Digestive Specializations for Herbivory in Marine Fishes: Genetic Adaptation and Phenotypic Plasticity in Gut Structure and Function in Pricklebacks

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Horn 9906857The requirements for herbivory in terms of gut structure and function remain poorly understood for marine fishes. The impetus for this research can be framed as a question: What does it take to be a herbivore? To answer this question, feeding experiments and accompanying histological and enzymatic analyses at different levels of biological organization will be performed. The work involves a two-part investigation addressing genetic adaptation and phenotypic plasticity of the digestive system in four related species of marine fishes. In the first part of the project, the objective is to test whether there are genetically programmed changes in gut structure and function that precede an ontogenetic dietary shift from carnivory to herbivory. To meet this objective, the activity, location, concentration and gene expression of a broad array of digestive and absorptive enzymes will be determined in the target fishes that have been fed a consistent animal diet during an interval in which two of the species naturally switch from carnivory to herbivory and the other two naturally remain carnivorous. In the second part of the project, the objective is to assess the degree of phenotypic plasticity in gut structure and function shown by carnivorous and herbivorous fishes. To meet this objective, gut structure and digestive enzyme adjustments and body composition changes will be determined during experiments in which the four target species are fed both a plant and an animal diet. The fishes chosen for this study are members of adjacent clades in a larger clade of a phylogeny that has been proposed for the family (Stichaeidae, pricklebacks) to which these species belong. These related taxa are viewed as model species for this type of research because all four begin life as carnivores, but two switch to herbivory as small juveniles while the other two remain carnivorous throughout life. The histological and enzymatic data obtained in the two-part study will be mapped as phenotypic traits onto the stichaeid phylogenetic tree using comparative biology methods to see whether adaptations for herbivory and the evolutionary trajectory of diet change can be inferred for this family of fishes. The research has the potential to make a substantial contribution to the understanding of gut structure and function in marine fishes and offers a model approach that can be applied to other marine systems. The use of an array of powerful in situ histochemical and molecular techniques in the context of both ontogeny and phylogeny should advance the knowledge of digestive processes among marine fishes in general and broaden the perspective on marine vertebrate herbivores. The project will also help advance science education and develop human resources by involving persons from a full range of academic levels and from different gender and ethnic backgrounds. Teaching, training and learning will be enhanced by this diversity of involvement and by the acquisition of new laboratory equipment.