Mammalian returns to the sea reveal broad genomic slowing rather than a fixed adaptive toolkit

Marine mammals — cetaceans, pinnipeds, sirenians, sea otters and polar bears — returned to the sea independently, yet whether their genomes converged on a shared adaptive programme or shifted in a common direction without a fixed toolkit has remained unclear. Here we separate marine specialization from general aquatic dependence across 302 mammals and 17,432 protein-coding genes and show that the dominant genomic signature of marine life is widespread evolutionary slowing, not acceleration: of 1,559 marine-associated genes, nearly 88% evolved more slowly, and this slow-direction bias persisted (98%) after removing cetaceans. Compact gene fingerprints that distinguish marine identity combine fast-rate remodeling of body-surface and sensory genes with slow-rate constraint on blood, metabolic and DNA-repair genes, but these fingerprints are sharpened by cetaceans and do not preserve a fixed functional toolkit across lineages. Species-level and ancestral-branch decompositions reveal that different marine mammals assembled marine-like genomic states through distinct gene combinations. Mammalian marine convergence is therefore directional rather than modular: a broad constraint landscape resolved into clade-weighted genomic fingerprints.