112 
PACIFIC SCIENCE, Vol XX, January 1966 
pect that preference orders would all tend to 
be similar among the grazing species examined. 
The consumption of some of the tougher algae 
(. Eisenia , Laminaria, Pterygophora, Gigartina, 
Cystoseira ) is greater than or equal to that of 
the relatively tender Macrocystis and Egregia in 
a sufficient number of cases to suggest that dif- 
ferential algal consumption could not be at- 
tributed solely to the degree of toughness of the 
algae. A more likely explanation of the observed 
results would involve selection of algal foods on 
the basis of chemical perception. 
Assuming that food preference, as demon- 
strated in the laboratory, must indeed contribute 
to the behavior of grazing populations in the 
field, one might expect Macrocystis to receive 
concentrated grazing and to provide support for 
a great number of animals of at least these 11 
species. Grazing is selective under certain condi- 
tions in the field (Leighton, 1964, Kelp Investi- 
gation, Final Report, in preparation). Selectivity 
disappears when grazing pressures become ex- 
treme (Leighton, 1960:28). Although the food 
preference relationships found for the 1 1 benthic 
invertebrate species in the present study cannot 
be said to restrict any species to a given associa- 
tion of algae, as has been shown for the inter- 
tidal periwinkle, Littorina ohtusata (Bakker, 
1959), the preponderance of first choices falls 
under the headings Macrocystis and Egregia. In 
field studies some herbivores have appeared col- 
lectively oriented toward these plants and away 
from Pterygophora and Cystoseira (Leighton, 
1964). A tendency may exist, therefore, for the 
grazing species to remain in Macrocystis and 
Egregia stands and, perhaps, to move into such 
stands from locations nearby which are less at- 
tractive with respect to food. 3 The attractiveness 
3 Experiments reported elsewhere (North, 1963), in- 
dicating the food value of Macrocystis as compared 
with other algae ( Egregia , Pterygophora, and Bossiella 
sp.) based on conversion efficiencies, place the former 
in a leading position as a food plant for Strongylo- 
centrotus purpuratus and S. franciscanus. 
TABLE 3 
Decigrams of Different Algae Consumed by 10 Adult Lytechinus anamesus 
in 24-hour Experimental Periods* 
EXPERIMENT 
Macrocystis 
Egregia 
Laminaria 
Eisenia 
Pterygophora 
Cystoseira 
Gigartina 
Lytechinus from La Jolla Submarine Canyon; depth 20 m; 
bottom sand 
1 
51 
0 
25 
3 
1 
7 
54 
2 
31 
9 
17 
4 
0 
0 
65 
3 
25 
3 
12 
0 
0 
0 
39 
4 
14 
10 
0 
0 
0 
0 
24 
5 
22 
3 
0 
0 
0 
0 
14 
Mean 
28.6 
5.0 
10.8 
1.4 
0.2 
1.4 
39.2 
Lytechinus from channel of Mission Bay; depth 2-3 m; bottom sand 
6 
39 
0 
8 
0 
0 
0 
39 
7 
50 
9 
9 
12 
6 
10 
35 
8 
50 
2 
55 
7 
6 
6 
99 
9 
15 
8 
9 
18 
18 
3 
64 
10 
19 
6 
0 
0 
1 
11 
63 
Mean 
34.6 
5.0 
16.2 
7.4 
6.2 
6.0 
60.0 
Results are given for groups collected from depths of 20 m and 2 m in different locations. 
