Polymorphism in resting egg size and hatching strategy in the rotifer Brachionus calyciflorus pallas

This study examined polymorphism in the resting egg size, hatching pattern, morphotype, and starvation tolerance of stem females in a local population of the freshwater rotifer Brachionus calyciflorus. Fifteen rotifer families were cultured under laboratory conditions to induce the production of inbred, homozygous resting eggs. Each family of resting eggs was measured for size and aliquots were then incubated at 10 degrees C or 20 degrees C to test the phenotypic plasticity of hatching dynamics in this population. The distribution of resting egg size was approximately normal in each family, and the sizes of the resting egg shell and embyro showed significant differences among families, ranging from 6.4-8.9 (x 10(5) microm(3)) and 3.7-5.2 (x 10(5) microm(3)), respectively. Hatching rates also varied greatly among families at each temperature, with average values ranging from 10.0-51.5% at 10 degrees C and 3.1- 67.2% at 20 degrees C. Hatching temperature clearly affected the hatching pattern. Resting eggs of most families hatched synchronously at 20 degrees C, whereas they were likely to hatch more sporadically at 10 degrees C. We observed no correlation between size of resting egg and subsequent hatching variability. This was the first time to find a large diversity in morphotype of B. calyciflorus stem females among different families. In some families, most stem females lacked posterolateral spines, but in other families, they had two short posterolateral spines. There was a considerable variation in survival time of stem females that had undergone starvation, and survival time was positively correlated with resting egg size. The divergence in these life history traits may have important implications for understanding the rotifer's adaptive strategy and life history evolution.