Description: Genetic drift and non-random mating seldom influence species with large breeding populations and high dispersal potential, characterized by unstructured gene pool and panmixia at a scale lower than the minimum dispersal range of individuals. In the present study, a set of nine microsatellite markers was developed and used to investigate the spatio-temporal genetic patterns of the holoplanktonic jellyfish Pelagia noctiluca (Scyphozoa) in the Southern Tyrrhenian Sea. Homozygote excess was detected at eight loci, and individuals exhibited intra-population relatedness higher than expected by chance in at least three samples. This result was supported by the presence of siblings in at least 5 out 8 samples, 4 of which contained full-sib in addition to half-sib dyads. Having tested and ruled out alternative explanations as null alleles, our results suggest the influence of reproductive and behavioural features in shaping the genetic structure of P. noctiluca, as outcomes of population genetics analyses pointed out. Indeed, the genetic differentiation among populations was globally small but highlighted: a) a spatial genetic patchiness uncorrelated with distance between sampling locations, and b) a significant genetic heterogeneity between samples collected in the same locations in different years. Therefore, despite its extreme dispersal potential, P. noctiluca does not maintain a single homogenous population, but rather these jellyfish appear to have intra-bloom localized recruitment and/or individual cohesiveness, whereby siblings more likely swarm together as a single group and remain close after spawning events. These findings provide the first evidence of family structures and consequent genetic patchiness in a species with highly dispersive potential throughout its whole life cycle, contributing to understanding the patterns of dispersal and connectivity in marine environments.

Description: Capitate hydrozoans are a morphologically and ecologically diverse hydrozoan suborder, currently including about 200 species.Being grouped in two clades, Corynida and Zancleida, these hydrozoans still show a number of taxonomic uncertainties at the species,genus and family levels. Many Capitata species established symbiotic relationships with other benthic organisms, including bryozoans,other cnidarians, molluscs and poriferans, as well as with planktonic dinoflagellates for mixotrophic relationships and with bacteria forthiotrophic ectosymbioses. Our study aimed at providing an updated and comprehensive phylogeny reconstruction of the suborder, atmodelling the evolution of selected morphological and ecological characters, and at testing evolutionary relationships between the sym-biotic lifestyle and the other characters, by integrating taxonomic, ecological and evolutionary data. The phylogenetic hypotheses herepresented shed light on the evolutionary relationships within Capitata, with most families and genera being recovered as monophyletic.The genusZancleaand family Zancleidae, however, were divided into four divergent clades, requiring the establishment of the newgenusApatizancleaand the new combinations for species inZancleaandHalocorynegenera. The ancestral state reconstructionsrevealed that symbiosis arose multiple times in the evolutionary history of the Capitata, and that homoplasy is a common phenomenonin the group. Correlations were found between the evolution of symbiosis and morphological characters, such as the perisarc. Overall,our results highlighted that the use of genetic data and a complete knowledge of the life cycles are strongly needed to disentangle taxo-nomic and systematic issues in capitate hydrozoans. Finally, the colonization of tropical habitat appears to have influenced the evolu-tion of a symbiotic lifestyle, playing important roles in the evolution of the group.