Life history traits influence marine species dispersal and habitat colonization. Medusozoans (jellyfish and siphonophores) exhibit diverse life cycles, evolved from an ancestral cycle alternating between a benthic polyp and a pelagic medusa. Despite their ecological importance, factors shaping medusozoan distribution remain poorly understood. By integrating metabarcoding and environmental data from the Tara Oceans expedition with life history traits, we provide global evidence supporting the longstanding hypothesis that benthic polyp presence/absence is a key factor influencing the distribution and abundance of planktonic medusozoans in the surface ocean. We inferred on a time-calibrated phylogeny of Medusozoa multiple transitions to a fully planktonic (holoplanktonic) life cycle, either through polyp loss, acquisition of drifting polyps, or development of polyps parasitizing pelagic organisms. We could associate each transition with a shift toward offshore habitats and the emergence of globally dominant Operational Taxonomic Units (OTUs), whose abundance far exceeds that of any nonholoplanktonic medusozoans in the planktonic realm. The prevalence of holoplanktonic medusozoans in terms of abundance and diversity is broadly observed in coastal and offshore environments, peaking over greater bathymetric depths in tropical and subtropical regions. We show that holoplanktonic and nonholoplanktonic groups interact with distinct yet compositionally similar planktonic communities. Holoplanktonic OTUs occupy more peripheral positions in a plankton interactome, suggesting greater flexibility in biotic interactions, an adaptive trait in rapidly changing planktonic ecosystems. These findings highlight how life cycle evolution shaped the global distribution of medusozoans and suggest that variations in life history may significantly influence how medusozoans respond to global environmental changes.

ABSTRACT Coastal marine megafauna faces increasing threats from habitat degradation, climate change, and human activities, making conservation efforts crucial for their survival. The New Caledonian dugong population was reclassified as Endangered on the IUCN Red List in 2021, following research on its abundance and genetic diversity. With fewer than 800 individuals estimated between 2008 and 2012, urgent conservation measures are needed to prevent further decline. Modern genetic tools provide critical insights into spatial genetic differentiation and gene flow across New Caledonia's extensive lagoon habitats. In this study, we analyzed 66 skin samples from live and stranded dugongs collected between 2003 and 2023, using a multiscale genetic approach. We examined mitochondrial DNA control region sequences at the Indo‐Pacific level, 13 microsatellite loci to compare New Caledonian and Australian populations, and 2499 single nucleotide polymorphisms (SNPs) to assess fine‐scale structure within New Caledonia. Our findings confirm that the New Caledonian dugong population has extremely low genetic diversity and is highly differentiated from its Australian counterpart. The effective population size ( N e ) was critically low, ranging between 95 and 160 individuals, depending on the analytical approach. Within New Caledonia, we identified two genetically distinct clusters along the west coast, north and south of Bourail, a division consistent with previous satellite tracking studies showing no movement across this natural boundary. These findings highlight the urgency of conservation action and suggest that the population's isolation and low genetic diversity may warrant an upgrade to Critically Endangered status.

Climate change is placing unprecedented stress on ecosystems, leading to extreme biodiversity loss. In this context, estimates of population connectivity (i.e., the exchange of individuals between geographically separated populations, a main driver of population recovery) are essential for designing effective conservation strategies. In coastal ecosystems, connectivity is primarily driven by larval dispersal, which depends on ocean currents and species-specific life-history traits. Both factors vary over time, with long-term shifts driven by climate change and shorter-term fluctuations due to meteorological variability. Yet, most empirical studies estimate connectivity from single-time-point datasets, overlooking potential changes in time. This thesis aims to investigate the temporal variability of connectivity in coastal environments. Using Mediterranean gorgonians as model species, I reanalyzed historical samples (2008 to 2014) alongside recent ones (2022 or 2023), collected by size class as a proxy for age. All individuals were genotyped using ddRADseq, generating a time-structured genomic dataset spanning approximately 20 years and enabling the investigation of such temporal changes. As a prerequisite, in the first chapter, I used simulated SNP datasets to evaluate the reliability of commonly used methods to infer connectivity networks from genomic data, under different demographic and sampling scenarios. Worryingly, none of the tested methods proved reliable under the low sampling rates classically used in empirical marine studies. These findings informed the methodological choices in the next chapters. The second chapter investigates connectivity of Paramuricea clavata populations in the Ligurian Sea. The results reveal a strong genetic structure and an absence of first-generation migrants among samples, across all size classes and sampling years. These results indicate a lack of contemporary connectivity, challenging earlier assumptions of significant larval dispersal in this species, and showing that recovery after disturbance likely depend entirely on self-recruitment. The third chapter focuses on Eunicella singularis in the northwestern Mediterranean Sea. I explore temporal variation of differentiation between population, genetic identity of the populations, and inter-population recruitment patterns. While the genetic identity of most populations stays stable over time, some low-density populations exhibit significant temporal shifts that influence broader patterns of genetic differentiation. Furthermore, first-generation migrants were detected in some size classes only, indicating episodic external recruitment events. These findings reveal the occurrence of rare, high-intensity connectivity events and emphasize their lasting impact, particularly in fragmented or declining populations, challenging the assumption of temporal stability underlying the usual snapshot studies. Overall, the thesis challenges the static view of marine connectivity by showing that, while population identities are often stable, rare connectivity events can have long lasting and significant effects. These events may become increasingly important under intensified climatic stress, habitat fragmentation, and mass mortality of adult populations. These findings contribute to a deeper understanding of marine connectivity and emphasize the need for more temporally explicit frameworks in ecological research and conservation strategies.

A model organism in developmental biology is defined by its experimental amenability and by resources created for the model system by the scientific community. For the most powerful invertebrate models, the combination of both has already yielded a thorough understanding of developmental processes. However, the number of developmental model systems is still limited, and their phylogenetic distribution heavily biased. Members of one of the largest animal lineages, the Spiralia, for example, have long been neglected. In order to remedy this shortcoming, we have produced a detailed developmental transcriptome for the bivalve mollusk Mytilus galloprovincialis, and have expanded the list of experimental protocols available for this species. Our high-quality transcriptome allowed us to identify transcriptomic signatures of developmental progression and to perform a first comparison with another bivalve mollusk: the Pacific oyster Crassostrea gigas. To allow co-labelling studies, we optimized and combined protocols for immunohistochemistry and hybridization chain reaction to create high-resolution co-expression maps of developmental genes. The resources and protocols described here represent an enormous boost for the establishment of Mytilus galloprovincialis as an alternative model system in developmental biology.

In marine environments, life cycle strategies strongly impact species dispersal and their ability to colonize new habitats. Pelagic medusozoans (jellyfish and siphonophores) exhibit various reproductive strategies, variations of meroplanktonic and holoplanktonic life cycles. In the ancestral meroplanktonic life cycle, a benthic polyp stage alternates with a pelagic medusa stage. During the course of evolution, some medusozoans lost their benthic stage, leading to a holoplanktonic life cycle. The ecological consequences of these losses have not been addressed at global scale. Here, integrating metabarcoding and environmental data from Tara Oceans into a phylogenetic framework, we show that each convergent transition toward a holoplanktonic life cycle is associated with a more offshore distribution compared to meroplanktonic medusozoans. Our analyses showed that holoplanktonic medusozoans are more globally distributed and relatively more abundant than meroplanktonic medusozoans, although they are less diversified and occupy a more peripheral position in a global plankton community interactome. This suggests that holoplanktonic medusozoans have acquired a greater tolerance to biotic and abiotic conditions. Overall, our results demonstrate the relationship between medusozoan life cycles, distribution, and biotic interactions, suggesting that the loss of the benthic stage promoted colonization of the open ocean.