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Algal morphology is considered to be influenced by physiological and environmental factors such as stress and disturbance; one key disturbance exerted on algae is grazing pressure from herbivores. The factors upon which herbivores base their food preferences include algal attractiveness and edibility. Steneck and Watling's (1982) functional group model advocates the combination of algal species into seven groups based upon morphology and ecological function; the basic premise being that algal attractiveness and edibility will decrease hierarchically from functional group one to functional group seven, and that species within a group will be similarly attractive and edible. Two grazers commonly found in the intertidal area are the gastropod Littorina littorea and the isopod !dotea granulosa. Attractiveness and edibility experiments were conducted in which both grazers were presented with algae representing functional groups two to seven in single- and two-way choice experiments, in order to determine whether their food preferences could be predicted using a functional group approach. Although L. littorea and I. granulosa exhibited preference for algae between functional groups with regards to attractiveness and edibility, preference for both functional group and algal species differed between grazers. When the effect of algal morphology was investigated by presenting grazers with algal homogenates in agar, both grazers displayed a preference for algal extracts in agar over whole plant material, and exhibited preference for algae both between- and within-functional groups. The fact that not all species within a functional group were consumed in similar amounts contradicts the functional group model. Further investigation of the effect of algal availability on the feeding preferences of L. Iittorea showed their food choices could not be predicted based upon the algal species dominant in their habitat of origin. Results from all experimental manipulations suggest that both the habitat and food requirements of a herbivore, along with algal characteristics such as morphological, structural and chemical defences, algal availability and nutritional composition all influence herbivore food choice. As such, the functional group model proposed by Steneck and Wading (1982) cannot be accurately used to predict the feeding preferences of L. littorea or I. granulosa. It is suggested that unless models are modified to meet the requirements of a specific question, current functional group approaches are not a useful tool for predicting algal-grazer interactions.