Stratil, Stephanie B. (2009) Do herbivores induce defenses in macroalgae?. (Diploma thesis), Christian-Albrechts-Universität, Kiel, Germany, 63 pp.

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Abstract

Alga-herbivore interactions have a great impact on the structure of coastal benthic communities. Herbivores, of different sizes and belonging to different taxa, can exert severe grazing pressure, thus, influencing seaweed abundance and distribution. This top-down control is counteracted by algal defense mechanisms, many of which are chemical deterrents. Previous studies have unveiled the existence of defenses which are triggered by grazer attack. These so-called inducible defenses have been reported for all alga divisions and have been detected in many temperate and tropical coastal regions. However, little is known about inducible defenses in macroalgae from the Lower St. Lawrence Estuary, East Canada. In a series of laboratory experiments, I investigated the potential of four brown and two red macroalgae species from the Lower St. Lawrence Estuary to regulate defenses against direct grazing by two littorinid snail and one amphipod species. Two-choice feeding assays with live algal material were conducted to detect changes in algal palatability after exposure to grazers for a time span of 2 to 4 weeks. Algal material that was kept in the absence of grazers served as controls. Grazer preference for the control over the previously grazed algae tissue should indicate defense induction.
Interestingly, none of the screened algae species reduced their palatability in response to herbivore attack. Furthermore, consumption rates of the gastropod L. littorea feeding on Fucus vesiculosus were lower than has been reported for other study systems, possibly due to a
constitutive defense of the algae, among other factors. A chemical analysis, however, did not confirm such a defense. This study adds information to the concept of inducible defenses with regard to its reported variability by suggesting that the absence of inducible defenses at the Lower St. Lawrence Estuary may reflect a large scale intraspecific biogeographic variation in defense induction. However, other factors that may have lead to the absence of defense induction must be discussed.

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