Many aquatic vascular plants show cosmopolitan geographical distributions at taxonomic levels ranging from family to species. The unusually widespread distributions of hydrophytes have intrigued botanists since De Candolle, who thought the pattern arose by multiple convergent origins. However, Darwin believed that the wide distribution of hydrophytes was due to long-distance avian dispersal, an explanation that continues to hold favor. Human introductions over long distances are also possible, especially in Lemnaceae where the minute plants are often transported with aquarium stock. We suggest that the great age of many aquatic families coupled with the effects of continental drift (its phytogeographical implications elucidated relatively recently), should also be considered as possible factors involved in the genesis of cosmopolitan hydrophyte distributions. Examples of both temperate and tropical intercontinental disjunctions are evident in the aquatic families Aponogetonaceae, Callitrichaceae, Ceratophyllaceae, Haloragaceae, Hydrocharitaceae, Lemnaceae, Nymphaeaceae and Podostemaceae. Analogous interoceanic disjunctions also occur in marine angiosperm (seagrass) families such as Cymodoceaceae, Posidoniaceae and Zosteraceae. We evaluated transoceanic disjunctions in cosmopolitan hydrophytes using molecular data to estimate divergence times for various aquatic taxa. Unlike oceanic islands which typically indicate recent plant colonization via long-distance dispersal, the estimated divergence times for disjunct aquatic plants are much more ancient in comparison. Our findings indicate that disjunctions in several aquatic groups are consistent with phytogeographical hypotheses invoking continental drift, and dismiss the possibility of recent long-distance dispersal by birds or humans. Phylogenetic data of some cosmopolitan hydrophytes also indicate phytogeographical patterns corresponding to age-area models. However, there are several instances (mainly in Lemnaceae) where widespread aquatic species exhibit much less molecular diversity than expected, where recent secondary dispersal is highly unlikely, but cannot be ruled out.

Key words: angiosperms, aquatic plants, molecular clock, phytogeography