Migration, speciation, and the evolution of diadromy in anguillid eels

Recent findings and hypotheses about the migration, spawning ecology, phylogenetic relationships, and possible mechanisms of speciation of anguillid eels are overviewed. The offshore distribution and otolith microstructure of small leptocephali suggest that the Japanese eel Anguilla japonica may spawn at seamounts west of the Mariana Islands in the western North Pacific during the new moon of each month from April to November. Some temperate eels have been found to remain in coastal areas after recruitment without a freshwater growth phase (ocean residents or sea eels), showing flexible patterns of migratory histories. Recent molecular phylogenetic studies suggest that the tropical eel Anguilla borneensis from the Borneo Island region is the most ancestral species. Every eel species or population has its own migration loop (migration route or life cycle) that connects their spawning area and growth habitats. Spatial and temporal shifts in these migration loops could cause separation into subpopulations, or speciation. Therefore, the large-scale migration of temperate eels probably evolved from local migrations of tropical eels as a result of long-distance dispersal of leptocephali from spawning sites in tropical waters of low latitude to temperate growth habitats at higher latitudes.