Feeding preferences and performance of a marine isopod on seaweed hosts: cost of habitat specialization

The evolutionary hypotheses on plant-herbivore interaction assume that plant secondary compounds, such as the phlorotannins of brown algae, function as feeding deterrents for herbivores. We studied the effect of seaweed quality on the feeding preferences and performance of the isopod Idotea baltica. We offered I. baltica 6 species of algae, abundant in the Fucus vesiculosus belts where this mesograzer lives, in simultaneous preference tests. The tests were conducted both with natural algae and with artificial food made of freeze-dried and powdered algae of the same species. We found clear feeding preferences among the natural algae: the order of decreasing preference was F. vesiculosus > Dictyosiphon foeniculaceus > Elachista fucicola > Pilayella littoralis > Enteromorpha intestinalis > Ceramium tenuicorne. The preferences in the test with artificial food, however, did not parallel those with natural algae, suggesting that the chemical quality of algae is not the major determinant of feeding preferences. Furthermore, performance of isopods when reared on a diet of single algal species did not match the feeding preferences of natural algae: the most preferred brown alga provided poor growth rate. Surprisingly, the more phlorotannin a seaweed species contained, the more it was preferred by I. baltica. Moreover, the assimilation efficiency of soluble sugars was generally high when isopods fed on brown algae, and in the 2 species richest in phlorotannins it was not correlated with the phlorotannin concentration of the algal individual. In contrast to the conventional assumption of the defensive function of phlorotannins, this study shows that phlorotannins in seaweeds do not function as feeding deterrents to I. baltica. Instead, this herbivore readily feeds on phenolic-rich host plants, which, however, carries a cost in terms of decreased growth rate. We suggest that feeding preferences and habitat choice behavior evolve together; habitat structure, in terms of predation avoidance, and the spatiotemporal stability of the host algae are more important factors selecting for feeding preferences in mesoherbivores than the chemical composition of algae.