とう脚類の休眠

Abstract

Dormancy is a widespread phenomenon in Copepoda. It is known from free-living and bottom dwelling representatives of three copepod taxa, namely Harpacticoida, Cyclopoida, and Calanoida. There are 7 genera of Harpacticoida, 9 genera of Cyclopoida and 31 genera of Calanoida that exhibit dormancy in both marine and freshwaters. Copepod dormancy is expressed in various developmental stage, i. e. egg, nauplus, copepodite and adult. The phenomenon of dormancy represents a spectrum of development ranging from diapause to quiescence. Diapause is induced at sensitive stage (s) by triggering factors, i. e. token stimuli (e. g. photoperiod, temperature, population density), prior to the onset of environmental adversity. While quiescence defines a state of retarded development which is induced by adverse environmental conditions (e. g. extremes of temperature, salinity, oxygen concentration). Dormancy is a strategy to avoid biotic (e. g. predation, competition, food shortage) or abiotic (e. g. desiccation, extreme low or high temperature, low oxygen concentration) harsh environmental conditions along time axis. Dormancy also plays a role to synchronize the seasonal life cycle of copepodes. Recent studies find the extreme abundance and long-term viability of resting eggs in the bottom sediment at any seasons and suggest that the pool of the resting eggs may be regarded as an egg bank. Further studies on physiological, endocrinological and neurological aspects of copepod dormancy are necessary.