Cryptic speciation in molluscs: to be or not to be the same species?

Many species of sea slugs are regarded as circumglobal occurring all around the World or across vast regions. Yet, these broad and global distributions are not consistent with the idea of distinct biogeographic regions in the Oceans and pose serious challenges to understand how species disperse across vast spans of open water. This is the case of the flamboyant and beautiful tropical species Micromello undatus known across the Atlantic, Indian, and Pacific Oceans and several Indo-West pacific nudibranchs of the family Phyllidiidae  that are extremely similar. Most assumptions of conspecificity are based on morphological similarities and remain to be tested using DNA sequence data. We want to use molecular phylogenetics and population genetic methods to (1) test whether these case studies are one or several species, (2) how the various species or populations relate to each other, (3) where were the ancestral distribution ranges, and (4) what processes of marine speciation and biogeography explain the present distribution of these species.