OFFSPRING SIZE AND PARENTAL FITNESS IN DAPHNIA-MAGNA

Variation in offspring size and number has been described for a wide r ange of organisms. Many theoretical models predict that in a given env ironment, the production of one single offspring size would yield the highest parental fitness. In most planktonic cladocerans, however, off spring size has been found to increase with size and age of the mother , and as individuals of variable size often co-occur within a populati on, offspring of variable sizes can be produced simultaneously. In thi s study, I investigated the relationship between age of the mother and size of her offspring to assess at what age of the mother the optimal offspring size was produced. Optimal offspring size was defined as th at size of the offspring yielding the highest parental fitness, which translates to a definition of optimal offspring size as the one having the highest juvenile fitness per unit effort put in these juveniles. I observed that the youngest females produced offspring with the highe st juvenile fitness per unit effort, and hence concluded that offsprin g produced by these females were of optimal size. Larger offspring pro duced by older females were estimated to yield only 70% of the potenti al fitness of optimally sized offspring.