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 med iated movements which occur at predictable life history phases in each diadromous species, they involve most members of a species' populatio ns, and they are usually obligatory. Around 250 fish species are regar ded as diadromous. A review of the life history strategies amongst fam ilies of fishes that include diadromous species provides little suppor t for a suggested scenario for their evolution that involves: (1) evol ution 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 orig ins amongst diverse fish groups. There is increasing confidence that b ehavioural characteristics of animals are heuristic in generating and interpreting phylogenies. However, examination of fishes shows wide va riability of diadromous life histories within closely related families and genera, within species, and there is even ontogenetic variation i n patterns of behaviour by individual fish. In addition, there is mult iple loss of diadromy in many diadromous fish species in which the lif e history becomes restricted to fresh waters. This variation suggests that diadromy is a behavioural character of dubious worth in determini ng phylogenetic relationships. Moreover, it appears to have been an an cestral condition in some fish families, such as Anguillidae, Salmonid ae, Galaxiidae, Osmeridae, and others, and perhaps in the whole salmon oid/osmeroid/galaxioid complex of families. This, too, makes diadromy of dubious worth in phylogenetic analysis.