Laboratory studies on egg and faecal pellet production of Centropages typicus: Effect of age, effect of temperature, individual variability

The rates of egg production of individually reared females of Centropages typicus have been observed from the moulting CS-adult to death under constant conditions of food (10 000 cells ml(-1) of Hymenomonas elongata) and temperature (15 and 20 degrees C). Preliminary experiments showed that virgin females could produce unfecunded eggs, but that the distinctive spines of the eggs of C.typicus were only obtained after fecundation occurred. High and continuous egg production needs several matings, and the following experiments were run with a couple. In all experiments, both males and females fed and produced faecal pellets. The females show a high variability in spawning life periods, in spawning rhythms, in daily rates and, consequently, in cumulated numbers of eggs spawned during their life. The highest daily rates of egg production were obtained in the first half of the female's life, and were as high at 20 degrees C as at 15 degrees C. Nevertheless, the females survived longer at 15 degrees C than at 20 degrees C, and finally produced 47.6% more eggs during their whole life at 15 degrees C than at 20 degrees C, which can be partially due to the difference in size (almost 10% bigger at 15 degrees C than at 20 degrees C). The production of faecal pellets occurred during the whole of life and was more regular than egg production, with a maximum daily rate in the first part of life, and showed a decrease until death. The decline in daily egg production with age was parallel to the decrease in daily faecal pellet production. The daily production of faecal pellets was higher at 15 degrees C than at 20 degrees C. At 20 degrees C, a good correlation was observed between the total egg production per female and the total faecal pellet production per female. The individual-based egg production rates of our experiments have been compared with rates obtained in different conditions by several authors, and were lower than in other published studies, probably due to the food offered. We emphasize the difficulties of comparing results which have been obtained for the entire life of the females with those based on daily egg production. The total egg production of a female is an important parameter in regard to the population dynamics because it fixes the maximal rate of losses (mortality plus dispersion) for which a population can continue to develop.