Studies of Food Preference in Algivorous Invertebrates 
of Southern California Kelp Beds 1 
David L. Leighton 
ABSTRACT : Stands of the giant kelp, Macrocystis pyrifera, frequently suffer attack 
by grazing organisms and in some cases complete destruction has been observed. The 
present investigation of the feeding habits of grazing organisms is primarily con- 
cerned with food preferences. Discrimination in choice of plant foods was examined 
in 1 1 common invertebrate species of the sublittoral rock bottom fauna of southern 
California by measuring the differential consumption of seven common algal species 
in the laboratory. The algae were selected as representing the major floral elements 
of the kelp bed ( Macrocystis pyrifera) community. All of the grazers exhibited high 
degrees of preference for Macrocystis. Shallow water grazers revealed stronger pref- 
erences for Egregia than for Macrocystis. Herbivores found at greater depths in- 
dicated strongest preferences for Macrocystis, Laminaria, and Pterygophora, plants 
that are generally common at these depths. The deepest-living herbivore, Lytechinus , 
showed greatest preference for a red alga, Gigartina; red algae generally supplant 
brown algae in dominance at greater depths. Some of the invertebrates refused cer- 
tain of the marine plants. A specific distaste factor may exist in these cases. 
The large laminarian brown alga, Macro- 
cystis pyrifera, has attracted interest during the 
past decade since it became apparent that the 
once extensive beds of this plant were on the 
decline in southern California and northern Baja 
California (North, 1962, 1963:7). A five-year 
study of the kelp beds was undertaken at the 
Scripps Institution of Oceanography (Institute 
of Marine Resources) to seek causes of the kelp 
regression. (The final report is in preparation.) 
Physical and chemical factors possibly detri- 
mental to kelp, e.g., elevated temperature and 
high turbidity of nearshore water and toxicity 
associated with industrial wastes discharged into 
the sea, are discussed elsewhere (North, 1963). 
Studies of food habits and behavior of kelp- 
associated fauna were prompted when it was 
found that grazing by organisms normally a part 
of the kelp community was frequently a cause 
of kelp destruction (Leighton, I960; North, 
1963). Because gut contents provide unreliable 
clues to food habits in algivorous animals, 
1 This contribution is from the Scripps Institution 
of Oceanography, University of California, San Diego. 
Support for these studies was provided by the National 
Science Foundation (Grants No. 785 and 10688) and 
by the Institute of Marine Resources, University of 
California. Manuscript received May 21, 1964. 
studies were carried out in marine laboratory 
aquaria where food selection could be observed. 
Studies of algal preferences in the marine 
snail, Littorina obtusata, considered chiefly scent 
perception and orientation to algae which were 
variously distributed within large tanks in which 
slow unidirectional water flow was usually main- 
tained (Barkman, 1955; Van Dongen, 1956; 
Bakker, 1959 ). The present study and an earlier 
one (Leighton and Boolootian, 1963) report 
gustatory preference and quantitative measure- 
ment of the amounts of algal foods ingested. The 
method reflects more certainly the animals’ 
genuine food preference in contrast to selection 
of algae through other motivation (e.g., for 
spawning substrate, specific plant shelter, etc.). 
Barkman ( 1955:50) states that L. obtusata de- 
posits its eggs on fronds of Fucaceae, particu- 
larly on Fucus serratus. 
MATERIALS AND METHODS 
The procedure employed was simple. Groups 
of a grazing species, held either in large concrete 
tanks or in glass aquaria, were provided equal 
weights of each of several common algae occur- 
ring in or near kelp beds. Amounts ingested 
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