Perspectives on the Grey Zone of Species Delimitation With Notes on Invertebrates in the Marine Environment

Defining species boundaries, or delimiting species, is a complex and often difficult task. Indeed, when such studies incorporate approaches that consider evolutionary mechanisms, there is much to be learned about species diversity and how the processes that play critical roles in speciation can impact species delineation. In 2021, a virtual workshop on species delimitation was held at the Smithsonian Institution National Museum of Natural History to train natural history scientists and taxonomists on the appropriate analytical tools that can be used to help delimit species when using molecular data. This perspective highlights some of the main themes discussed during that workshop while detailing three processes that can challenge any species delimitation study. Specifically, we discuss incomplete lineage sorting, gene flow, and population structure when delimiting species boundaries using molecular data. We highlight empirical studies and methodological approaches that have successfully met these challenges under various scenarios. Finally, we provide recommendations and considerations for undertaking species delimitation studies in a variety of taxa. To this end, we recommend that taxonomists fully embrace process-based species delimitation, which can provide important insights into speciation in their study systems. For those developing analytical approaches, we hope they consider incorporating less well-known taxa, such as marine invertebrates, into method testing. Marine invertebrates encompass many dark taxa across the tree of life yet represent the majority of animal phyla, many of which are vulnerable to extinction due to global ocean change. Thus, advancing species delimitation to address taxonomic revisions in these organisms will support conservation decisions on keystone ecosystems. Furthermore, the diversity of their life history strategies, the lack of obvious barriers to gene flow in the ocean environment, and their occurrence in isolated habitat patches can better inform our knowledge of speciation and the evolutionary processes that play a role in generating diversity in nature.