Abstract
Species delimitation is central to understanding biodiversity and its conservation, yet genetic divergence among sister lineages is often insufficient to demonstrate reproductive isolation or to resolve speciation unambiguously. This limitation is especially pronounced in marine invertebrates such as corals. Their slowly evolving genomes, cryptic diversity, and complex reproductive traits can obscure species boundaries. These challenges also characterize the Paramuricea-species complex occurring in the Iberian-Atlantic, whose members, including those studied here, commonly dominate deep coral forests. Within this complex, sympatrically occurring yellow and purple morphs exhibit little mitochondrial differentiation, despite evidence of partial genetic structuring. By characterizing gametogenesis, spawning time, and early life-history stages, we reveal pronounced prezygotic barriers between the yellow (broadcast spawning) and purple (surface-brooding) morphs, including consistent multi-year temporal separation and strongly contrasting fertilization environments. These differences extend into embryonic development, larval morphology, behavior, and settlement dynamics and reflect divergent dispersal strategies. Overall, our results provide direct biological evidence that the two morphs represent independent evolutionary lineages, supporting their recognition as distinct species. This model system represents a contemporary example of ecological speciation and may serve as a powerful model for future research on the genetic coupling between reproductive modes and life-history traits.