Papers in Marine Biology and Oceanography, Suppl. to vol. 3 of Deep-Sea Research, pp. 1 10-114. 



Experimental feeding of the copepod Calanus finmarchicus (Gunner) 

 on phytoplankton cultures labelled with radioactive carbon (^*C) 



By S. M. Marshall and A, P. Orr 

 The Marine Station, Millport 



Summary — A small number of experiments were made on phytoplankton cultures labelled with ^*C; 

 they confirmed earlier results obtained when using ^^P. With female Calanus the volume filtered varied 

 from about 1 -40 ml per day, and the digestion of a diatom and two flagellates lay between 50 and 80 %. 

 Since both carbon and phosphorus are important constituents of the algal cell, it is fair to assume 

 that the major part of the organic material is digested. It may be concluded therefore that, during 

 a period of diatom abundance in the sea, most of the food ingested is utihzed. 



Qualitative WORK on the feeding of Ca/a«M5 (Dakin, 1908; Esterly, 1916; Lebour, 

 1922; Marshall, 1924) has shown that it can ingest most of the small organisms 

 present in the marine plankton, but that its diet consists predominantly of diatoms. 

 It is possible that small naked flagellates are also important as food, but, since they 

 have no indigestible skeleton, they can rarely be recognized in the gut, and not at all 

 in the faecal pellets. 



It has been possible to measure the quantity of different food organisms consumed 

 by counting the numbers of these in a known volume of culture before and after a 

 known period of feeding (Clarke and Gellis, 1935; Fuller and Clarke, 1936; 

 Fuller, 1937; Clarke and Bonnet, 1939; Gauld, 1951). The earlier workers 

 obtained values for the volume of water filtered of a few ml per day, but Gauld 

 obtained an average of about 70 ml per day, using cultures of Dunaliella sp. (Chlamy- 

 domonas). 



Recently some experiments have been carried out by Marshall and Orr (1955) 

 on feeding Calanus with cultures of diatoms and flagellates labelled with radioactive 

 phosphorus (*^P). These cultures were grown with a limiting quantity of phosphorus 

 present, and were in most cases used after the phosphorus had been almost entirely 

 taken up by the organisms. The initial concentration of ^^P in the culture was 

 measured as well as that present at the end of the experiment in the body, faeces and 

 eggs laid (if any). It was then possible to calculate the volume swept clear by the 

 Calanus in 24 hours. This varied from less than 1 ml up to about 40 ml with a maxi- 

 mum in a single instance of a little over 80 ml. 



The digestion of most of the food organisms was unexpectedly complete, the figures 

 being usually over 60 % and often over 90 %. With a few organisms only was it appar- 

 ently very low. It was high even when the Calanus were feeding rapidly in rich 

 concentrations of food cells. These experiments show therefore that the phosphorus- 

 containing portion of the algal cell is readily assimilated. 



It seemed possible that the high digestion figures might be misleading, since various 

 workers have shown that the phosphorus in algal cells is partly labile, and that this 

 fraction may be either adsorbed on the cell surface or present in inorganic solution 

 within the cell (Gest and Kamen, 1948; Kamen and Spiegelman, 1948; Goldberg, 



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