Ontogenetic Changes in Defensive and Nutritional Characteristics of Juvenile Kelps and their Effects on Community Structure

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9513988 Van Alstyne Kelp forests are habitats that are important nursery grounds for many commercially important species of fish and shellfish. The kelps that make up these forests serve as food and shelter from predators for a variety of marine animals. Large numbers of kelps within kelp forests die each year creating open patches that can be colonized by juvenile kelps. In order to understand how kelp forests are sustained, it is necessary to know how juvenile kelps become established in open areas and how environmental factors affect their establishment. Like many marine organisms, kelps produce large numbers of offspring but only a few survive to become adults. Many kelp offspring die within a few weeks after they settle to the bottom because they are consumed by snails and sea urchins. When they first settle, juvenile kelps are very small relative to their predators and their tissues are thin and easily consumed. However, some species of kelps are less likely to be consumed than others and these species often become the dominant species in kelp beds. Little is currently known about why these species are less likely to be consumed by grazers, but preliminary data suggest their survival may be due to high concentrations of chemical defenses in their tissues. Previous research has shown that seaweed chemical defenses are distasteful to snails and sea urchins, and that many grazers will avoid chemically defended seaweed individulas. This study will use field and laboratory experiments to examine the biochemical and structural features of juvenile kelps that affect grazing by herbivores. The following questions will be addressed (1) Do juvenile kelps invest greater amounts of resources in defending themselves against attacks by herbivores than adult kelps, or do they preferentially allocate resources for growth rather than defense?; (2) Do kelps at different locations use different strategies in allocating resources towards defense versus growth? If so, is the strategy use d at a particular location determined by environmental features of the site?; (3). Are the polyphenolic defensive compounds that are produced by kelps effective against all types of herbivores?; and (4) Are juvenile kelps that do not invest heavily in defenses better competitors because they have more resources to invest in growth?. The results of this study will provide insights into the mechanisms by which these important habitats are sustained. They will also serve as a basis for determining the effects of changes in the environment on interactions between predators and their prey.