Contrasting digestive strategies in four New Zealand herbivorous fishes as reflected by carbohydrase activity profiles

Enzymatic degradation of algal carbohydrates was examined in the New Zealand herbivorous fishes Parma alboscapularis (Pomacentridae), Aplodactylus etheridgii (Aplodactylidae), Girella tricuspidata and G. cyanea (Girellidae). Enzyme extract taken from the anterior gut wall, gut fluid and microbial pellet from sections sampled along the gut were tested for activity against starch, carrageenan, agarose and carboxymethylcellulose. Hydrolysis of starch was greater than for all other substrates tested. Endogenous (host-produced) activity in the anterior gut fluid varied between species in the order G. tricuspidata (7700 units mL− 1) > G. cyanea (2300 units mL− 1) > P. alboscapularis (2000) > A. etheridgii (1400 units mL−1) where one unit is equivalent to 1 μg of reducing sugar released per minute. Activity decreased markedly along the gut in all cases, so that at the posterior end of the gut only 0.3–8% of the anterior activity remained in the gut fluid. Enzyme activity against structural carbohydrates was lower than that against starch, and was of exogenous (produced by resident microbiota) origin in all species although the location of activity along the gut differed. The microbial extract of A. etheridgii displayed the highest activity against carrageenan and agarose in all gut sections, reaching maxima of 47 units mL− 1 against carrageenan and 35 units mL− 1 against agarose in the mid-gut microbial extract. Carrageenase and agarase activity in the other three species was < 10 units mL− 1 for all gut sections. Results suggest that carrageenan and agarose are potentially important substrates for microbial fermentation, particularly in A. etheridgii, and that there is microbial activity in the mid-gut of this species, rather than primarily in the hind-gut as in other herbivorous species.