Oscillations in the reproductive condition of Diaptomus leptopus (Copepoda: Calanoida) and their relation to rates of egg-clutch production.

Isolated female D. leptopus oscillate between gravid and nongravid reproductive conditions. When gravid, the oviducts are visible as a pair of dark bands lateral to the digestive tract. Periodically the females revert to a nongravid condition by releasing all the unfertilized oocytes into the environment. The oviducts then remain transparent until they refill with ripe gametes.The characteristics of this gametogenic cycle were defined at 18° C using females cultured under controlled conditions. The average period of the cycle was 4.4 days, with a gravid phase (dark oviducts) of 3.4 days and a nongravid phase (clear oviducts) of 0.9 days. Observations made in situ confirmed the oscillatory nature of the changes in reproductive status.The duration of the nongravid phase was shown to represent the minimal time interval between successive clutches of fertilized eggs. Estimates of maximal rates of clutch production based on this measure were more than three times higher than those based on the duration of embryonic development.Since unfertilized oocytes disintegrate upon extrusion, a considerable amount of reproductive material may be wasted regularly. A turnover rate of 5.6% body weight (dry) day-1 was calculated for isolated females at 18° C. A model used to estimate the frequency of mating interactions indicated that the probability of oocyte extrusion in lakes may be high and the impact on nutrient pools could be substantial. Up to 0.12 μg phosphorus mg-1 h-1 may be released in the form of unfertilized oocytes.