Formal establishment of Delphinus mediterraneus sp. nov.: Reticulate speciation within the evolutionary laboratory of the Mediterranean Sea

Abstract

The understanding of the evolutionary dynamics of the genus Delphinus within the Mediterranean Sea has long been confounded by the orthodox paradigm of global panmixia. This study dismantles this assumption, demonstrating that the Strait of Gibraltar acts as a strict phylogenetic barrier, effectively isolating the entire Mediterranean Sea complex from Atlantic oceanic populations. By integrating population mitogenomics, isotopic ecology, and comparative three-dimensional (3D) geometric morphometrics, we formalize the designation of Delphinus mediterraneus sp. nov., a profoundly derived endemic entity (Holotype: Inv. 684, complete skeleton, Museo Civico di Storia Naturale di Trieste). Unlike its oceanic congeners, the skull of D. mediterraneus does not conform to classic pelagic or coastal morphotypes; rather, it exhibits an intermediate autapomorphic architecture characterized by an occipital narrowing and the highest shape variability documented globally.Ecological investigations indicate that this phenotype is the product of a drastic niche shift imposed by the basin's intrinsic oligotrophy. Such an ecological squeeze has forced the species into a pronounced trophic downgrading, leading its isotopic niche to overlap by more than 80% with that of the striped dolphin (Stenella mediterranea). To counteract dramatic demographic bottlenecks, D. mediterraneus has evolved a formidable genomic permeability: introgressive hybridization with the sympatric genera Stenella and Tursiopsdoes not represent accidental genetic pollution, but rather an intrinsic and structural mechanism of genetic rescue driven by reticulate speciation.Building upon this innovative evolutionary framework, the Black Sea ecotype is formally reclassified as Delphinus mediterraneus ponticus subsp. nov., identifiable through pronounced insular dwarfism that evolved in response to the severe constraints of the semienclosed basin, without any clinal intergradation with the parent stock. The establishment of D. mediterraneus radically redefines the status of the apex pelagic predators of the MareNostrum, demonstrating that their evolutionary resilience is dictated not by impermeable reproductive barriers, but by their vital participation in a complex network of genomic exchange.