Female Calanus helgolandicus from L4 were captured during mid-morning and incubations were initiated at mid-day. Five replicates of 5 healthy females were randomly picked from a net sample and incubated for 24 h in 2 l glass beakers. Each beaker contained a plexiglass egg production tube (mesh 500 µm) and was filled with 0.6 µm filtered seawater at ambient sea temperature.