The Seasonal Life Cycle of Sinocalanus tenellus (Copepoda : Calanoida) in a Brackish-water Pond

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The seasonal life cycle of a calanoid copepod Sinocalanus tenellus (Kikuchi) was studied during the period from June 1983 to July 1984 in a brackish-water pond in Fukuyama, Hiroshima Prefecture. Except for summer, the species was present in the plankton throughout this period, with a small peak in April and a large peak in November. The seasonal variation in abundance of benthic resting eggs was directly related to that of the adult population. Most eggs isolated from the bottom sediments and those spawned directly by females in the laboratory hatched out subitaneously without any seasonal difference in egg physiology. Since considerable numbers of quiescent resting eggs were present in the bottom sediments during summer, the planktonic population could be reestablished in fall from these eggs. During the period when the planktonic population was abundant, the population was predominantly composed of adults. Sex ratio (male: female) was generally 1.0. The prosome length of S. tenellus was larger in fall and spring and smaller in winter and early summer.