On the Nutrition and Metabolism of Zooplankton IX. Studies Relating to the Nutrition of Over-WinteringCalanus

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Abstract
Studies were made relating to the problem of howCalanusfeeds during winter in the Clyde sea-area. Different diets were assessed in terms of sustaining the levels of body nitrogen and phosphorus inCalanus helgolandicus(Claus) over a period of several days. The test diets, all equivalent to the same level of particulate nitrogen in sea water, were: (1) suspended matter collected from the Clyde sea-area in winter; (2) particulate material produced in a foam-tower by bubbling sea water enriched with soluble extracts of plant cells; (3) living nauplii of the barnacleElminius modestusDarwin; (4) dead nauplii of this species.It was found that neither body nitrogen nor body phosphorus was sustained by diet 1; that body nitrogen, but not body phosphorus, was sustained by diet 2; that both were sustained by either of diets 3 and 4.With livingElminiusnauplii as the food, eachCalanuscaptured the equivalent of 25 % of its body nitrogen and 47·3 % of its body phosphorus daily: with dead nauplii as the food the corresponding values were 34·4 and 44·5%. These rations are much higher than those found in an earlier study ofCalanusgrazing on a spring diatom increase in the Clyde (Butler, Corner & Marshall, 1970) and demonstrate that animal diets are readily captured.In general, the results indicate thatCalanuscould survive the winter in the Clyde sea-area by feeding carnivorously.