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

The evolutionary hypotheses on plant-herbivore interaction assume that plan t secondary compounds, such as the phlorotannins of brown algae, function a s feeding deterrents for herbivores. We studied the effect of seaweed quali ty on the feeding preferences and performance of the isopod Idotea baltica, We offered I. baltica 6 species of algae, abundant in the Fucus vesiculosu s 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 feed ing preferences among the natural algae: the order of decreasing preference was F. vesiculosus > Dictyosiphon foeniculaceus > Elachista fucicola > Pil ayella littoralis > Enteromorpha intestinalis > Ceramium tenuicorne. The pr eferences in the test with artificial food, however, did not parallel those with natural algae, suggesting that the chemical quality of algae is not t he major determinant of feeding preferences. Furthermore, performance of is opods when reared on a diet of single algal species did not match the feedi ng preferences of natural algae: the most preferred brown alga provided poo r growth rate. Surprisingly, the more phlorotannin a seaweed species contai ned, the more it was preferred by I. baltica. Moreover, the assimilation ef ficiency of soluble sugars was generally high when isopods fed on brown alg ae, and in the 2 species richest in phlorotannins it was not correlated wit h the phlorotannin concentration of the algal individual. In contrast to th e conventional assumption of the defensive function of phlorotannins, this study shows that phlorotannins in seaweeds do not function as feeding deter rents to I. baltica. Instead, this herbivore readily feeds on phenolic-rich host plants, which, however, carries a cost in terms of decreased growth r ate. We suggest that feeding preferences and habitat choice behavior evolve together; habitat structure, in terms of predation avoidance, and the spat iotemporal stability of the host algae are more important factors selecting for feeding preferences in mesoherbivores than the chemical composition of algae.