MEPS 566:79-90 (2017)  -  DOI: https://doi.org/10.3354/meps12004

ABSTRACT: Phlorotannins are a putative anti-herbivory defense trait in brown algae (Phaeophyceae), but their benefits and trade-offs with other fitness components have seldom been studied in the wild. To address this, we manipulated herbivore access to cloned genotypes of the brown alga Fucus vesiculosus in the marine sublittoral over 2 seasons varying in the magnitude of natural herbivory. We measured grazing damage, growth, and the content of induced and constitutive phlorotannins of algae. In addition, we used herbivore preference bioassays to quantify induced resistance. Under weak spring herbivory, there was no induction of resistance or increase in phlorotannin content. However, under intense autumn herbivory, algae induced resistance with an 18% increase in phlorotannins. The relative amounts of constitutive and induced phlorotannins varied among genotypes, and grazing decreased with an increase in constitutive phlorotannins, both in the wild and in preference bioassays. However, the variation among genotypes in the induced phlorotannins did not covary with the variation in induced resistance, indicating that possible contribution of phlorotannins to induced resistance is based on a small quantitative threshold or on a qualitative change in polymer composition. A trade-off between constitutive and induced phlorotannins provides evidence for costs of phlorotannin production, while results also show that in nature trade-offs may vary with seasonal variation in herbivory and growth allocation patterns. Seasonality of herbivory and costs of phlorotannins likely contribute to the adoption of induced resistance, which serves as a cost-saving strategy when herbivory remains low and as an efficient defense during periods of intense herbivory.

KEY WORDS: Field experiment · Trade-off · Induced defense · Phaeophyceae · Fucus vesiculosus · Idotea balthica