The “marine-speciation paradox” describes the observation that there is still considerable speciation of marine organisms in a vast ocean with few geographic barriers and consequently few barriers to gene flow. For marine mammals, unique lineages still manage to evolve despite this lack of geographic barriers and additionally, the documented capability of cross-species hybrids. To illustrate this point, approximately 20% of cetacean species and several pinnipeds have been recorded to produce hybrids, with some even capable of producing backcrossed offspring. This paradigm has produced an interesting field of research focused on identifying which factors limit gene flow and thus, permit unique lineages to arise. Observations of marine mammal hybrids have been documented through at-sea observations but also from museum specimens with anatomical anomalies suggesting hybridization. Here, we propose to use whole genomes from ringed, harbour and gray seals to investigate hybridization and introgression between three marine mammals with overlapping ranges in the Baltic Sea. We will first first screen genomes derived from field biopsies to investigate background signatures of introgression between all three species. Next, we will investigate introgression between genomes derived from museum specimens. These specimens have been flagged as hybrids due to anatomical abnormalities, particularly, in dental morphology - which has previously been used as a discriminant proxy for seal hybridization. We will characterize the contribution of each parental lineage to the offspring, assess reproductive success of hybrids by calculating each sample's filial generation and determine whether hybridization increases the offspring’s fitness by annotating the impact of variants on protein function. This project will inform the degree to which marine mammals hybridize in the wild and assess whether there is any phenotypic advantage to hybridization. Overarchingly, it will contribute to the broader understanding of the marine-speciation paradox and how marine organisms speciate despite the lack of geographical barriers and ability to hybridize.