cricket per time-point). We identified 284 significant circatidal cycling transcripts (period length 12–15 h). Almost half of them were annotated with known genes in the NCBI nr database, including enzymes related to metabolic processes and molecular chaperones. There were less transcripts with circadian rhythmicity than with circatidal rhythmicity, and the expression of core circadian clock genes did not show significant rhythmicity. This may reflect the nature of the mangrove cricket or may be due to the paucity of the sampling repeats: only two periods for circadian cycle with no replications. We evaluated for the first time the rhythmic transcriptome of an insect that shows circatidal rhythmic activity; our findings will contribute to future studies of circatidal clock genes.

Annual rhythms of temporal niche partitioning in the Sparidae family are correlated to different environmental variables

The seasonal timing of recurring biological processes is essential for organisms living in temperate regions. While ample knowledge of these processes exists for terrestrial environments, seasonal timing in the marine environment is relatively understudied. Here, we characterized the annual rhythm of habitat use in six fish species belonging to the Sparidae family, highlighting the main environmental variables that correlate to such rhythms. The study was conducted at a coastal artificial reef through a cabled observatory system, which allowed gathering underwater time-lapse images every 30 minutes consecutively over 3 years. Rhythms of fish counts had a significant annual periodicity in four out of the six studied species. Species-specific temporal patterns were found, demonstrating a clear annual temporal niche partitioning within the studied family. Temperature was the most important environmental variable correlated with fish counts in the proximity of the artificial reef, while daily photoperiod and salinity were not important. In a scenario of human-induced rapid environmental change, tracking phenological shifts may provide key indications about the effects of climate change at both species and ecosystem level. Our study reinforces the efficacy of underwater cabled video-observatories as a reliable tool for long-term monitoring of phenological events.

Resource landscapes explain contrasting patterns of aggregation and site fidelity by red knots at two wintering sites

BACKGROUND

Space use strategies by foraging animals are often considered to be species-specific. However, similarity between conspecific strategies may also result from similar resource environments. Here, we revisit classic predictions of the relationships between the resource distribution and foragers' space use by tracking free-living foragers of a single species in two contrasting resource landscapes. At two main non-breeding areas along the East-Atlantic flyway (Wadden Sea, The Netherlands and Banc d'Arguin, Mauritania), we mapped prey distributions and derived resource landscapes in terms of the predicted intake rate of red knots (Calidris canutus), migratory molluscivore shorebirds. We tracked the foraging paths of 13 and 38 individual red knots at intervals of 1 s over two and five weeks in the Wadden Sea and at Banc d'Arguin, respectively. Mediated by competition for resources, we expected aggregation to be strong and site fidelity weak in an environment with large resource patches. The opposite was expected for small resource patches, but only if local resource abundances were high.

RESULTS

Compared with Banc d'Arguin, resource patches in the Wadden Sea were larger and the maximum local resource abundance was higher. However, because of constraints set by digestive capacity, the average potential intake rates by red knots were similar at the two study sites. Space-use patterns differed as predicted from these differences in resource landscapes. Whereas foraging red knots in the Wadden Sea roamed the mudflats in high aggregation without site fidelity (i.e. grouping nomads), at Banc d'Arguin they showed less aggregation but were strongly site-faithful (i.e. solitary residents).

CONCLUSION

The space use pattern of red knots in the two study areas showed diametrically opposite patterns. These differences could be explained from the distribution of resources in the two areas. Our findings imply that intraspecific similarities in space use patterns represent responses to similar resource environments rather than species-specificity. To predict how environmental change affects space use, we need to understand the degree to which space-use strategies result from developmental plasticity and behavioural flexibility. This requires not only tracking foragers throughout their development, but also tracking their environment in sufficient spatial and temporal detail.

Keeping time without a spine: what can the insect clock teach us about seasonal adaptation?

Seasonal change in daylength (photoperiod) is widely used by insects to regulate temporal patterns of development and behaviour, including the timing of diapause (dormancy) and migration. Flexibility of the photoperiodic response is critical for rapid shifts to new hosts, survival in the face of global climate change and to reproductive isolation. At the same time, the daily circadian clock is also essential for development, diapause and multiple behaviours, including correct flight orientation during long-distance migration. Although studied for decades, how these two critical biological timing mechanisms are integrated is poorly understood, in part because the core circadian clock genes are all transcription factors or regulators that are able to exert multiple effects throughout the genome. In this chapter, we discuss clocks in the wild from the perspective of diverse insect groups across eco-geographic contexts from the Antarctic to the tropical regions of Earth. Application of the expanding tool box of molecular techniques will lead us to distinguish universal from unique mechanisms underlying the evolution of circadian and photoperiodic timing, and their interaction across taxonomic and ecological contexts represented by insects.This article is part of the themed issue 'Wild clocks: integrating chronobiology and ecology to understand timekeeping in free-living animals'.

Marine biorhythms: bridging chronobiology and ecology

Marine organisms adapt to complex temporal environments that include daily, tidal, semi-lunar, lunar and seasonal cycles. However, our understanding of marine biological rhythms and their underlying molecular basis is mainly confined to a few model organisms in rather simplistic laboratory settings. Here, we use new empirical data and recent examples of marine biorhythms to highlight how field ecologists and laboratory chronobiologists can complement each other's efforts. First, with continuous tracking of intertidal shorebirds in the field, we reveal individual differences in tidal and circadian foraging rhythms. Second, we demonstrate that shorebird species that spend 8–10 months in tidal environments rarely maintain such tidal or circadian rhythms during breeding, likely because of other, more pertinent, temporally structured, local ecological pressures such as predation or social environment. Finally, we use examples of initial findings from invertebrates (arthropods and polychaete worms) that are being developed as model species to study the molecular bases of lunar-related rhythms. These examples indicate that canonical circadian clock genes (i.e. the homologous clock genes identified in many higher organisms) may not be involved in lunar/tidal phenotypes. Together, our results and the examples we describe emphasize that linking field and laboratory studies is likely to generate a better ecological appreciation of lunar-related rhythms in the wild.

This article is part of the themed issue ‘Wild clocks: integrating chronobiology and ecology to understand timekeeping in free-living animals’.

Two sides of a coin: ecological and chronobiological perspectives of timing in the wild

Most processes within organisms, and most interactions between organisms and their environment, have distinct time profiles. The temporal coordination of such processes is crucial across levels of biological organization, but disciplines differ widely in their approaches to study timing. Such differences are accentuated between ecologists, who are centrally concerned with a holistic view of an organism in relation to its external environment, and chronobiologists, who emphasize internal timekeeping within an organism and the mechanisms of its adjustment to the environment. We argue that ecological and chronobiological perspectives are complementary, and that studies at the intersection will enable both fields to jointly overcome obstacles that currently hinder progress. However, to achieve this integration, we first have to cross some conceptual barriers, clarifying prohibitively inaccessible terminologies. We critically assess main assumptions and concepts in either field, as well as their common interests. Both approaches intersect in their need to understand the extent and regulation of temporal plasticity, and in the concept of 'chronotype', i.e. the characteristic temporal properties of individuals which are the targets of natural and sexual selection. We then highlight promising developments, point out open questions, acknowledge difficulties and propose directions for further integration of ecological and chronobiological perspectives through Wild Clock research.This article is part of the themed issue 'Wild Clocks: integrating chronobiology and ecology to understand timekeeping in free-living animals'.