Bockhorn, Kerstin (2009) Flexibility in macroalgal response to herbivory: the influence of grazing pressure on the induction of chemical defenses in seaweeds. (Diploma thesis), Leuphana Universität , Lüneburg, Germany, 28 pp.

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Abstract

Abundance and distribution of seaweeds are controlled by grazing pressure of various marine organisms. This top-down control is counteracted by defence mechanisms of macroalgae, such as changes in nutritive value or morphology, and the production of chemical compounds. Chemical defence is differentiated in constitutive (permanently present to deter grazers), and inducible defences (triggered by herbivore attack). An inducible defence is an effective strategy against herbivores: it prevents grazer adaptation and self-intoxication as well as minimizing costs for the production of secondary metabolites. So far only little is known about the flexibility of defence induction in seaweeds. We examined responses of the red alga Grateloupia imbricata and the brown seaweed Stypopodium zonale to one macro-grazer, the sea urchin Paracentrotus lividus, on the Portuguese island of Madeira in the Atlantic Ocean. Different experiments were conducted to investigate whether (1) total consumption is a positive function of grazer density, (2) whether grazing can induce a defence in the investigated algal species and (3) whether grazer density has an influence on the time span that it takes the investigated algae to respond to grazing with an upregulated defence. Living algal material was exposed to different grazer densities for up to 20 days while ungrazed algal material served as a control. We assume that previous grazing reduces the palatability because of the production of deterrent chemicals in response to grazing. To examine differences in the palatability of previously grazed and ungrazed seaweed material, two-way choice feeding assays were conducted with fresh algal material and naïve individuals of the same herbivore species P. lividus every second day. G. imbricata responded to grazing of P. lividus by upregulating a defence, while the sea urchins failed to induce a defence in S. zonale. The finding of an inducible defence in G. imbricata was confirmed by HPLC analyses. For none of the two grazer-algae combinations could we support the hypothesis that higher grazing pressure leads to a faster induction of a defence. However, our results indicated that macroalgal responses to herbivores are even more complex than currently understood.

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