Ecology and phylogeography of supratidal rockpool beetles

Abstract

Understanding how biotic, abiotic, and historical factors interact to shape species distribution is a central theme in ecology and evolution, with significant implications for biodiversity conservation. Coastal habitats, which are ecotones affected by marine and terrestrial conditions, include supratidal rockpools, which are extremely dynamic and fragmented environments, unpredictable in terms of water quantity, temperature, and salinity. Thus, these habitats sustain a community poor in species number but formed by unique organisms adapted to withstand harsh environmental conditions. The beetle genus Ochthebius, one of the few aquatic insect lineages capable of living in these environments, represents a good model to study the ecological and evolutionary processes that shape the occurrence, distribution, and genetic structure of organisms living in one of the planet's most inhospitable habitats, and thus predict responses to future environmental change scenarios. This thesis aims to determine the spatial patterns in the population genetic structure of three Ochthebius species (O. lejolsii, O. subinteger and O. quadricollis) inhabiting the supratidal rockpools of the Iberian Mediterranean coast, and to characterise their ecological niche and their patterns of co-occurrence. Moreover, the thesis identifies the abiotic factors that enable their coexistence in such extreme habitat and tries to disentangle the drivers behind the patterns of genetic diversity and population structure. In chapter 1, we analyse the spatio-temporal occurrence patterns of O. lejolisii and O. quadricollis, estimate and compare the environmental niches of adults and larvae of these species, and determine the environmental variables driving their distribution and abundance patterns. Negative correlations between the abundances of both species were found, suggesting spatial-temporal segregation based on their different environmental responses. These results suggest that subtle interspecific differences in their ecological niches, greater in larvae than in adults, and mainly related to the hydroperiod and pool salinity, could determine the storage effect as one of the main coexistence mechanisms at local and regional scales. In chapter 2, we explore the potential role of oceanic currents and fronts in shaping the contemporary genetic structure of the three species in the western Mediterranean basin, combining population genetics and biophysical modelling. Results showed that the biophysical model based on ocean currents and fronts in the Western Mediterranean predicted the observed genetic structure better than simple isolation by distance in both species. Strong genetic discontinuities were observed in populations separated by the Ibiza Channel and the interface between the Alborán Sea's anticyclonic gyres, suggesting these oceanographic features act as significant barriers to dispersal. In chapter 3, we examine the genome of Ochthebius and develop and characterise microsatellite markers (SSR) loci through next-generation high-throughput sequencing (NGS) to facilitate population identification and genetic structure studies, valid for the three species of Ochthebius coexisting in the Iberian Mediterranean. Finally, in chapter 4, we determine the genetic diversity and population structure of these species using microsatellites as molecular markers, in order to elucidate population connectivity, and discern the underlying factors shaping their patterns of population genetic differentiation. The results revealed low genetic diversity and high connectivity and gene flow among populations of these Ochthebius species, likely due to passive dispersal mechanisms. O. quadricollis was the only one presenting a marked genetic structure, with significant genetic break at the Alboran gyres interfaceand isolation by distance without significant influence from the environmental factors explored at the studied spatial scale. Overall, the results of this thesis make a significant contribution to the understanding of the abiotic factors that enable the coexistence and determine the genetic structure in organisms inhabiting supratidal rockpool, integrating ecological studies and phylogeography, and enhancing our understanding of their distribution patterns and genetic variation, which reflect their distinct evolutionary histories, microhabitat preferences, and dispersal mechanisms.