Diversified Egg and Clutch Sizes Among Local Populations of the Fresh-Water Prawn Macrobrachium Nipponense (De Haan)

Egg and clutch sizes of the fresh-water prawn Macrobrachium nipponense (de Haan) varied remarkably among local populations, corresponding to the hydrogeographic features of their habitats; spawning of many small eggs (approximately 0.05 mm3 in volume per egg) in a single clutch at river mouths, a few but large eggs (approximately 0.1 mm3) in inland fresh waters, and a moderate number of intermediate-sized eggs in brackish-water lagoons. The weight of egg mass deposited in one spawning also varied among populations from 15.6-24.3% in dry weight ratio to the female body weight. This change seemed to come largely from temporary environmental factors. Accordingly, the variation of clutch size among populations was partly due to temporary environmental factors, and partly due to the difference in egg size. This species is considered to be splitting into local populations with distinct reproductive traits through reproductive isolation in patchily distributed inland waters. Adaptive significance of varied egg and clutch sizes is discussed with emphasis on the role of larval dispersal.