Cyclomorphosis in Daphnia cucullata: morphometric and population genetic analyses

The cyclomorphic change of head and tailspine of Daphnia cucullatahas been analysed morphometrically and compared with temporalanalyses of enzyme variability. During June, round-headed animalsare rapidly replaced by specimens with high helmets; the rateof increase of the relative helmet length is more rapid thanthe rate of decrease later in the year. The relative sizes ofhelmet and tail spine are not always coupled. The tailspinelength remains approximately the same year round and grows withnegative allometry, so that small animals have the relativelylongest spines. In contrast, the helmet grows in a positivelyallometric fashion and exhibits high seasonal variation. Helmetgrowth is strongly correlated with water temperature but notwith food abundance. The morphometric results are consistentwith the hypothesis that helmet formation is a response to fishpredation while the tail spine provides protection against invertebratepredators. There is no indication of changes in the gene orgenotype frequencies at two loci during the period of rapidmorphological change. Thus cyclomorphosis apparently appearsin the absence of clonal replacement.