The mating and reproductive biology of the freshwater planktonic calanoid copepod Eudiaptomus gracilis

1. Quantitative aspects of the mating and reproductive biology of the fresh water planktonic calanoid copepod Eudiaptomus gracilis, including duration and frequency of mating, duration of various phases of the oviducal cycle, egg production rate and adult longevity were studied under laboratory condi tions. One set of copepods was fed the alga Chlamydomonas reinhardtii whose density was adjusted to 2 x 10(5) cells mL(-1) (about 10 mg C L-1), anothe r set was fed a mixed diet consisting of natural plankton (copepod nauplii, small rotifers and large algae) in the size range of 50-150 mum (dry mass approximately 90 mg L-1). 2. The entire mating process, from the grasping of the female by the male's right geniculate antennule to the separation of the pair, lasted about 2 m in. Spermatophore placement started at about 30 s to 1 min after mating beg an and took approximately 1 min. Immediately after the spermatophore had be en fixed in the female's genital segment, the pair separated. 3. The total oviducal cycle, including the gravid phase where the female ca rried ripe oocytes and the non-gravid phase where the female did not carry ripe oocytes, lasted about 5-6 days. The non-gravid phase was particularly long; it was longer than the gravid phase and constituted 62-72% of the tot al cycle. 4. Mating and spermatophore placement usually occurred with gravid females although occasionally (in 30 of 200 observations) spermatophores were attac hed in the genital segment of non-gravid females. Generally two to four, bu t up to seven, spermatophores were observed at a female's genital segment a t the same time. 5. Clutch size, rate of egg production and adult longevity depended on food . When fed on C. reinhardtii, females carried 7-8 eggs clutch(-1), produced a mean of 1.3 clutches and lived 14 days on average. When fed natural mixe d food, females carried 10 eggs clutch(-1), produced 5.6 clutches and lived 37 days on average. 6. Removal of males after the first clutch resulted in no further egg produ ction. Re-mating is necessary in E. gracilis for continuous clutch producti on and the production of fertile eggs. 7. Mating duration is comparatively short and the non-gravid phase comparat ively long in E. gracilis. This could be an adaption to the life in the pel agic zone of the lake, where fish predators are present. Fish select oviger ous females, pairs in copula and, probably, females with ripe oocytes which make them conspicuous. Thus, a short mating duration and a prolonged perio d without conspicuous oocytes, can be advantageous.