Nancy TARGETT, Anne BOETTCHER, Timothy TARGETT, Thomas ARNOLD
University of Delaware, Graduate College of Marine Studies,
700 Pilottown Rd., Lewes. DE 19958, U.S.A.
Brown algal polyphenolics (phlorotannins) are polymers of phloroglucinoi (1,3,5trihydroxybenzene) with a wide range of molecular sizes (up to 650kDa). Phlorotannins differ from terrestrial polyphenolics in that they are derived via the acetate-malonate pathway (vs shikimic acid pathway for terrestrial polyphenolics) and occur as a single structural class. They have been shown to deter feeding in some, but not all, marine herbivores. The activity of phlorotannins, like their terrestrial counterparts, the condensed tannins, is thought to be related to their ability to complex with proteins and render the algae indigestible. Studies in terrestrial systems have suggested that herbivore gut characteristics play a major role in this complexation. We tested this hypothesis in several marine herbivores (fishes, urchins, snails, a crab and a chiton) by examining the relationship between herbivore gut characteristics (pH, redox state, anatomical features) and herbivore assimilation of phlorotannin-rich algae.
We examined gut characteristics in 9 marine fishes representing the four recognized gut types and related them to assimilation efficiencies in fishes representinc. three of the four fish gut types. Type I fishes have an acid stomach, neutral intestine generally oxidizing gut redox state, and low assimilation efficiency. Type I fishes have a pharyngeal mill, no stomach, neutral to basic gut pH, slightly reducing redox state in the posterior portion of the intestine, and high assimilation efficiency. Type IV fishes, typified by kyphosids, have an acidic stomach, a hindgutfermentation caecum with very reducing redox state, and high assimilation efficiency. Type II fishes have a gizzard-iike stomach with a near neutral stomach pH and a slightly reducing redox state. Although no data is available on assimilation efficiencies of phlorotannin-rich food by Type II fishes, we predict that it would be high. Exposure to low stomach pH appears to be important to formation of the phenolic-protein complexes. Such complexes apparently do not form in Type III fishes. The hindgut fermentation caecum, with highly reducing conditions, in Type iV fishes appears to create conditions which facilitate digestion, reversing the effects of iow stomach pH.
We also examined gut characteristics for 8 species of marine invertebrates an evaluated assimilation efficiency values for phlorotannin-rich food in three of the eight species. For all eight species examined, pH values across the guts were neutral to slightly basic. The redox states in all but one species (Tegula funebralis) were oxidizing. Assimilation efficiency values on phlorotannin- rich food, available for three of the eight species tested (including Tegula funebralis), were high, again suggesting the importance of exposure to low stomach pH in the formation of protein- phlorotannin complexes.
Comparisons are made between the results obtained for these marine herbivores and various terrestrial herbivores.