アイテム詳細

要旨

The cyclomorphic change of head and tailspine of Daphnia cucullata has been analysed morphometrically and compared with temporal analyses of enzyme variability. During June, round-headed animals are rapidly replaced by specimens with high helmets; the rate of increase of the relative helmet length is more rapid than the rate of decrease later in the year. The relative sizes of helmet and tail spine are not always coupled. The tailspine length remains approximately the same year round and grows with negative allometry, so that small animals have the relatively longest spines. In contrast, the helmet grows in a positively allometric fashion and exhibits high seasonal variation. Helmet growth is strongly correlated with water temperature but not with food abundance. The morphometric results are consistent with the hypothesis that helmet formation is a response to fish predation while the tail spine provides protection against invertebrate predators. There is no indication of changes in the gene or genotype frequencies at two loci during the period of rapid morphological change. Thus cyclomorphosis apparently appears in the absence of clonal replacement.