GROWTH AND VARIABILITY IN DAPHNIA PULEX

¹ From the Biological Laboratory, Western Reserve University

Eighty-two individually reared female Daphnia pulex were observed from the time they were released from the brood chamber of their mothers until they died. Measurements of total length, carapace length, and height were made daily on each animal. The number of young released during each instar was recorded.

Seventy-one animals were primiparous during the fifth instar and 9 during the sixth. Consequently the number of pre-adult instars is variable, the minimum being four.

Data from the 47 animals which were primiparous during the fifth instar and which lived for twenty instars or more were used in constructing growth curves. Growth in the three dimensions studied is sigmoid. The point of inflection in all curves comes during the fourth instar, the last pre-adult.

The Robertson and the Gompertz equations do not fit the data satisfactorily. This may be due to the time unit employed, which in this case is the instar.

The growth increment is greatest during the fourth instar. The increments increase up to the fourth instar, then decrease gradually until the eleventh instar, after which they remain low and relatively constant.

A significant negative correlation exists between initial body size and initial growth rate, also between duration of growth and final body size. Other characteristics of the growth process investigated appear to vary independently.

The number of young released during the adult instars increases to a maximum at the tenth instar followed by a gradual decrease.

The number of young released during any adult instar is significantly correlated with the growth increment for the instar preceding the one during which the young are released.

Relative growth in the dimensions studied may be expressed satisfactorily by the equation

y = bx.

No marked changes occur in the relations between total length and carapace length. Marked changes in the relations between carapace length and height and between total length and height occur at the thirteenth instar.

The standard deviation and coefficient of variation of total length tend to increase somewhat during the early instars, but after the eighth instar both decrease rather steadily.

Relative variability in body size, as measured by the coefficient of variation of total length, is roughly directly proportional to the logarithm of the growth rate. The existence of this relationship is ascribed largely to the mode of action of environmental factors during growth. It may perhaps also be explicable in terms of Rahn's theory of the physico-chemical origin of variability in growth rate.

Size-frequency analyses of natural populations as methods of studying growth are shown to be inferior to the method using individually reared animals.

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