Polar extracts of the brown alga Fucus vesiculosus (L.) reduce assimilation efficiency but do not deter the herbivorous isopod Idotea baltica (Pallas)

Plant secondary metabolites provide resistance against herbivory if they are capable of decreasing herbivore performance or deterring feeding. Here we test the effects of phlorotannin-containing polar extracts from the brown alga Fucus vesiculosus on the assimilation efficiency and feeding preferences of the herbivorous isopod Idotea baltica. We test the effects separately for each sex of the isopod to test the hypothesis that the digestive efficiencies of the sexes may have diverged. Phlorotannin-containing polar extracts were incorporated into an agar-based artificial food in varying amounts. We also varied the quality of the food, using high-quality food containing animal proteins and low-quality food containing only algal material. The total assimilation efficiency as well as the assimilation efficiencies of carbon and nitrogen decreased with an increasing concentration of phlorotannin-containing extracts. The negative effects on carbon and nitrogen assimilation were higher when extracts were incorporated in the high-quality food. Females were consistently more efficient than males in all assimilation measures; there were no interactions between sex and food quality or phlorotannin concentration, suggesting that the digestive efficiency of the sexes had diverged but that the two sexes responded similarly to phlorotannin enriched artificial food. Although phlorotannin-containing extracts were harmful to performance, they did not deter feeding in preference tests. On the contrary: the isopods preferred the food containing more phlorotannins, suggesting that phlorotannins may act as cues in host recognition. Thus phlorotannins were inefficient deterrents, and I. baltica seems to be adapted to utilizing phlorotannin-containing foods despite their detrimental effects on assimilation. We suggest that F. vesiculosus provides a predation refuge and temporal constancy in availability for I. baltica, both of which select for the ability to utilize it as a food. Phlorotannin-containing polar extracts from F. vesiculosus are still harmful digestibility reducers, and may thereby prevent complete specialization and generate selection for utilization of a wider host range.