Food habits, gut morphology and pH, and assimilation efficiency of the zebraperch Hermosilla azurea, an herbivorous kyphosid fish of temperate marine waters

Abstract

Adult zebraperch, Hermosilla azurea, were found to be functional herbivores in that animal matter constituted <0.01% of the total dry weight of stomach contents of fish collected off Santa Catalina Island in southern California waters. The diet of these fish consisted mainly of red 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 red algae Polysiphonia spp., was found in >78% of the stomachs and comprised >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 composed of the stomach, pyloric caeca, intestine, hindgut chamber with a blind 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, respectively). Algal foods are apparently digested by acid lysis in the stomach and by microbial fermentation in the hindgut. Zebraperch assimilated nutritional constituents from six species of algae with varying degrees of efficiency: carbon (73.7 to 89.7%), nitrogen (72.4 to 84.5%), and 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 results show that zebraperch, like their tropical and subtropical relatives (members of the genus Kyphosus), can digest a wide variety of algae including brown algae containing defensive secondary compounds.