Chlorophyte and rhodophyte starches as factors in diet choice by marine herbivorous fish

The primary aim of this study was to investigate differences in the digestibility of algal starches that could affect the diet choices of marine herbivorous fish. We examined nine species of algae from New Zealand. Five of these species were dietary (i.e. species eaten by herbivorous fish); three chlorophytes Enteromorpha sp., Ulva rigida, and Caulerpa geminata, and two rhodophytes Champia novae-zelandiae, and Gigartina livida. The other four species were nondietary, one chlorophyte Caulerpa flexilis, and three rhodophytes, Osmundaria colensoi, Plocamium costatum, and Asparagopsis armata. This study consisted of three main parts. First, we measured the ash, protein, lipid and starch content of the algae. Second, we examined the susceptibility of the algal starch to hydrolysis by digestive enzymes from four species of New Zealand marine fish. Samples of the dried, ground algae were digested with an extract of digestive enzymes from the herbivores Parma alboscapularis, Aplodactylus arctidens and Aplodactylus etheridgii, and the omnivore Girella tricuspidata. Third, we determined the ability of the enzyme extracts from the fish to hydrolyse the starch components amylose, amylopectin, and maltose. The ash, protein, lipid and starch content varied significantly among the algal species. The rate of starch hydrolysis from all four enzyme extracts also varied significantly among the algal species. Enzyme extracts from all fish species hydrolysed amylose more readily than amylopectin, and maltase activity was found in all enzyme extracts. There was no significant separation between dietary and nondietary algae on the basis of ash, lipid, or starch content, or rate of starch digestion. There was significant separation between dietary and nondietary algae on the basis of two variables: (1) protein content, which was highest in nondietary algae, and (2) when both starch content and the rate of starch digestion (glucose produced g(-1) algae h(-1)) were taken into account, dietary algae was significantly higher than nondietary algae. We draw two main conclusions from this study. First, both starch content and starch digestibility may play a role in structuring the diet choices of marine herbivorous fishes. Second, measurements of either total soluble carbohydrates, or total storage polysaccharides will not reflect assimilable energy in algivorous fish. Estimation of assimilable energy requires an examination of the nutritional components of the algae in relation to the digestive physiology of the herbivore. (C) 1999 Elsevier Science B.V. All rights reserved.