The egg of Calanus is denser than sea water and will begin to sink as soon as it has been laid. Gross and Raymont (1942) have calculated that at a temperature of about 13°C it will sink at the rate of about 2.5 cm/min. A Calanus egg takes about 24 hours to develop and this means that if it is laid at the surface the nauplius will hatch at a depth of about 36 m. Eggs, nauplii and young copepodites are found mainly in the top 30 m (Nicholls, 1933b) and it seems probable that the restriction of egg laying in early spring to a short time at night (pp. 39–40), when the females are near the surface, may be an adaptation to allow the eggs to hatch as near the surface as possible.
KeywordsPolar Body Endoderm Cell Thoracic Segment Mesoderm Cell Masticatory Process
Unable to display preview. Download preview PDF.