X-radiographic observations of food passage and nutrient absorption along the alimentary tract of archerfish, Toxotes jaculatrix

X-radiography can be used to estimate gastric emptying time, which is valuable in calculating food consumption and growth rates in fishes. The movement of food through the alimentary tract (gastric motility) of banded archerfish, Toxotes jaculatrix (Pallas, 1767), was observed in the laboratory under natural conditions of temperature (27 °C) and salinity (28) using X-radiography, with barium sulphate (BaSO₄) as an inert food marker. Nutrient absorption along the alimentary tract (expressed as relative percentage absorption gradient) was calculated using the apparent digestibility coefficient (ADC) based on ash contents of adjacent samples. We found that initial voiding of fecal matter began 6–12 hrs after feeding commenced, and that alimentary tracts were completely emptied within 96 hrs. Considerable proportions of all macronutrients were absorbed in the short region (anterior intestine/pyloric caeca), but active absorption continued along the longer middle and posterior intestinal segments. The macro- nutrient in wild-fed T. jaculatrix differed (51% protein, 63% lipid, 20% carbohydrate, and 40% energy) from that of laboratory-fed fish (75% protein, 96% lipid, 50% carbohydrate, and 87% energy), due to the fact that the proximate contents of macronutriet in the wild-fed fish were unknown, while the diet of the laboratory-fed fish was initially known. Gut passage times in T. jaculatrix was longer than those reported in various temperate carnivorous fishes, which is surprising given the higher temperatures in the tropics, however, all previous studies were performed on cultured fish fed commercial pellets and therefore direct comparisons are not possible. Slow gut passage may depend on the types of food ingested (e.g., high chitinous mealworm) that correlated to lower food consumption. Our results generally indicated that T. jaculatrix is opportunistic feeders regardless of the macronutrient content of the prey.