Time of Egg-laying in the Planktonic Copepod Calanus

IN the course of some observations on the cytology of Calanus made in Plymouth in June 1950, the eggs in the oviduct of a fairly high proportion of females examined were found to have reached full metaphase of the first reduction division (this is the stage reached when the eggs are laid). On the other hand, in March 1951 at Millport it was difficult to find eggs in the oviduct of Calanus in which the chromosomes had reached further than the pachytene stage. This was in spite of the fact that a high proportion of the females brought into the laboratory at this date laid a batch of eggs overnight. It seemed as if few of these Calanus would be ready to lay before night. In order to find at what time the actual spawning takes place at Millport, 104 ripe female Calanus were put singly into small glass dishes containing about 20 ml. of filtered sea water and examined hourly. The Calanus were caught between 50 and 100 metres at 4 p.m. and examination was begun at about 6 p.m. The results are shown in diagram A ; 68 per cent spawned between midnight and 3 a.m. and the bulk of the rest by 4 a.m. Only five of the 104 did not lay. The average number of eggs was very constant at about forty per female. There was no difficulty in finding primary oocysts with full metaphase plates in females fixed at 2 a.m.