Killifishes are widely distributed across the globe and have successfully colonized a range of habitats, from coastal lagoons to rivers, streams, seasonal water ponds, as well as remote tropical islands, such as the Seychelles (Murphy & Collier, 1997). While annual killifish can colonize arid savanna environments by virtue of their desiccation-resistant embryos that can sustain life during monthslong dry seasons (Cellerino et al., 2016;Dolfi et al., 2019;Hu et al., 2020), we know little about the biotic and abiotic factors that enabled killifish to colonize and adapt in islands that are hundreds of kilometres distant from the mainland, such as Madagascar and the Seychelles.

How freshwater fish colonize remote islands remains an evolutionary puzzle. Tectonic drift and trans-oceanic dispersal models have been proposed as possible alternative mechanisms. Integrating dating of known tectonic events with population genetics and experimental test of salinity tolerance in the Seychelles islands golden panchax (Pachypanchax playfairii), we found support for trans-oceanic dispersal being the most likely scenario. At the macroevolutionary scale, the non-annual killifish golden panchax shows stronger genome-wide purifying selection compared to annual killifishes from continental Africa. Reconstructing past demographies in isolated golden panchax populations provides support for decline in effective population size, which could have allowed slightly deleterious mutations to segregate in the population. Unlike annual killifishes, where relaxed selection preferentially targets aging-related genes, relaxation of purifying selection in golden panchax affects genes involved in developmental processes, including fgf10.