Abstract

In biogeography, related lineages often share similar range sizes and species richness due to inherited traits and shared geographic history. This expectation is based on phylogenetic niche conservatism and shared dispersal constraints. However, within the subfamily Laevilitorininae, this pattern is not observed: Laevilitorina shows broader distributions and higher species richness than Laevilacunaria despite similar developmental mode and habitat preferences. Combining multilocus genetic data, radular morphology and model-based historical biogeography, we aim to understand the origins of these disparities in range extent. The paraphyly detected in both Laevilitorina and Laevilacunaria genus indicates a more complex history than a simple older-younger genus contrast, with age and diversification distributed across four clades. Our dated analyses place the time to the most recent common ancestor (TMRCA) of Laevilitorininae at 52.2 Mya, with diversification within clades from the Miocene to Pleistocene. The sister genera show process-level contrasts: in Laevilitorina, broad ranges in some lineages are consistent with intermittent, macroalgal raft-mediated connectivity, whereas Laevilacunaria exhibits restricted ranges consistent with rare, storm-facilitated rafting that enables colonization without sustained gene flow. Overall, our findings support how stochastic long-distance dispersal and establishment can shape biogeographic patterns in the Southern Ocean and underscore the need to update systematic classifications.