The evolution of diadromy in fishes (revisited) and its place in phylogenetic analysis

Diadromy is a term used to describe migrations of fishes between fresh waters and the sea; these migrations are regular, physiologically mediated movements which occur at predictable life history phases in each diadromous species, they involve most members of a species' populations, and they are usually obligatory. Around 250 fish species are regarded as diadromous. A review of the life history strategies amongst families of fishes that include diadromous species provides little support for a suggested scenario for their evolution that involves: (1) evolution of anadromy via amphidromy from fishes of marine origins, and (2) evolution of catadromy through amphidromy from fishes of freshwater origins, even though these scenarios seem intuitively reasonable. The various forms of diadromy appear to have had multiple independent origins amongst diverse fish groups. There is increasing confidence that behavioural characteristics of animals are heuristic in generating and interpreting phylogenies. However, examination of fishes shows wide variability of diadromous life histories within closely related families and genera, within species, and there is even ontogenetic variation in patterns of behaviour by individual fish. In addition, there is multiple loss of diadromy in many diadromous fish species in which the life history becomes restricted to fresh waters. This variation suggests that diadromy is a behavioural character of dubious worth in determining phylogenetic relationships. Moreover, it appears to have been an ancestral condition in some fish families, such as Anguillidae, Salmonidae, Galaxiidae, Osmeridae, and others, and perhaps in the whole salmonoid/osmeroid/galaxioid complex of families. This, too, makes diadromy of dubious worth in phylogenetic analysis.