INTESTINE LENGTH IN THE FISHES OF A TROPICAL STREAM .2. RELATIONSHIPSTO DIET - THE LONG AND SHORT OF A CONVOLUTED ISSUE

We examined the relationship between the intestine length and the amou nt of plant material in the diet of 21 species of fish from forest str eams in Panama. Alimentary tract analyses supplemented by literature r eports showed that four loricariid catfish species and one poeciliid w ere specialized herbivores consuming almost exclusively periphyton and detritus. Four species, including one erythrinid, one characid, one t richomyctycterid and one eleotrid, were carnivores consuming almost en tirely food of animal origin. Twelve species, including five characids , one lebiasinid, two pimelodelids, three cichlids and one poeciliid, were omnivores consuming food of both plant and animal origin, but the average proportion of food of plant origin (detritus and algae plus h igher plant parts) varied from 4-60%. Most omnivores increased plant f ood consumption with increasing size. Because intestine length increas es allometrically with body size and the pattern of increase differs c onsiderably among species and is influenced by length:mass relationshi ps, we compared species at the same size and took both length and mass into account. At a given size, intestine lengths of herbivores were l onger than those of omnivores, and these were longer than those of car nivores. Differences in intestine length among the dietary categories were greater at larger body sizes and when the common size was defined by body mass than when it was defined by body length. There was no tr end for the average proportion of plant material consumed to be relate d to intestine length among the omnivores, when confounding effects of body mass were taken into account. The slopes of the allometric equat ions relating log(10) intestine length to log(10) body size for herbiv ores tended to be higher than for omnivores and higher for omnivores t han for carnivores, but both herbivores and omnivores showed extensive variation and overlap with the other dietary categories. Among the om nivores, there was no trend for slopes to be steeper for species consu ming more plant material on average or for species showing larger onto genetic increases in plant consumption. These results permit increased precision in describing diet-intestine length relationships, but indi cate that the widely held belief that intestine length reflects diet i n fishes should only be applied to broad dietary categories and not to finer divisions among omnivores.