Kazuo Mashiko

DIVERSIFIED EGG AND CLUTCH SIZES AMONG LOCAL POPULATIONS OF THE FRESH-WATER PRAWN MACROBRACHIUM NIPPONENSE (DE HAAN)

ABSTRACT 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.

Social relation in a small experimental population of Odontobutis obscurus (Temminck et Schlegel) as related to individual growth and food intake

SummarySocial relations among even-sized male individuals of Odontobutis obscurus, a carnivorous eleotrid goby, were observed in a small experimental population with measurements of individual body size and food intake. 1.Nipping, chasing, threatening, fighting and territorial defence were always observed. The dominance order of an incomplete peck-right type was recognized all the time. The dominant and the second-rank fish widely surpassed other subordinate individuals in the frequency of their attacking behaviour.2.The dominant fish occupied and defended the whole bottom area of the aquarium (ca. 0.36 m2) as his territory throughout the experimental period except only 1 day, though he was frequently challenged by the second-rank fish. Introducing nest-holes in the aquarium did not cause either increase of territory number or decrease of territory size. The holes were used by the dominant fish as the center of his territory.3.All individuals increased their body weight in the initial period, but the growth of two subordinate fish became stagnant in the middle stage and later on, resulting in a wider difference between the growth rates of dominant and subordinate groups.4.The second-rank fish showed the greatest food (live goldfish) intake and the highest gross efficiency of food/growth conversion, and his body weight surpassed that of the dominant fish in the later half of the experimental period. Thus the social ranking did not agree with the body-size ranking.5.Both kinds of ranking did not change during the following periods of starvation and limited feeding.6.O. obscurus showed predatory behaviour even at midnight in darkness whenever fed with gold fish, which was followed by a short period of aggressive behaviour.

Relationships between egg size and incubation time among the populations of two freshwater prawns

It is a prevailing (partly demonstrated) view that, in aquatic organisms, large newborns derived from large eggs generally show greater viability than small newborns from small eggs (e.g. Lonsdale and Levinton, 1985; Mashiko, 1985). On the other hand, incubation time (embryonic duration until hatching) is said to be longer for large eggs than for small eggs among closely related species (Corkett, 1972; Wear, 1974), possibly due to the relatively low metabolic rate of a large body mass (McLaren, 1966; Steele and Steele, 1975; Steele, 1977). Such a prolonged incubation time for large eggs will result in an increased risk of mortality, for instance, by predation for both the eggs (Spight, 1975) and the egg-carrying females (Winfield and Townsend, 1983). Is this therefore an evolutionary dilemma in producing large eggs? The present paper deals with contrasting results which have been obtained regarding the relationship between egg size and incubation time among local populations of the two Palaemoninae prawns, Palaemon paucidens de Haan and Macrobrachium nipponense (de Haan).