Comparison between the omnivorous jundiá catfish (Rhamdia quelen) and Nile tilapia (Oreochromis niloticus) on the utilization of dietary starch sources: Digestibility, enzyme activity and starch microstructure

Omnivorous fish utilize dietary carbohydrates better than carnivorous ones due to striking morphological and physiological differences in their digestive tracts. However, digestive tract morphology and physiology also varies among omnivorous fishes, which can lead to different dietary utilization of plant sources as well. Starch, energy, and dry matter apparent digestibility coefficient (ADC) of wheat bran, cassava residue, ground corn, and broken rice were compared between the omnivorous freshwater jundiá catfish (93.9±34.0g; mean±standard deviation) and Nile tilapia (93.7±51.6g) by using 5gkg−1 chromic oxide as a diet marker. Starch ADCs were significantly higher in tilapia for all plant sources (92.02% to 99.74% versus 55.87% to 90.61%), except for wheat bran, which was similar to that found in jundiá. Starch-richer plant sources (ground corn and broken rice) showed the lowest starch digestibility for jundiá. However, ground corn provided significantly higher energy digestibility in jundiá (55.35%), as opposed to broken rice (86.59%) and ground corn (71.68%), in tilapia. Dry matter ADCs ranged from 22.89% for cassava residue in jundiá to 89.17% for broken rice in tilapia. Higher dry matter ADCs were registered in tilapia for all plant sources, except for wheat bran. Despite presenting lower starch and energy digestibilities, jundiá catfish showed an adaptive capacity to utilize starch sources by presenting significantly higher specific activity of amylase (58 U mg protein−1 versus 29 U mg protein−1) and maltase than Nile tilapia for all plant sources, except for wheat bran. Maltase activities in the gut of jundiá varied from 2.5 to 3.6 U mg protein−1, depending on the plant source, while in tilapia it averaged 1.7 U mg protein−1, regardless of the plant source. The scanning electron microscopy approach used to study the influence of fish digestion on the structure of starch granules from different plant sources revealed the presence of spherulites in the fish feces. Starch spherulite formation in jundiá intestines could be related to lower starch digestibility. Our findings demonstrate different carbohydrate digestion abilities among omnivores.