Photoperiodism and diapause in Daphnia: a strategy for all seasons

Polymorphism and diapause in arthropods may be evoked by photoperiod, although control is sometimes conditional to a certain range of temperature or an adequate diet (Lees, 1953; Paris and Jenner, 1959; Adkisson et al., 1963; Danilev skii, 1965). Further intricacy of control of terrestrial diapause is provided by the critical photoperiod of the adjustment to provide latitudinal adaptation, and by the number of instars separating the photosensitive and responding stages (Danilevskii, 1965; de Wilde, 1962; Beck, 1963). A recent entry to the list of examples of photoperiodism is the genus Daphnia, a fresh-water member of the Cladocera (Crustacea). Expression is reported to be conditional, and with a permissively short photoperiod the culture may shift from parthenogenic to sexual reproduction but only with the provision of a second stimulus which is associated with density of the culture (Stross and Hill, 1965). Details of inception of sexual reproduction and termination of diapause in the overwintering embryos are pre sented below. Embryonic diapause in Daphnia follows the shift from parthenogenic to sexual reproduction. The sexually derived embryo is structurally conspicuous as are its enveloping (ephippial) membranes. In reality the diapause may be obligatory to the sexually determined embryo, in which case the expression of a polymorphism rather than of diapause is under study. Sexual reproduction in Daphnia (and embryonic diapause) has been observed in all seasons of the year (Berg, 1931). Berg, among others, distinguishes three types of life cycles which include shifts to sexuality in both short- and long-day photoperiods. The acyclic type reproduces exclusively by parthenogenesis, and a motile, reproductive diapause has been described for over-wintering individuals (Brooks, 1965). The acyclic type which has been described for the larger, deeper lakes, may retain the capacity for sexual reproduction, however, as Berg (1931) demonstrated. The monocycle contains a single period of sexual reproduction, and this seems to occur primarily if not exclusively in autumn; it is found in small but permanent bodies of water. A third type is the polycyclic, and in Berg's reported observations, it consists mainly of two periods that are spring- and autumn-initiated. The autumn period is reported to be the more intense, and one species (D. pulex) may produce only the sexual (ephippial) eggs in some lakes. The polycycle is apparently re stricted to ponds where there are at least two periods of unfavorable conditions. A single species, D. pulex, is reported to exhibit all types of life cycles and to pursue simultaneously both kinds of reproduction with varying degrees of intensity (Berg, 1931).