Algal Structure Defenses: Form and Calcification in Resistance to Tropical Limpets

Growth form and calcification in marine macrophytes have been suggested to act as structural defenses against grazers. Support for this hypothesis has been largely indirect, consisting of negative correlations of plant and herbivore abundance and the observation that some herbivores prefer less calcified food plants. The hypothesized defensive role of calcification and growth form has generally not been directly tested. Experiments were conducted to test this hypothesis directly for an important group of grazers, the docoglossan limpets, and a large variety of tropical algae with varying degrees of calcification and different thallus forms. This was accomplished by mimicking the mode of feeding of the herbivore using its feeding apparatus (the radula) and directly measuring the forces required to remove tissue. The results were opposite to the predictions. Neither calcification nor the thallus forms that had been predicted to be structural defenses resulted in increased amounts of force to remove tissue. For these particular herbivores, and possibly others, these morphological features are not acting as simple structural defenses. Other explanations for observed correlations of herbivore and plant abundance and the food preferences of herbivores must be considered.