One of the most remarkable types of migration found in animals is diadromy. Diadromy is a life-history behavior in which individuals move between oceans and freshwater habitats for feeding and reproduction. Diadromous fishes include iconic species such as salmon, sturgeon, eels, and shad, and have long fascinated biologists because they undergo extraordinary physiological modifications to survive in very different habitats. We are investigating the evolutionary origins of diadromy: why, where, and when does this behavior evolve? Does diadromy facilitate macroevolutionary transitions between marine and freshwaters? How does diadromy influence diversification? What are the ecological differences between different modes of diadromy (catadromy and anadromy), marine and freshwater fishes. Are there differences in genetic structure between diadromous and non-diadromous species?
Thus far our work on diadromy has focused on Clupeiformes, a clade that includes some of the world’s most important fisheries, such as sardines, herring, shads, and anchovies. Clupeiformes are an ideal system for studying diadromy because there are anadromous, catadromous, fully marine and fully freshwater species in this group. Using a time-calibrated phylogeny we rejected a long-standing hypothesis that suggested primary productivity was the major determinant for the origins of the different modes of diadromy (Bloom & Lovejoy, 2014). We also showed that there is no evidence that diadromy is an intermediate step in marine/freshwater transitions. This study helps clarify the evolutionary origins of diadromy and the framework developed in our study establishes a clear approach to test hypotheses about the origins of diadromy in other diadromous lineages. By integrating phylogeny with ecology we have also developed an ideal system for future work investigating the evolutionary ecology of migrating versus non-migrating lineages.