FOOD-HABITS, GUT MORPHOLOGY AND PH, AND ASSIMILATION EFFICIENCY OF THE ZEBRAPERCH HERMOSILLA-AZUREA, AN HERBIVOROUS KYPHOSID FISH OF TEMPERATE MARINE WATERS

Adult zebraperch, Hermosilla azurea, were found to be functional herbi vores in that animal matter constituted <0.01% of the total dry weight of stomach contents of fish collected off Santa Catalina Island in so uthern California waters. The diet of these fish consisted mainly of r ed algae (88.2% by dry wt) and also small amounts of brown (7.8%) and green (4.0%) algae. The most important dietary item, the filamentous r ed algae Polysiphonia spp., was found in >78% of the stomachs and comp rised >60% of the contents by dry weight. The digestive tract was long , on average 4.0 times the standard length of the fish, and was compos ed of the stomach, pyloric caeca, intestine, hindgut chamber with a bl ind caecum, and rectum. The mean pH of the cardiac stomach was acidic (3.9), whereas that of the intestine was nearly neutral (6.9) and that of the hindgut and blind caecum slightly acidic (6.3 and 6.6, respect ively). Algal foods are apparently digested by acid lysis in the stoma ch and by microbial fermentation in the hindgut. Zebraperch assimilate d nutritional constituents from six species of algae with varying degr ees of efficiency:carbon (73.7 to 89.7%), nitrogen (72.4 to 84.5%), an d protein (71.9 to 94.9%). The fish assimilated these constituents as efficiently or more efficiently from three species of nondietary brown algae as from three species of dietary red and green algae. These res ults show that zebraperch, like their tropical and subtropical relativ es (members of the genus Kyphosus), can digest a wide variety of algae including brown algae containing defensive secondary compounds.