Female Japanese Medaka Research Articles

Resource defence, monopolization and variation of fitness in groups of female Japanese medaka depend on the synchrony of food arrival

The predictions that a decrease in the synchrony of food arrival leads to an increase in aggression, resource monopolization and, ultimately, variation in fitness within groups were tested. Groups of four female medaka, Oryzias latipes, were allowed to compete over a 2-week period for prey ( Artemia sp.)_that arrived either synchronously (every 5 s) or asynchronously (ever 60 s). As predicted, both the monopolization of food and the frequency of aggression were higher when food arrived asynchronously than when it arrived synchronously. Fitness of individual fish was measured as the number of eggs spawned, growth, and surplus power, the sum of the energy allocated to eggs and growth. Variation of within-group fitness was greater in the asynchronous treatment than in the synchronous treatment when measured by growth and surplus power, but not by number of eggs. The frequency of aggression by an individual was positively correlated with feeding success and ultimately to fitness in the asynchronous treatment but was not correlated with either in the synchronous treatment. These results suggest that food was economically defendable only in the asynchronous treatment.

An Experimental Analysis of Reproductive Effort and Cost in the Japanese Medaka, Oryzias Latipes

Experiments measured the effects of differences in controlled temperature and food intake on reproduction, growth, and survival of female Japanese medaka, Oryzias latipes, a daily spawning killifish. Maintenance of isolated breeding females at nine different food—temperature treatments allowed the construction of individual energy budgets and hence the measurement of reproductive effort (the proportion of an individual's total energy intake devoted to reproduction). Increased temperature was associated with increased total reproduction and decreased growth at all food levels, although total production (sum of reproduction + growth) was essentially the same at all temperatures. Increased food intake led to higher fecundity at two of the three temperatures, while reproductive effort actually declined with increased food intake at two temperatures. Growth (change in female length, total mass, ovary size, and total energy content) increased with greater food intake at all temperatures. There was no evidence of an increase in reproductive effort with size or age; fecundity declined during the experiments although food intake was constant. Costs of reproduction included reduction in energy content of fish tissue and substantial loss of mass in many fish that nevertheless continued to reproduce at high levels. Reproduction and growth were in general negatively correlated, suggesting that individuals reproducing the most were not able to grow as rapidly as individuals reproducing less. Mortality varied substantially (0—30%) among the treatments and was highest at high temperatures and high levels of reproductive effort.