No abstract

Apart from a few scattered observations (Hensen, 1887; Gran, 1902) the study of the food of Calanus began with Dakin (1908). He examined the remains present in the gut, and this was the method used also by Esterly (1916), Lebour (1922) and Marshall (1924). They all found that these remains consisted of a greenish mush containing the skeletons of numerous planktonic organisms, chiefly diatoms and dinoflagellates. Naked flagellates were, how-ever, occasionally seen, and it was realized that the food might in reality consist largely of organisms which had no skeleton and could leave no recognizable remains.

1. The seasonal changes in the population of Calanus in Loch Striven in 1933 have been investigated.2. There were three main breeding periods. The first was in February and March, an unsuccessful beginning of the second in April, the true second brood in May to June, the third in July, and subsidiary broods in August and September.3. Numbers were very low until April when the copepodites of the first brood caused a large increase; there was a second increase in May consisting mainly of the eggs of the second brood, and the maximum for the year came in July. The last and subsidiary broods provided the stock for the autumn and winter of 1933.

Comparatively little is known about the factors affecting egg-laying in Calanus. Deductions from work in the field have been made by Nicholls (1933a, b) and Marshall, Nicholls & Orr (1934), and experimental work in the laboratory has been done by Raymont & Gross (1942).Emgg-LayingInCalanusThe anatomy of the reproductive system has been described in detail by Lowe (1935). In the female there is a pair of diverticula extending from the proximal end of the oviduct into the head region, and into this the developing eggs pass. Each diverticulum has a dorsal and a ventral channel, and the eggs pass from the dorsal into the ventral channel and then into the oviduct proper. The oviduct rises from the forward end of the ovary and crosses the body laterally and ventrally so that a short part of it can be clearly seen.

The length, weight, calorific value and organic matter content of Calanus finmarchicus, Stage V, males and females, were measured in monthly samples taken from April 1962 to June 1963.There is considerable variation throughout the year and all stages had their maxima of weight and calorific value in spring, 1962, and winter 1962–63.The calorific value and weight are closely correlated so that, knowing the dry weight, the calorific value can be calculated from it. The correlation is closest in Stage V and, for these, only the dry weight need be known. Since the relation-ship is slightly different for males and females the sex of the adult must be known as well as the dry weight; curves showing these relationships are given.The calorific value obtained from organic-matter analysis at times deviates widely from the values obtained in direct bomb calorimetry, and it is concluded that in the method used not all the fat is accounted for.In weight, calorific value, and organic matter, but not in length, Stage V for a good part of the year show higher figures than adults.In comparing the weights with those of other years and other places, the annual pattern is found to vary considerably. The spring maximum seems to occur every year, but there may be either a maximum or a minimum in autumn and winter.