Variation in the genus Keratella

The literature on variation in Keratella is reviewed. The old idea of a thorough endogenous control has to be rejected, but internal factors ought to play a certain role beside influences from current and previous environment. In certain cases there is probably a succession of genetically different clones during the course of the year (cf. King, 1972, 1977), but the seasonal variation in lake populations of, e.g., K. cochlearis ought to be mainly non-genetical. There is some evidence that temperature and food exert an influence on the morphology, via rate of growth, but probably other abiotic and biotic factors are at work as well. The existence of allometric relationships is clearly demonstrated for several species. The variation in spine length has been suspected by some authors to consititute just the function of size variation which is thus considered primary. Some of the variation found is obviously non-adaptive. An attempt is made at explaining the existence of discontinuous variation within a single lake. Implications on taxonomy and speciation are briefly discussed.