Early anatomical development of the digestive system and high intestinal digestive enzyme activity in two silverside species, Odontesthes bonariensis and O. hatcheri (Atherinomorpha: Atherinopsidae) Elva

Integrative studies are important for a better understanding of the functional development of fish digestive systems and the consequent design of suitable feeding regimes and ad-hoc diets. To characterize the ontogeny of digestive function in larvae and juveniles of two atherinopsids from South America, Odontesthes bonariensis and O. hatcheri, morphologic, histologic, and intestinal enzyme biochemical analyses were performed. Both species showed a functional digestive system from one week after hatching (wah) with a sustained high-level activity of the cytosolic enzyme leucine alanine peptidase (leu-ala) until 9 and 13 wah in O. hatcheri and O. bonariensis, respectively. Maximum specific leu-ala activities in both species were higher compared to other agastric and most gastric fish. These results and the abundant pinocytic vacuoles in the posterior intestine suggest that cytosolic digestion is crucial for these species. It seems that their apparent anatomically simple digestive system is compensated by an uncommonly early development and function of the digestive tract and accessory organs accompanied by high intestinal cytosolic activity. The above findings contribute to the understanding of the Atherinopsidae digestive model and, for the first time, suggest that under farming conditions, these species could be weaned at an early developmental stage with an appropriate balanced diet.