On the Biology of Calanus finmarchicus, I. Reproduction and Seasonal Distribution in the Clyde Sea-Area during 1932

Abstract

1. Fortnightly samples were taken with a fine-meshed net in single vertical hauls at different positions in the Clyde Sea-Area.2. Each stage of Calanus present, from ovum to adult, was counted, and charts were constructed expressing these results for one year as total numbers and as percentage composition.3. Calanus passes the autumn and early winter mainly in its Stage V copepodite form.4. In general, total numbers were high in September, 1931, and fell steadily to a minimum in March; in Loch Fyne the minimum occurred i n April.5. A sudden increase was observed in May and numbers rose to maximal values; the decline also was sudden.6. Reproduction began in February and three plainly marked breeding periods were observed between February and July.7. The time taken by the egg in developing into the adult was four weeks.8. The total life of a Calanus during the summer was estimated to be about two and a half months; in winter it is five to six months.9. In Loch Fyne a noticeable lag occurred during the winter and early spring.10. The number of Calanus present in the autumn of 1932 was considerably less than.at the same time in 1931. This may have been due to a shortage of food.

On the biology of Calanus finmarchicus. VII. Factors affecting egg production

“…Adult females of the new brood appeared on 12 April as immature and gradually replaced the over-wintering stock. Nicholls (1933a) estimated from field work that a female after moulting takes about a month before the eggs are ready to lay. The egg-laying experiments, however, have shown that the time taken will depend very much on the food present in the sea.…”

“…The present observations extend and amplify their findings. Nicholls (1933b) suggested that Calanus spawns at night, since eggs and young stages are always found above 3°m., and this has now been confirmed for the over-wintering generation. The eggs are heavier than sea water, and the rate of sinking is given by as 36 m. in 24 hr.…”

“…When summed over the year the number of nauplii above and below 10 m. was approximately the same, but very few were found below 3°m. The young copepodites do not show diurnal vertical migration (Nicholls, 1933b) and are confined to the surface layer so that the nauplii must swim up towards the surface. Since females show a marked diurnal vertical migration, the fact that the eggs are laid during the night ensures that they will be laid fairly near the surface.…”

The Nansen closing method with vertical plankton nets

“…The net hauls were taken by means of a modified International net fitted with a Nansen (1915) closing mechanism. The net, according to Nicholls (1933), differed from the standard pattern of Ostenfeld & Jespersen (1924) in that 'the cylindrical part of the net was made of canvas instead of netting and the conical part of fine-meshed bolting-silk (77 strands in I cm). An ordinary townet bucket was used.…”

On the Biology ofCalanus finmarchicus. Part IV. Seasonal Changes in the Weight and Chemical Composition in Loch Fyne

“…In the Clyde Sea-Area it constitutes a considerable percentage of the zooplankton as a whole. The object of the work was to ascertain, in conjunction with the observations on size and distribution by Marshall (1933) and Nicholls (1933) …”