A Quantitative Study of Reproduction in Some Species of Ceriodaphnia (Crustacea: Cladocera)

1. An account has been given of the occurrence of Ceriodaphnia pulchella, C. reticulata and C. laticaudata in the Long Water, and of C. megalops in the Wick Pond, Hampton Court; also of their males and periods of sexual reproduction in these two habitats. 2. The seasonal variation in body size and egg number of parthenogenetic females of these species has been recorded throughout the period March 1963 to July 1964. The mean body length of C. reticulata is consistently greater than that of C. pulchella. C. megalops is larger, on average, than both these species. 3. Egg number increases with body length at the same rate in C. reticulata and C. megalops but more slowly in C. pulchella. C. megalops, the largest species, carries fewer eggs than C. reticulata of the same size and the smallest females of the latter species carry fewer eggs than large C. pulchella of the same size. The egg numbers of C. laticaudata are relatively low although its length range overlaps those of C. pulchella and C. reticulata. 4. Total egg volume increases with body length at the same rate in C. pulchella and C. megalops but more rapidly in C. reticulata. C. megalops therefore carries a smaller amount of egg material than either of the other two species, despite its greater size, while the reproductive material of C. pulchella is divided into more, smaller units than that of the other two species. Absolute egg production by C. reticulata is, however, greater than in C. pulchella due to its greater average size. 5. Body length in C. pulchella and C. reticulata varies inversely with temperature. When the egg numbers of these two species are corrected to the grand mean length of the species, there is no correlation between corrected mean egg number and temperature. Any apparent inverse relationship between egg number and temperature is, therefore, a consequence of the effect of temperature on body length. 6. Corrected mean egg numbers of C. pulchella and C. reticulata plotted against the chlorophyll content of the Long Water indicate that, up to a certain level, concentration of available food, unlike temperature, does have a direct effect on egg number. Above this level food concentration makes little difference to egg production. 7. The larger eggs of C. megalops are more nearly spherical than those of C. pulchella and C. reticulata. This is a trend found in many Cladocera: the larger the eggs the more spherical they tend to be. C. laticaudata does not fit this pattern since it has l