Reanalysis of aligned sequence data from the mitochondrial cytochrome oxidase I gene region across Crustacea has generated a number of new insights into how we can interpret genetic variation in these species. Some insights, such as the relationship between genetic diversity and the latitudinal biodiversity gradient, have been predicted in earlier studies. Other patterns – such as the bias toward more ‘rare’ allelic variants than expected under null models – across the 255 data sets analyzed here suggest that a more nuanced understanding of molecular evolution is necessary for interpreting the phylogeography and population biology of species in this group.