LAUSR

ABSTRACT Egg dormancy is a key life history strategy in copepods to temporarily escape catastrophic or repetitive detrimental events and ensure permanent colonization. In Lake Biwa, although the copepod Eodiaptomus japonicus dominates the zooplankton community year-round, it has been known to produce resting eggs, yet little is known about its dormancy. To clarify the nature of dormancy in E. japonicus, we collected a sediment core from Lake Biwa, isolated resting eggs, and conducted hatching experiments to determine the life-cycle traits of the hatched individuals. We isolated 242 eggs (maximum age, 60 years) from the upper 17 cm depth sediments, and although no strict catastrophic event such as seasonal drying up was found, the findings implied that E. japonicus might have a “bet-hedging” strategy. Accumulated egg density above 17 cm was 2.5 × 104 m−2, and mean annual egg flux was 103 m−2 per year. Over the first 10 cm sediments, average egg hatching success was 17%. No egg hatchings were confirmed below 10 cm depth. Embryonic development times of the resting eggs were longer than those of subitaneous eggs, indicating that a lag phase existed until development resumed; 33% of hatched nauplii molted to adulthood, and 80% of females produced offspring. These results suggest potential recruitment from resting eggs in the bottom sediments, which partly serve as an egg bank in this copepod.