Digestion without a stomach: Comparative studies on the digestive physiology, nutritional ecology and energetics of stomachlessness in fishes

The lack of the stomach is surprisingly common amongst teleost fishes, though little is known about how this reductive specialisation effects the processes involved with prey processing, digestion and nutrient acquisition. The digestive physiology and morphology of a stomachless herbivore (grass carp, Ctenopharyngodon idella), an omnivore with a stomach (channel catfish, Ictalurus punctatus) and a carnivore with a stomach (largemouth bass, Micropterus salmoides) were compared to determine whether diet and feeding frequency influence the ability to modulate digestion. Digestive tract form, assessed through gross anatomical measurements and histology of the intestine, and function, assessed through assays of brush border digestive enzyme activities and measurements of nutrient uptake, were compared between fishes that had been fasted for 7 days and fishes that were recently fed and actively digesting. The grass carp response was primarily morphological, with a significant decrease in microvillar length, and little down regulation in the function of the digestive system with stable digestive enzyme activities and nutrient uptake rates. Both the catfish and the bass showed a similar degree of response by the intestinal morphology, with significant differences at the gross and cellular level. The catfish demonstrated the greatest degree of biochemical modulation, with significantly reduced digestive enzyme activities and nutrient uptake capabilities. Bass showed a modest physiological response, with decreases in enzyme activities and uptake rates. Based on these results, it appears that natural feeding frequency may not play as significant a role in determining fasting response in fishes as has been demonstrated in other animals. The effect of diet on stomachless fish digestive function was further investigated through the comparison of digestive enzyme activities between two herbivorous halfbeak (Hemiramphidae) taxa, Hyporhamphus regularis ardelio and Arrhamphus sclerolepis krefftii and two carnivorous needlefish (Belonidae) taxa, Tylosurus gavialoides and Strongylura krefftii. Although the halfbeaks showed significantly greater α-amylase activity than did the needlefish (P