Digestive system development and study of acid and alkaline protease digestive capacities using biochemical and molecular approaches in totoaba (Totoaba macdonaldi) larvae.

Abstract

The present study aimed to describe and understand the development of the digestive system in totoaba (Totoaba macdonaldi) larvae from hatching to 40days post-hatch (dph) from morphological and functional perspectives. At hatch, the digestive system of totoaba was undifferentiated. The anus and the mouth opened at 4 and 5dph, respectively. During exogenous feeding, development of the esophagus, pancreas, liver and intestine was observed with a complete differentiation of all digestive organs. Expression and activity of trypsin and chymotrypsin were observed as early as at 1dph, and increments in their expression and activity coincided with changes in food items (live and compound diets) and morpho-physiological development of the accessory digestive glands. In contrast, pepsin was detected later during development, which includes the appearance of the gastric glands between 24 and 28dph. One peak in gene expression was detected at 16dph, few days before the initial development of the stomach at 20dph. A second peak of pepsin expression was detected at day 35, followed by a peak of activity at day 40, coinciding with the change from live to artificial food. Totoaba larvae showed a fully morphologically developed digestive system between 24 and 28dph, as demonstrated by histological observations. However, gene expression and activity of alkaline and acid proteases were detected earlier, indicating the functionality of the exocrine pancreas and stomach before the complete morphological development of the digestive organs. These results showed that integrative studies are needed to fully understand the development of the digestive system from a morphological and functional point of views, since the histological organization of digestive structures does not reflect their real functionality. These results indicate that the digestive system of totoaba develops rapidly during the first days post-hatch, especially for alkaline proteases, and the stomach becomes functional between 20 and 24dph allowing the weaning process to begin at this age.