Herbivory And Its Effects On Brown Algae
Seaweed serves as a primary food source for a number of marine herbivores. Various fish, sea urchins, gastropods, and crabs, as well as smaller herbivores such as amphipods, isopods, and polychaetes all eat the plants. In many aquatic habitats, this consumption is intense, and has been going on throughout most of evolutionary history. Consequently, in response to these pressures many seaweeds have evolved elaborate defense mechanisms. In the first of three articles which will be presented, Duffy and Hay (1990) describe the different protective strategies employed by marine plants. Of these, perhaps the most common are the chemical defense mechanisms. Such defense systems are generally most elaborate among tropical seaweeds. However, in tropical plants certain strategies are also conspicuously absent. For example, the article by Steinberg et al. (1991) notes that tropical brown algae produce very low levels of protective polyphenolic compounds. The paper also gives several possible explanations for this observation. A third article by Tugwell and Branch (1992), however, may provide the answer. It suggests that many fish species have evolved countermeasures to the polyphenolics. Thus, it may be that the compounds offer no survival advantage against tropical fish. Since most defensive measures also involve some inherent cost to the organism, the production of polyphenolics by tropical seaweeds has subsequently been selected against and therefore diminished over time.
The paper, Duffy and Hay (1990), describes an assortment of various defenses employed by seaweeds against herbivores. The authors have, for the most part, presented a review of the experimental studies performed by others. The three primary defensive strategies used by plants include the following: (1) escape; (2) tolerance; and (3) deterrence (1:368).
Escape tactics generally involve either spatial,...