Changes in the gastrointestinal pH from larvae to adult in Senegal sole (Solea senegalensis) M. Yúfera ⁎ , M.J. Darías Instituto Ciencias Marinas Andalucía (C.S.I.C.), Apartado Oficial, 11510 Puerto Real, Cádiz, Spain Received 10 November 2006; received in revised form 4 February 2007; accepted 5 February 2007 Abstract This study examines the changes of pH in the lumen of the digestive tract during the transition from larvae to juveniles and in adults of the invertebrate-feeder marine flatfish Solea senegalensis. Digestive pH was determined in the different segments of the gut with a pH microelectrode (tip diameter 600 μm). The smaller larvae in which the measurement was possible ranged between 20 and 40 mg wet weight. This size roughly coincides with the apparition of the first gastric glands. The juveniles and adults of this species have a small stomach, long intestine and lack a pyloric caeca. The gastric pH declined slightly with an increase in weight and never decreased below 6.0 irrespective of the age. In older fish the gastrointestinal pH was significantly lower than in younger ones. The values in the anterior part of the intestine were close to those measured in the stomach, but an increase in pH was observed progressing through the medium and posterior intestine. Acid digestion and proteolysis in the stomach seems to be residual in the Senegal sole. The digestion occurs primarily in its long intestine in a slightly alkaline environment. © 2007 Elsevier B.V. All rights reserved. Keywords: Gastrointestinal pH; Senegal sole; Stomach; Development 1. Introduction One of the main features which determine the end of the transformation from larvae to juvenile stage in teleosts is the development of a complete functional adult-style digestive system. Generally, in marine fish larvae that hatch from small pelagic eggs, the digestive system is not fully developed at first-feeding but already functional (Segner et al., 1994; Moyano et al., 1996; Martínez et al., 1999, Falk-Petersen, 2005). Digestion during the first stages takes place in the intestine with the contribution of pancreatic and cytosolic intestinal enzymes in an alkaline- neutral pH environment. Protein digestion occurs mainly by the action of alkaline proteases such as trypsin and chymotrypsin and cytosolic peptidases (Zambonino- Infante and Cahu, 2001). These enzymes have limited capacity for macromolecules digestion that are absorbed by the pinocytotic activity of the hindgut enterocytes for their intracellular digestion. In species with a stomach, the adult stage of food processing implies an acid digestion and consequently a more efficient extracellular digestion of proteins (Karpoor et al., 1975; Segner et al., 1994). The first gastric glands can be detected a few days or weeks after hatching and their number increases progressively partially or completely covering the stomach epithelium depending on the species (Ribeiro et al., 1999; Ortiz- Delgado et al., 2003; Elbal et al., 2004). Accordingly, the secretions of these glands, pepsinogen and hydrochloric acid, induce a progressively lower pH environment in the Aquaculture 267 (2007) 94 – 99 www.elsevier.com/locate/aqua-online ⁎ Corresponding author. Fax: +34 956 832612. E-mail address: manuel.yufera@icman.csic.es (M. Yúfera). 0044-8486/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.aquaculture.2007.02.009