Digestion in Herbivorous Reef Fishes
Most algae (like terrestrial plants) have complex polysaccharide cell walls which must be broken down after consumption so the fish can access their nutrients. In herbivorous fishes this can be achieved by mechanical, chemical, or enzymatic stress[i]. Mechanical breakdown of the cell wall can be performed using a pharyngeal mill or gizzard-like stomach, both of which grind and shred algal cells, an action that is analogous to our own chewing. Chemical digestion is attained using acid lysis of the cells in the stomach. Finally, enzymatic digestion of algal cells is most likely accomplished by microbial fermentation; enzymes produced by microorganisms inhabiting the gastrointestinal tract can breakdown the otherwise unusable polysaccharides and convert them to organic molecules that the fish can absorb and utilize.
The Role of Microbes in the Digestion in Reef Fishes
There is increasing evidence for the contribution of intestinal microbes (and their enzymes) to the digestion of algae in herbivorous reef fishes. Microbial fermentation has been detected using short-chain fatty acid (SCFA) analyses in the intestines of a variety of species of tropical herbivorous fishes, including many Epulopiscium-harboring Acanthurids[ii],[iii]. SCFAs are products of microbial fermentation and are thought to contribute to the nutrition of some herbivorous fishesV. A more direct study of the intestinal microbiota of the herbivorous king angelfish (Family: Pomacanthidae) demonstrated that a variety of gut microorganisms could be cultured from angelfish intestinal contents, and some of these microbes were able to grow on components of algal cell walls (cellulose, agar, and alginic acid)[iv]. The Angert lab·s own work on the bacterial community in the gut of Naso tonganus (Bulbnose Unicornfish) has yielded gene sequences from bacteria that are closely related to species such as Clostridium, Bacteroidetes, and Verrucomicrobia.
[i] Choat, J.H. and K.D. Clements 1998. Vertebrate Herbivores in Marine and Terrestrial Environments: A Nutritional Ecology Perspective. Annual Review of Ecological Systems 29: 375-403
[ii] Clements, K.D. and J.H. Choat. 1995. Fermentation in Tropical Marine Herbivorous Fishes. Physiological Zoology 68(3): 355-378
[iii] Choat, J.H., K.D. Clements, and W.D. Robbins. 2002. The trophic status of herbivorous fishes on coral reefs: 1. Dietary Analyses. Marine Biology 140:613-623
[iv] Martinez-Diaz, S.F. and H. Perez-Espana. 1999. Feasible mechanisms for algal digestion in the king angelfish. Journal of Fish Biology 55: 692-703
[v] Kuiter, Rudie H. and Helmut Debelius. 2001. Surgeonfishes, Rabbitfishes, and their Relatives: A comprehensive guide to Acanthuroidei. TMC Publishing: Chorleywood, UK. 18