Rates of egg production by the copepod Calanus marshallae in the laboratory and in the sea off Oregon, USA

Abstract

Egg production by adult female Calanus marshallae Frost was studied in the laboratory by making a census of the number of eggs produced each day by individuals maintained on a diet of the diatom Thalassiosira weissflogii. When fed ad libitum individuals produced a clutch of 30 eggs every 30 h on average. Egg production rates were influenced by food concentration, ranqng from 0 d-' when females were kept in filtered seawater up to 24 eggs d-' at food concentrations 2 3500 cells m]-'. Maximum rates of ingestion of phytoplankton cells also occurred at 3500 cells ml-l. Females maintained at food concentrations ranging from 2000 to 3500 cells ml-' produced the same number of clutches per unit time as females maintained at >3500 cells rnl-l, but produced fewer eggs per clutch. At food concentrations < 2000 cells ml-' the frequency of clutch production declined to one clutch per 4 d. Egg production declined to 0 after l d when females were starved. When starved for periods up to 10 d, an interval of 0.4x the starvation interval was required before egg production was re-initiated. Females maintained at 3500 cells ml-' had a gross efficiency of egg production of 29.0 % and a reproductive output of 6.g0/0 of their body weight per day. The latter is 3 x the CS-to-female growth coefficient. In situ egg production rates were also determined, using the egg-ratio method, and were usually food-limited. In the sea the functional response of egg production to food concentration in situ was similar to that measured in the laboratory.