Effects of Photoperiod, Temperature, and Population Density on Induction of Diapause Egg Production in Eurytemora Affinis (Copepoda: Calanoida) in Lake Ohnuma, Hokkaido, Japan

Abstract

Two types of eggs (diapause or subitaneous) produced by females of the copepod Eurytemora affinis from both natural and laboratory populations collected from Lake Ohnuma, Hokkaido, Japan, were examined under various photoperiod and temperature conditions. In the lake, females produce subitaneous eggs from May to mid-October and switch to producing diapause eggs in late October. In the laboratory, females raised from nauplii collected in late autumn (November) under spring conditions (15°C, 12L:12D) produced exclusively subitaneous eggs, whereas those raised from nauplii collected in spring (May) under late autumn conditions (10°C, 10L:14D) produced exclusively diapause eggs. More than 79% of the females raised from nauplii under 10L:14D, at both 10 and 15°C, produced diapause eggs, but more diapause eggs were produced at 10°C. These results indicate that the type of eggs to be produced by females of E. affinis is determined in response to environmental conditions experienced during their naupliar stages, and that a short-day photoperiod (10L:14D) is the primary cue for inducing diapause egg production, and low temperature (10°C) promotes the effect. Even under spring or summer conditions (15°C, 12L:12D or 14L:10D), females reared in crowded cultures (>80 females-l-1) partially produced diapause eggs, suggesting that a high population density also induces the diapause egg production.