Coral Reefs

Reef restoration practitioners aim to preserve coral genetic diversity by protecting reefs and cultivating diverse genotypes in coral nurseries. However, cryptic genetic lineages in most corals complicate restoration strategies, as the role of between-lineage genetic divergence remains unclear regarding adaptation. In Montastraea cavernosa, researchers have identified cryptic lineages, some strongly segregated by depth. We conducted a ten-week reciprocal transplantation experiment using two cryptic lineages restricted to shallow water (<10m depth), with one lineage more common on nearshore reefs and the other on offshore reefs. We aimed to quantify lineage-specific responses to the environment that explain the genetic and ecological divergence between the two lineages. Surprisingly, the strongest response was not lineage-specific. Instead, both lineages exhibited strong and similar changes in growth and metabolomic profiles, depending on the transplantation habitat. These results suggest that cryptic lineages employ similar mechanisms of adaptation and acclimatization to environmental challenges, despite their genetic distinction.

Near-shore coral reefs in southern Guam (Mariana Islands) experience severe sedimentation, in particular during the wet season when rainfall and erosion are high. We sampled fragments of the reef-forming coral Porites lobata from opposite ends of a sedimentation gradient in Fouha Bay, southern Guam, during dry and wet seasons. Using DNA metabarcoding, we characterized the diversity and composition of P. lobata-associated Symbiodiniaceae and bacterial microbiome communities. As in many species of Porites, Symbiodiniaceae communities of P. lobata were dominated by variants of Cladocopium C15 with sites showing differences in Symbiodiniaceae communities attributable to variation in these Cladocopium C15 variants. Bacterial microbiomes of P. lobata were dominated by Endozoicomonadaceae, a family of putative coral bacterial endosymbionts involved in nutrient cycling. Site and seasonal differences in bacterial diversity and community composition were apparent. In close proximity to the mouth of the river draining into Fouha Bay, bacterial diversity was highest during the wet season when sedimentation is generally severe. Microbiome reorganization in response to sedimentation may explain this result, but we also found overrepresentation of bacteria associated with terrestrial origin close to the river mouth and/or during the wet season. Together these patterns highlight that coral Symbiodiniaceae and bacterial communities are both spatially and temporally structured in this disturbed system. IMPORTANCEThis study provides a time series dataset of coral-associated microorganisms, including dinoflagellate algae and bacteria, from a tropical bay impacted by sedimentation that results from upstream erosion of disturbed soils. Characterizing temporal patterns of coral-associated microbes provides insights into the dynamic nature of these communities. While microbiome variability across sites and seasons may be a result of acclimatization to different environmental conditions, we identified bacterial groups of putative terrestrial origin in sampled coral microbiomes that may have been exported from eroded soils to the near-shore reef. Considering that disturbed soils act as hotspots for the proliferation of potentially harmful substances, such as antimicrobial resistance genes, understanding microbial community connections at the marine-freshwater-terrestrial interface is an important step toward evaluating environmental impacts across connected ecosystems from ridge to reef.

Coral reefs deliver vital services via a complex three-dimensional framework sustained by the balance between calcium carbonate production and erosion, or the net carbonate budget state. In many tropical western Atlantic reefs, ecological decline has reduced carbonate production, yielding near-neutral or negative budgets. Yet some reefs retain high coral cover and, theoretically, should also have high net positive budgets, yet often show modest carbonate accumulation. We used the remote reef of Cayo Arenas in the Campeche Bank, Gulf of Mexico, to test whether in reefs under suboptimal (variable) environmental conditions, high coral production is offset by robust bioeroder communities, producing neutral budgets. At 14 sites, we quantified carbonate producers and bioeroders to estimate gross production, bioerosion, and net budget states. Despite relatively high live coral cover, mean net carbonate budgets were approximately neutral. Crucially, this neutrality arose not from depressed biological activity (as in degraded reefs) but from an active equilibrium: vigorous carbonate production coupled with substantial bioerosion. These reefs, therefore, represent a contemporary, functional reef state in net stasis. Distinguishing active-neutral from impoverishment-neutral regimes is critical for predicting reef trajectories under environmental change and for targeting management, although near-stasis emerging from high carbonate turnover can appear functionally intact yet operate with limited buffering capacity against net carbonate loss.

Ocean warming is altering abiotic environments and biotic interactions experienced by marine organisms, where sensitive early developmental windows occur in biologically complex seawater communities. The impact of these interactions on developmental processes and fitness in hosts is not well understood, but likely contingent on the establishment of a host-associated microbiome. Here, we hypothesize that temperature and microbial exposure during embryogenesis influence larval microbiome assembly and host morphology. Strongylocentrotus purpuratus embryos were raised in low microbial richness (LMR) or high microbial richness (HMR) seawater at ambient (14 {degrees}C) or elevated (18 {degrees}C) temperature, then collected at 2, 4, and 6 days post-fertilization (dpf) following multiple feedings. Higher microbial diversity was observed in larvae that developed in HMR seawater when compared to LMR. Differences in relative abundances of dominant microbial families between seawater and larvae suggest some degree of host selectivity in microbiome assembly. Temperature did not strongly alter microbiome composition, but both temperature and microbial condition led to differences in larval morphology by 6 dpf, potentially due to enrichment of microbes with chemoheterotrophic functions. By linking how temperature and microbial communities interact with host development, we contribute novel insights into how early-life environmental conditions impact holobiont formation and morphology. One sentence summaryEarly developmental temperature and microbial conditions shape larval microbiome establishment and morphology.

Linking animal movements to environmental drivers is essential for understanding ecological processes and anticipating species responses to climate change. We investigated habitat-specific movements in a globally significant aggregation of loggerhead turtles (Caretta caretta) nesting in Cabo Verde. Satellite tags on 15 adults (12 females, 3 males) provided multi-year tracks spanning breeding, migration, and foraging habitats. Movements and phenology differed by habitat. During the breeding season, females used either coastal areas, remaining within [~]20 m depth, or undertook long looping forays up to 360 km. Males showed two strategies: two remained resident in Cabo Verde waters, including Fra, the largest male tracked (Curved carapace length of 105 cm compared with a male mean of 90.7 {+/-} 10.3 cm), while the third migrated annually to distant foraging grounds and returned ahead of the subsequent breeding season. In foraging habitats, turtles adopted neritic or oceanic strategies: neritic turtles remained localised in warm, productive waters, whereas oceanic turtles ranged widely in deeper, less productive areas. Time- and space-shift analyses showed that oceanic foragers used intermediate sea surface temperature and chlorophyll-a conditions relative to nearby or temporally shifted alternatives, consistent with movement within a thermal-trophic trade-off. Together, these results show how sex, body size, and energy balance drive habitat-specific movement dynamics in a changing ocean.

Global marine heatwaves have devastated tropical coral reefs, and further mortality is projected under ongoing climate change. Identifying thermally tolerant coral colonies is therefore a priority for conservation, restoration, and research. Portable acute heat stress assays (e.g., CBASS) enable rapid, standardized estimates of coral thermal tolerance under field conditions. However, it remains unresolved whether such experimentally derived metrics (ED5, ED50, DW) predict bleaching and mortality in situ. Here, we quantified acute thermal tolerance metrics for 2,068 coral colonies across 12 common Indo-Pacific species, six months prior to an unprecedented heat stress event in northeastern Peninsular Malaysia and compared experimentally derived ED and DW values to subsequent bleaching severity and mortality in the field. Experimental thermal tolerance metrics explained only a limited proportion of variation in bleaching outcomes and survival. Predictive power varied among species and was higher in slow-growing species. Our findings suggest that while acute heat stress assays capture substantial variation in coral thermal tolerance, their ability to predict in situ outcomes is context-dependent and diminishes under severe thermal stress. Ultimately, in situ coral bleaching under severe heat stress may reduce the discriminatory capacity of acute assay-derived tolerance metrics.

Corals harbor a diverse bacterial community that facilitates adaptation and sustains their health. In coral holobiont research, culture-independent approaches have transformed the existing paradigm. Molecular techniques, such as metabarcoding, revealed a high diversity of previously unrecognized bacterial symbionts. Coral microbiota characterization has relied on these techniques over the last decade, but relying solely on them does not provide a detailed understanding of the dynamics of the coral holobiont complex. Returning to classic microbiological methods and in vitro experimentation can yield novel insights into symbiont roles, physiology, and interactions within the holobiont. Under this premise, we aimed to isolate and culture bacteria from four Mediterranean corals. The recovery of 84 pure bacterial isolates and their initial classification based on the 16S rRNA gene revealed substantial diversity among symbionts amenable to culture. Several isolates represent novel species within relevant genera, such as Vibrio, underscoring the value of culture-based studies. All cultures were cryopreserved to guarantee long-term accessibility for future projects. This represents a key step towards describing the roles of bacteria within the coral holobiont, as cultures enable in-depth morphological and physiological characterization of the symbionts and experimental ecology studies.

O_LIThe frequency and intensity of heat events is increasing across marine and terrestrial ecosystems. Within the same ecological community, the relative exposure and sensitivity to heat stress may vary considerably among interacting species, like predators and prey. This can be especially true for species that interact at the aquatic-terrestrial interface, as well as for interactions between primarily nocturnal and diurnal species, making it difficult to predict how such communities will respond to habitat warming. C_LIO_LIThermal limit metrics such as CTmax are often assumed to equate with ecological death because such temperatures impair behavioral activity and/or physiological functioning. Prey that are diurnally active can be more frequently exposed to temperatures that approach CTmax compared to their nocturnal predators, which may use thermal refuges during the day. Yet the impacts of daytime heat exposure on nighttime predation risk remain unknown. C_LIO_LIHere, we compared the thermal environment, performance, and heat tolerance between the predatory blue crab, Callinectus sapidus and one of its prey species, the mangrove periwinkle Littoraria anguilifera in a tropical mangrove ecosystem. We examined how exposing prey to heat stress at and below their CTmax affected their capacity to avoid predation in the field at night when predation risk is highest. C_LIO_LIWe found that acute exposure to temperatures near CTmax during the day increased the prey species susceptibility to predation during recovery at night. Although both interacting predator and prey have high thermal tolerance, prey are exposed to conditions that already reach CTmax, suggesting that current extremes in temperatures may already be influencing vulnerability to predation in this ecosystem. C_LIO_LIOur results suggest that differential exposure to sublethal heat stress in diurnal prey relative to their predator, along with the subsequent impact of these exposures on predation risk, will play a role in shaping these interacting as climate warms. C_LI

The mechanisms driving host-symbiont associations across space and time in contemporary mutualisms can give insight into the capacity for symbiotic organisms to respond to environmental change. High specificity between partners can increase cooperation and facilitate efficient holobiont selection, whereas low specificity could reduce host benefit, but facilitate adaptive associations across heterogeneous environments. The present study explores specificity in natural populations of a cnidarian-algal model, Exaiptasia diaphana, across a latitudinal gradient to understand the genetic and environmental effects driving host-symbiont associations, and their relation to heritable and/or environmental symbiont acquisition. We found that symbiotic associations were extremely flexible in E. diaphana, regardless of transmission mode. E. diaphana were capable of associating with diverse symbiont communities across genetically identical hosts seeded with vertically transmitted symbionts, as well as across highly connected host populations which acquire symbionts horizontally. Host population connectivity was complex and unrelated to geographic distance, whereas symbiont community composition tracked the thermal gradient, potentially due to context dependent biotic interactions. These results indicate that in a flexible symbiosis, symbiont communities are environmentally-determined, suggesting the future of this symbiosis will likely depend on climate adaptation of symbionts.

Interactions between cleaner fish Labroides dimidiatus and client fish, from which cleaners remove ectoparasites and mucus, represent a textbook example of mutualism involving sophisticated strategic decision-making. However, cleaners must also face intraspecific social challenges within a size-based hierarchy, where the largest females may eventually change sex and become males with higher reproductive rates. Following 540 individuals over 11 months, we found that, contrary to expectations, slow-growing females spent more time cleaning and cheated more frequently, without causing more negative client responses than fast-growing females did. Instead, variation in growth was best explained by social factors: fast-growing individuals experienced reduced social control, while slow growers spent more time in proximity to dominant individuals. As there was no evidence that spawning activity affected growth patterns, it appears that fast growth as a viable strategy for becoming a male largely depends on the lack of control by dominants.

Animal weapons are ecologically important traits that mediate contests over limiting resources and can strongly influence survival and reproduction. Weapon traits often exhibit substantial intraspecific morphological diversity, raising questions about the ecological drivers of this variation. Acrorhagi are weapons produced by sea anemones that are used in intraspecific territorial encounters. Although acrorhagial morphology varies widely within species, patterns of intraspecific variation remain poorly characterized, and the extent to which such variation reflects differences in local intraspecific competition is unclear. Here, we conduct morphometric analyses to characterize within-population variation and allometry in acrorhagial traits of the solitary anemone Anthopleura sola. We show that these traits covary with habitats differing in conspecific density. The number of acrorhagi scaled positively with body size, and individuals occupying a high-density habitat tended to possess more acrorhagi than did similar sized individuals from a low-density habitat. In addition, anemones from high-density habitats exhibited longer acrorhagial cnidae, a pattern that was not explained by differences in body size or acrorhagial density. Together, these results suggest that competitive context influences weapon-related traits at multiple levels of biological organization, potentially via phenotypic plasticity or selective processes. More broadly, our findings highlight how fine-scale ecological variation may contribute to the maintenance of trait diversity within and across species.

The Canadian Arctic Archipelago (CAA) is warming at an unprecedented rate, leading to sea ice loss and glacial retreat. Marine-terminating (tidewater) glaciers can fuel summertime marine productivity by delivering nutrient-rich deep waters via upwelling to the surface ocean. While the impact of glacier-induced upwelling has been well-studied in the context of phytoplankton and primary productivity, its effects on broader marine microbial communities remain poorly understood. We investigated how glacier-driven upwelling shapes marine microbial (bacterial and archaeal) communities across a series of sites in the CAA. At upwelling sites, the upper 50 m of the water column exhibited elevated nutrient concentrations and physical characteristics that resembled deeper waters, which were associated with differences in microbial community composition relative to non-upwelling sites. Our results indicate that upwelling influences microbial communities in surface waters in two ways. It directly introduces typically deeper-water-associated taxa into surface waters and reshapes ecological niches by enhancing nutrient supply and stimulating primary production, indirectly driving changes in microbial communities. The enrichment of Candidatus Nitrosopumilus, a deep water nitrifier, likely affects nitrogen cycling and raises the possibility of active nitrification in surface waters. Likewise, the increased abundance of taxa known to be associated with phytoplankton-derived organic matter in upwelling regions suggests an enhanced capacity to process organic matter generated from elevated primary productivity. Ultimately, as tidewater glaciers continue to retreat, the resulting changes in the glacially-driven upwelling regime will likely shift marine microbial communities towards assemblages adapted to less productive ecosystems, with implications for nutrient cycling in these systems. ImportanceClimate change has a disproportionate impact on the Arctic, with rising temperatures causing increased marine-terminating glacier retreat and changes in the marine water column structure. The consequent loss of the ability of these glaciers to upwell deep water to the surface ocean results in a reduction of nutrient delivery and mixing in these ecosystems. Previous work has highlighted the importance of marine-terminating glaciers in sustaining phytoplankton productivity during the summer season through this delivery of deep-water nutrients to the surface ocean. The impact of glacially-induced upwelling on marine bacterial and archaeal communities, however, remains underexplored. We found that in regions with glacially-driven upwelling, the surface ocean showed enrichment of phytoplankton-associated taxa and nitrifiers commonly associated with deep waters. This work underscores the role of glacially-driven upwelling in structuring both microbial communities and nutrient cycling, suggesting that glacier loss could reshape community composition and biogeochemical processes in a rapidly changing Arctic.

Multimodal communication plays a central role in animal behavior, particularly when individuals must integrate information from different sensory channels to make rapid decisions. In aquatic environments, chemical and visual cues differ markedly in their spatial and temporal properties, such that chemical signals may be constrained by limited spatial resolution and temporal instability, potentially requiring visual information to reliably guide social decisions. In decapod crustaceans, both cue types are known to mediate reproduction, yet their relative contribution to mate-location behavior remains unclear. Here, we tested how visual and chemical cues from males influence mate-location behavior in females of the prawn Macrobrachium rosenbergii. Females were placed in a central arena and exposed to four stimulus configurations combining visual cues (a life-size photograph of a male or a control background) and chemical cues (water from an aquarium with or without a male). Attraction was quantified as the time spent in each half of the arena. Females showed no directional preference when exposed to chemical cues alone or when visual and chemical cues were spatially incongruent. In contrast, females spent significantly more time near male-associated stimuli only when visual and chemical cues were spatially congruent. These results indicate that mate-location behavior in this species depends on multimodal integration with a strong contextual dependence on visual information, which appears to gate the effectiveness of chemical cues. Spatially congruent multimodal signals are therefore necessary to guide orientation during mate search, suggesting that disruption of visual or chemical information in aquaculture systems may impair mating efficiency.

Reef manta rays (Mobula alfredi) are threatened by fishing and other anthropogenic threats. Which, when coupled with conservative life history traits, have made this species vulnerable to extinction. Spatiotemporal ecological knowledge, such as site fidelity and visitation patterns to key aggregation sites, are imperative for effective conservation management of M. alfredi. A novel method of environmental sensing, remote underwater photo systems (RUPs), was employed to understand drivers of M. alfredi habitat use and resighting patterns. RUPs were deployed at four cleaning sites around Laamu Atoll, Maldives. Between March 2021 and May 2023, 455,458 photos were analysed. Generalised linear models revealed increases in M. alfredi presence in response to high chlorophyll-a concentrations, low illumination moon states, the Southwest Monsoon, and in the morning, while human presence had no effect. Branchial spot patterns allowed for 81 M. alfredi individuals to be identified, from 629 sightings, representing 51.59% of Laamu Atolls previously identified population (n = 157). Cleaning stations are visited more intensively during periods of increased productivity of the Southwest Monsoon, likely in response to greater foraging opportunities near the study areas. Additionally, moon state, used as a proxy for tidal strength, was associated with increased visitation during new moon periods, suggesting that weaker tidal states may facilitate presence. These data support integrating RUPs with observational surveys to improve inferences about habitat use and our understanding of cleaning sites frequented by M. alfredi. This study aims to inform the implementation of Laamu Atolls first marine protected area management plan.

Environmental DNA (eDNA) surveys are increasingly used to assess marine biodiversity and inform deep-sea environmental decision-making, including mineral resource management and fisheries oversight. Yet standard low-volume protocols inherited from coastal work may be inadequate at depth, and no quantitative framework links depth and ecosystem context to defensible filtration volume targets. We compiled 841 eDNA samples from eight expeditions across the North Atlantic, Wider Caribbean, and Pacific (surface to 4000 m) to quantify how recoverable eDNA scales with depth and surface productivity, and to derive depth- and productivity-aware sampling targets. Total eDNA concentration declined with depth as a power law, with attenuation exponents (b) modulated by surface productivity: most gradual in eutrophic waters (b = 0.67), intermediate in mesotrophic (b = 0.90), and steepest in oligotrophic systems (b = 1.25); volume-weighted models explained 66-88% of the variance. At a fixed extract-concentration target, required filtration volumes diverged ~7-fold between oligotrophic and eutrophic systems at 200 m and ~38-fold at 4000 m. Conventional Niskin sampling, therefore, undersamples deep-sea biodiversity, particularly in mid- to low-productivity systems. Among laboratory parameters, the assay-specific extract-concentration target exerted greater leverage on required volume than extraction efficiency or elution volume. Volume-aware sampling paired with optimized recovery should be routine in deep-sea eDNA surveys.

Many marine invertebrates are characterized by broad and highly plastic thermal limits, though the dynamic molecular mechanisms that enable extended thermal acclimation remain poorly understood. A classic example is the green crab (Carcinus maenas), which is a prolific and damaging non-indigenous species. Using a 22-day thermal exposure to cold (5{degrees}C), ambient (13{degrees}C), or warm (30{degrees}C) temperatures, we characterized plastic shifts in C. maenas performance using respirometry and time-to-right. We then used untargeted metabolomics and lipidomics analysis of heart tissues from days 4 and 22 to identify the molecular mechanisms underpinning plastic responses over time. Crabs at 30{degrees}C exhibited higher oxygen consumption rates than counterparts at 5{degrees}C. Interestingly, oxygen consumption rate increased over time at both temperatures, indicating thermal plasticity of aerobic respiration. Temperature-dependent metabolic reprogramming was employed by crabs to sustain aerobic respiration across temperature. Catabolism of branched-chain amino acids was important for energy production at elevated temperatures, while catabolism of arginine may have sustained the minimal energy needs of crabs exhibiting metabolic depression at cold temperatures. Righting response was positively correlated with temperature, and did not exhibit any changes over time. Lipidome remodeling consistent with homeoviscous adaptation could have enabled motor activity across temperature. Higher abundances of saturated and monounsaturated lipids likely provided structural integrity to cell membranes at 30{degrees}C, while lower abundances of these compounds may have enabled membrane fluidity at 5{degrees}C. Our work demonstrates the importance of ongoing molecular reprogramming in long-term acclimation, even when whole-animal physiology remains relatively stable. Summary StatementThis study demonstrates how the highly invasive green crab regulates metabolite and lipid pathways over time to maintain physiological performance across different temperatures.

Acropora spp. are the dominant reef-builders of the Indo-Pacific but are also amongst the most stress-sensitive corals. For these reasons, Acropora spp. have become the most studied of corals, two species (A. digitifera and A. millepora) often essentially serving as the basis for understanding molecular responses and processes across the sub-order Refertina and corals in general. The early development of these species has been well-characterised in terms of morphology and gene expression but as yet we have a limited understanding of how transcription is regulated during development. In "higher" animals (bilaterians) microRNAs (miRNAs) are critical regulators of gene expression but until now their involvement in coral development has not been investigated. Building on the existing developmental data for Acropora spp., we catalogued microRNAs (miRNAs) expressed during the early development of Acropora digitifera and profiled their expression in 21 stages from unfertilised eggs to 24h after treatment with a natural settlement cue (CCA chips). 157 miRNAs were recognised, many of which ([~]60%) were novel. These fell into three distinct groups, corresponding to three distinct developmental phases: (1) those present in eggs through to gastrulation (2) a larvally expressed group and (3) those expressed following settlement induction. Exposure of competent larvae to a natural settlement inducer resulted in major changes in the miRNA profile within 10 minutes, indicating that miRNAs may be particularly important in mediating the larva/polyp transition but are also likely to play important regulatory roles throughout early coral development in addition to possible roles in disease resistance.

While Antarctic terrestrial ecosystems support low metazoan diversity, the surrounding marine macrobenthos is rich. However, marine meiofauna remains historically neglected, leaving its diversity patterns unclear. In this study, we used 18S rRNA gene metabarcoding alongside an enhanced taxonomic annotation pipeline to characterize marine meiofauna diversity in the Ross Sea, comparing it to global datasets. We evaluated how depth, habitat type, and mesh size influence community structures to test if habitat heterogeneity drives diversity despite the harsh Southern Ocean conditions. Our results revealed exceptionally high diversity, with metazoans richness comparable to or higher than temperate regions. Although environmental variables had limited effects on taxonomic richness, they significantly shaped community composition, with habitat type explaining the highest proportion of variance. Interestingly, we detected several ASVs 100% identical to North Sea and North Atlantic sequences, likely reflecting the limited taxonomic resolution of the 18S marker rather than global dispersal (the "meiofaunal paradox"). Overall, these findings demonstrate that Antarctic marine sediments host rich meiofaunal communities where ecological processes operate similarly to other global regions, contrasting sharply with depauperate continental Antarctic ecosystems.

BackgroundAdult vestimentiferan tubeworms inhabiting hydrothermal vents and cold seeps lack a mouth and anus and rely entirely on organic matter produced by sulfur -oxidizing autotrophic bacterial symbionts in their trophosomes. These symbionts, which predominantly belong to the genus Proteobacteria, are acquired horizontally from the environment. However, the effects of rearing conditions that differ from natural habitats on the microbiome composition or abundance of these bacteria remain unclear. MethodsWe conducted a metagenomic analysis of Lamellibrachia satsuma reared in an aquarium under sulfide-supplemented and sulfide-free conditions. ResultsImmediately after collection, the microbiome was dominated by known symbionts within {gamma}-Proteobacteria, exhibiting low species diversity. After 6 months of rearing, the abundance of these symbionts significantly decreased under both conditions, whereas overall bacterial diversity increased. In particular, -Proteobacteria became more abundant under sulfide-supplemented conditions, while {delta}-Proteobacteria predominated in the absence of sulfide. Despite these changes, symbionts were not entirely lost, and the hosts survived for 6 months, likely due to their low metabolic rate. These findings suggest that the microbiome of L. satsuma can respond flexibly to changes in the rearing environment. They also indicate that the hosts metabolism can be maintained even with a smaller quantity of symbiotic bacteria.

Within species, individuals of the same age can differ in size. Previously, parental genetics, nutrition, space, and social interactions have been suggested to explain different growth rates. However, direct effects of larger individuals on the physiology and growth of smaller individuals are poorly understood. In this study, we investigated how larger individuals of the marine worm Platynereis dumerilii can impact the growth of smaller conspecifics. Comparing growth distributions in communally and individually reared worms, we show that larger worms suppress the growth of smaller ones. Furthermore, we were able to demonstrate that this suppression is chemically mediated. The chemical cue does not originate from faeces but is water soluble, stable for several days and smaller than 3 kDa. Our findings highlight the importance of non-reproduction related chemical signalling, showing evidence that dominant individuals can chemically suppress the growth of their conspecifics. This study provides new insights into how hierarchy can be established and maintained in a population and is particularly relevant for the growing community studying this model species.