Gut anatomy and pH in a Red Sea surgeon-fish, Acanthurus nigrofuscus

Abstract

Gut length, stomach pH and intestinal pH of the brown surgeonfish Acanthurus nigrofuscus change according to conditions of feeding or starvation, time of day, and presence or absence of intestinal protozoan symbionts. Relative gut length declines by 30 to 50 % during short periods of starvation (several hours to ca 2 d). Pyloric stomach pH exceeds that of the cardiac stomach during both daytime feeding periods and night-time resting periods. Fluids in the pyloric caecae are near-neutral (mean = pH 7.1), exhibit little variation in pH, and appear to buffer intestinal contents. Intestinal pH rises rapidly from the pyloric sphincter for approximately 35 % of total intestine length, after which pH drops almost a full pH unit. This decline in pH correlates with the presence of large numbers of protozoa, and disappears when the symbionts are eliminated by starvation. The pattern of local differences in pH within the anterior and middle intestine changes during the night, when symbionts retreat to a bolus of food retained in the posterior intestine. Local differences and short-term changes in stomach or intestinal pH of herbivorous surgeonfishes may be due to several causes, but they caution against using spot checks of pH in stomach or intestine as indicators of gut physiology and indicate that we lack in-depth understanding of digestive processes in an ecologically important group of fishes.