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.
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