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 gardens. 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, behaviour, 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.

Environmental gradients can influence morpho-physiological and life-history differences in natural populations. It is unclear, however, to what extent such gradients can also modulate phenotypic differences in other organismal characteristics such as the structure and function of host-associated microbial communities. In this work, we addressed this question by assessing intra-specific variation in the diversity, structure and function of environmental-associated (sediment and water) and animal-associated (skin and gut) microbiota along an environmental gradient of pollution in one of the most urbanized coastal areas in the world. Using the tropical sea cucumber Holothuria leucospilota, we tested the interplay between deterministic (e.g., environmental/host filtering) and stochastic (e.g., random microbial dispersal) processes underpinning host-microbiome interactions and microbial assemblages. Overall, our results indicate that microbial communities are complex and vary in structure and function between the environment and the animal hosts. However, these differences are modulated by the level of pollution across the gradient with marked clines in alpha and beta diversity. Yet, such clines and overall differences showed opposite directions when comparing environmental- and animal-associated microbial communities. In the sea cucumbers, intrinsic characteristics (e.g., body compartments, biochemistry composition, immune systems), may underpin the observed intra-individual differences in the associated microbiomes, and their divergence from the environmental source. Such regulation favours specific microbial functional pathways that may play an important role in the survival and physiology of the animal host, particularly in high polluted areas. These findings suggest that the interplay between both, environmental and host filtering underpins microbial community assembly in H. leucospilota along the pollution gradient in Hong Kong.