FEEDING RATE OF CALANUS FINMARCHICUS IN RELATION TO ENVIRONMENTAL CONDITIONS 1 JOHN L. FULLER (From the Woods Hole. Oceanographic Institution and the Biological Laboratories, Clark University) OBJECTIVES The amount of food available to a plankton-feeding animal is deter-mined by the concentration of suitable food material in the water, and by the rate of the animal's feeding activity. The calanoid copepods, like many other zooplanktonts, are generally considered to feed by filtering out particles from a current of water generated by the animals (Cannon, 1928). By determining the number of food organisms re-moved by an animal from a suspension of known concentration, the volume of water which has been filtered clear of the organisms can be calculated and, if the chemical composition of the organisms be known, the amount of nutriment made available to the animal may be esti-mated. Preliminary measurements of the filtering rate of Calanus finmarchicus have been reported by Fuller and Clarke (1936). Further investigation of changes in the feeding rate induced by varying environ-mental factors of ecological importance was expected to yield informa-tion useful in the quantitative study of aquatic food cycles. Diatom concentration, light and temperature were expected to be variable factors in the sea and were chosen for study. METHODS In the experiments here reported the rate of feeding of the copepod Calanus finmarchicus, collected in Vineyard Sound near the whistle buoy, was measured in a suspension of the diatom Nitzschia closterium (Plymouth strain). It seems probable that Nitzschia is a suitable food for Calanus, as Crawshay (1913-15) kept individual copepods alive for as long as 80 days in persistent cultures of this diatom. Allen and Nelson (1910, p. 470) reared Calanus from eggs to copepodid stages in a mixed culture in which Nitzschia was predominant. Three stage V Calanus were placed in 15 cc. of sea water containing a known concentration of diatoms. Changes in concentration were followed for two to four days, counts being made in a hemacytometer. 1 Contribution No. 132 of the Woods Hole Occanographic Institution. 233
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