Adaptive Significance of Inter- and Intrapopulational Egg Size Variation in Ayu Plecoglossus altivelis (Osmeridae)

Abstract

We examined the reproductive traits and early survival of ayu, Plecoglossus altivelis (Osmeridae), from amphidromous and land-locked populations under controlled rearing conditions to evaluate the adaptive significance of egg size variation. Spawning was earlier for land-locked than for amphidromous ayu and was earlier for larger individuals within a population. Amphidromous fish produced larger eggs than land-locked fish; and for both forms, smaller individuals produced larger eggs within a population. Water temperature declined through the spawning season, and both salinity and low water temperature markedly damaged small offspring from small eggs. The large egg size of amphidromous fish promotes salinity tolerance of offspring, so lower salinity may have resulted in evolution of smaller eggs in land-locked populations. Smaller individuals within a population had larger egg sizes, which may be an adaptive response to enhance offspring survival at low water temperatures under changing environments.