Abstract The levels of various ions (Na + , Cl ) , Ca 2+ , Mg 2+ ,K + ) in the fluid phase of the gastro- intestinal contents and of the plasma were determined in four teleosts with different feeding habits: traira (Hoplias malabaricus), hassar (Hoplosternum littorale), silver catfish (Rhamdia quelen), and grass carp (Ctenopharyngodon idella). The results showed that most ionic levels in the fluid phase of the gastrointestinal tract seem to be affected by feeding habit and that these levels can be different from those found in the plasma. In practical terms, these results suggest that ionic levels of the gastrointestinal contents should be considered when preparing solutions for studies of intestinal absorption in fishes. Keywords Feeding habit Æ Gastrointestinal tract Æ Ionic levels Æ Plasma Introduction Freshwater teleosts are always in a hyperos- motic state in relation to their surrounding environment and, consequently, they have two basic problems: the loss of ions by diffusion, and the inflow of water by osmosis (Baldisser- otto 2003). Ions are absorbed by the gills (Wood 2001) and urinary bladder, and excess water is eliminated by abundant and diluted urine (Curtis and Wood 1991). The drinking rate of freshwater teleosts is low (Flik et al. 1995), but independently of whether freshwater teleosts drink water or not, the importance of the digestive tract on osmoregulation cannot be disregarded, since a portion of the ion absorp- tion could occur in the intestine (Baldisserotto and Mimura 1994). The intestine (or pyloric ceca, when present) absorbs Na + , Cl ) , Ca 2+ , and Mg 2+ (and probably other ions) that is present in the food (Dabrowski et al. 1986; Buddington and Diamond 1987; Boge ´ et al. 1988; Baldisserotto et al. 1993; Kerstetter and White 1994; Baldisserotto and Mimura 1995; Bijvelds et al. 1998). The intestine also has to reabsorb ions discharged by gastric secretion (such as Cl ) ) (Hirano and Mayer-Gostan 1976) as well as by bile when fat enters the intestine. The intestine of several species of freshwater teleosts either absorbs (Smith 1964; Skadhauge 1969; Nakamura 1985; Baldisserotto et al. 1996b) or A.G. Becker Æ J.F. Gonc ¸alves Æ L.O. Garcia Æ E.R. Behr Æ B. Baldisserotto (&) Departamento de Fisiologia e Farmacologia, Universidade Federal de Santa Maria, 97105-900 Santa Maria, RS, Brazil e-mail: bernardo@smail.ufsm.br F.J.K. Mallmann Departamento de Solos, Universidade Federal de Santa Maria, 97105-900 Santa Maria, RS, Brazil D.L. Grac ¸a Departamento de Patologia, Universidade Federal de Santa Maria, 97105-900 Santa Maria, RS, Brazil Fish Physiol Biochem (2006) 32:105–112 DOI 10.1007/s10695-006-9102-9 123 ORIGINAL PAPER Ion levels in the gastrointestinal tract content and plasma of four teleosts with different feeding habits A.G. Becker Æ J.F. Gonc ¸alves Æ L.O. Garcia Æ E.R. Behr Æ F.J.K. Mallmann Æ D.L. Grac ¸a Æ B. Baldisserotto Received: 28 September 2005 / Accepted: 17 March 2006 / Published online: 14 June 2006 Ó Springer Science+Business Media B.V. 2006