Utilisation of mannitol by temperate marine herbivorous fishes

The sugar alcohol mannitol is the primary photosynthate of phaeophytes, and as such is an important dietary constituent for many marine herbivorous fishes. Sugar alcohols are not thought to be efficiently digested by vertebrates. Although vertebrates lack intestinal transporters for sugar alcohols, these compounds are thought to serve as substrates for fermentation by hindgut flora. Here, we measured mannitol in twelve species of New Zealand phaeophytes and in gut contents and gut fluid from five points along the gut of eight species of New Zealand marine herbivorous fishes: Odax pullus, Kyphosus sydneyanus, Kyphosus vaigiensis, Girella tricuspidata, Girella cyanea, Parma alboscapularis, Aplodactylus arctidens and Aplodactylus etheridgii. The mannitol content of the algae varied from 6.76% dry wt. ± 0.83 SE in Cystophora scalaris to 28.81% dry wt. ± 2.03 SE in Marginariella boryana. The fishes consumed varying amounts of mannitol, correlating with their consumption of phaeophytes. In O. pullus and K. sydneyanus the amount of mannitol in gut contents decreased markedly in the hindgut. The correlation of this decrease with the density of gut microbiota and fermentation products suggests that mannitol is fermented to short-chain fatty acids in the hindgut, rather than being directly taken up by the fish. Hindgut fermentation of mannitol may thus serve as an important mechanism for energy salvage in hindgut-fermenting fishes that feed on phaeophytes.