Simultaneous maturation of two discrete oocyte batches in ayu

Abstract

Ayu Plecoglossus altivelis including amphidromous and landlocked populations, are known for batch spawning. Unusual clutches at ovulation were observed from rearing stocks that were naturally induced to sexual maturation. Two peaks occurred simultaneously in the frequency distribution of egg size, compared to normal fish, which have clutches with unimodal size distribution. The eggs in the small size class could be fertilized, and small-sized larvae hatched from these eggs. To explain the alteration in egg size through divergence from amphidromous to landlocked populations, the role of batch allocation among oocytes is discussed.