The Oscarella sponge genus: a hidden yet colorful diversity

The identification and ranking of new taxa are crucial for advancing our understanding of biodiversity. However, assigning samples to new taxa requires a cautious and rigorous approach. Historically, taxonomy has heavily relied on morphological traits, which can be subjective and may not always correspond to underlying genetic divergence. This challenge is especially pronounced in organisms with limited diagnostic morphological traits. A prime example is the poriferans of the genus Oscarella (Homoscleromorpha), where species delimitation often depends on insufficient morphological features and genetic data. In this study, we addressed these challenges by integrating a comprehensive genetic dataset, including both mitochondrial and nuclear markers, with systematic photographic documentation of 192 specimens collected in Western Mediterranean Sea (Bay of Marseille, France). This method allowed us to provide a more robust assessment of Oscarella diversity, leading to the identification of four species new to science. In addition, our phylogenetic multigene analyses enabled us to propose a meaningful evolutionary scenario for color polymorphism. Additionally, our data showed the critical limitation in current methodologies for Oscarella studies, such as the low resolution of the widely used cytochrome c oxidase subunit I (cox1/COI) gene, the lack of genetic data for many species, and the insufficient information regarding their geographical distribution. These findings underscore the need for more rigorous genetic frameworks and comprehensive datasets to enhance species delimitation and taxonomy.