al 



100 



10 



o 



en 



1 



0.1 



0.1 



O chl c, water 

 • chl b, water 



V 



j_ 



bl 



CO 



E 



c 

 < 



O 



1 10 



Km from Shore 



100 



0.1 



100 



Figure 3.— a) Mean chlorophyll b and c concentrations in water samples plotted relative to distance from 

 shore, b) As in a) but for pigments in the gut contents of Acartia tonsa and Calanus pacificus. 



If the ratio of the chlorophyll b or c to its sum, 

 T (= b + c), is the same in the gut of a copepod as 

 it is in the water, then it might be reasoned that 

 feeding on phytoplankton was not selective. Vari- 

 ations from unity would be interpreted as an in- 

 dication of food selectivity. We define relative 

 selectivity indices for chlorophyll b (RSIb) and 

 chlorophyll c (RSI C ) as: 



RSIt 



RSIc = 



(b/T). 

 (b/T) w 



(c/T) e 

 (c/T) w 



(1) 



(2) 



where g and w represent the ratios in the gut and 

 water, respectively. 



RSI values, presented in Table 3, indicate selec- 

 tivity for chlorophyll b-bearing organisms by 

 Acartia at stations N4 and N6. At station 7B, 

 Acartia evidenced a weak selection of chlorophyll 

 c-bearing organisms and Calanus pacificus 

 selected for chlorophyll b-bearing organisms. 



Discussion 



There were clear differences in the gut con- 

 tents of Acartia from near- and offshore loca- 

 tions. Gut fullness was higher in copepods from 

 nearshore than from those offshore (Fig. 2), but 

 the amount of chlorophyll a in the guts of animals 

 collected nearshore was substantially lower than 

 in the guts of animals from offshore locations 

 (Table 2). Apparently materials other than phyto- 

 plankton composed a relatively large portion of 



Table 3.— Relative selectivity indi- 

 ces for chlorophyll b (RSI b ) and 

 chlorophyll c (RSI C ) by Acartia tonsa 

 and Calanus pacificus. 



1 Data from cruise on 26 September 1980 

 2 Data from cruise on 12 September 1980. 



the diet of the nearshore animals. Evidence from 

 laboratory studies (Poulet 1973; Heinle and 

 Flemer 1975; Richman et al. 1977; Roman 1977) 

 suggests the possibility of a detrital or animal 

 component in the diet of Acartia when these 

 foods are available. 



The RSI indicates that C. pacificus was feeding 

 selectively on phytoplankton containing chloro- 

 phyll b at station 7B. The only green alga detected 

 by microscopic analysis of water samples at this 

 location was Halosphaera sp. Although it is pos- 

 sible, even likely, that other green algae were 

 present, the typical chlorophytes and euglenoids 

 were not observed, and, unlike the nearshore sta- 

 tions, nanoplanktonic green algae appeared to be 

 absent. We assume, therefore, that Halosphaera 

 was at least the dominant source of chlorophyll b 

 in the water, and constituted the greater portion 

 of the chlorophyll b signal in the C. pacificus gut. 

 Since we cannot test this assumption, what fol- 

 lows must be considered somewhat speculative. 

 However, we suggest that under the conditions 

 observed in Santa Monica Bay at the time, selec- 

 tive feeding by Calanus on Halosphaera cysts 



158 



