Ancestry and amphidromy in island freshwater fish faunas

Amphidromy is a frequent attribute of fish faunas of remote islands, where the presence of freshwater fishes creates perplexity as to how such remote places came to have ‘freshwater fish’. Not infrequently, the fact that amphidromous species spend part of their lives in the sea is invoked as indicating that such species have marine ancestries, and this is the implied explanation for presence of freshwater fishes on islands. However, examination of the ranges of some amphidromous species, and of the distributions of genera to which amphidromous fishes belong, strongly suggests that amphidromy, especially in the gobies of the subfamily Sicydiinae, is a widespread, probably ancestral trait. Rather than such amphidromous fish having a marine ancestry, their marine life stages are themselves the likely key element in explaining their distributions.