Department of Anatomy, Histology and Embryology, Faculty of Veterinary Medicine, University of Zagreb, Heinzelova, Zagreb, Croatia The Development of the Digestive Tract in Larval European Catfish (Silurus glanis L.) Z. Kozaric ´ 1 *, S. Kuz ˇir 1 , Z. Petrinec 2 , E. Gjurc ˇevic ´ 2 and M. Boz ˇic ´ 3 Addresses of authors: 1 Department of Anatomy, Histology and Embryology, Faculty of Veterinary Medicine, 2 Department for Biology and Pathology of Fish and Bees, Faculty of Veterinary Medicine, University of Zagreb, 10000 Zagreb, Croatia; 3 Fish Farm ‘‘Poljana’’, Kanisˇka Iva, Croatia; *Corresponding author: Tel.: +385 (01) 2390245; fax: +385 (01) 2441390; e-mail: zvonimir.kozaric@vef.hr With 9 figures and 1 table Received November 2006; accepted for publication July 2007 Summary The European catfish, Silurus glanis L., has become an important aqua cultural fish in Croatia, and it is cultivated extensively in ponds in polyculture with carps. The develop- ment of the digestive tract in S. glanis was studied with the aim of improving intensive fish production. Research was carried out on S. glanis larval stadium from 1- to 19-day post-hatch- ing (DPH). The main histological methods used were: hae- matoxylin and eosin staining, periodic acid Schiff staining (PAS), Alcian blue (AB) and toluidin blue staining (TB). A yolk sac was present during the first 5 days (1–5-DPH). During the initial 3-DPH period, there was no trace PAS and AB activity in the digestive tract. Differentiation of the digestive tract began at 3- to 5-DPH. The oesophagus was positive for AB at 5-DPH, PAS and TB after 7-DPH. Differentiation of enterocytes began at 5-DPG and the intestines were complete at 11-DPH. Development of liver and pancreas was also studied. The analysis of data obtained in this study suggests that after 5-DPH catfish larvae have morphologically com- pleted digestive tracts. Introduction The European catfish, Silurus glanis, has become an important aquaculture fish in Croatia, and it is cultivated extensively in ponds in polyculture with carps. The biology and technology of the European catfish have been presented in the study by Maresˇ and Kourˇil (1988). Various forms of pelleted feeds and nutrient content have an effect on larval and adult growth. European catfish fry fed with universal starter feed manifested excellent survival but grow slowly (Hamacˇkova et al., 1997; Prokesˇ et al., 1999). The optimal concentrations of lipid, proteins and addition of soya bean oil to fish food have beneficial effects on catfish growth indicators and meat quality (Bogut et al., 2002; Has-Scho¨n et al., 2004). Knowledge of differentiation of the digestive tract during ontogenetic development is essential for understanding the nutritional physiology of larval fishes and synchronizing the physiological stage of development with feeding practices and rearing protocol (Senger et al., 1993). Descriptions have been reported of the ontogenetic devel- opment of the digestive tract of a number of commercial sea and freshwater fish species. After hatching, the digestive tract is a simple tube. Larval period begins with the opening of the mouth, and the appearance of the gastric glands indicates a change in digestion and passage through the juvenile period (Kjørsvik et al., 1991; Boulhic and Gabaudan, 1992; Gawlicka et al., 1995; Santamaria et al., 2004). In general, larvae have a straight intestine after hatching, formed by simple epithelia that remain unchanged until the yolk sac and lipid globules are absorbed (Govoni et al., 1986; Ferrais et al., 1987; Verreth et al., 1992; Sarasquete et al., 1995). The critical stage in rearing larvae is the weaning of larvae from natural to formulated feeds. At the start of exogenous feeding, teleostean larvae may have a straight or convoluted digestive tract, and from this moment onwards several rapid morphological changes occur and larvae structurally resemble the adults of stomachless species (Verreth et al., 1992; Senger et al., 1994; Kaji et al., 1996).This transition is important as a source of larval mortality (Buddington and Cristofferson, 1985; Gisbert and Williot, 1997). Ezeasor and Stokoe (1981) studied the ultrastructure of rainbow trout intestinal absorptive cells in the intestine; caeca and rectum. Most of these cells showed morphological characteristics of lipid absorption. The vacuo- lated cells founded in the rectal mucosa showed structural indications for absorption of protein macromolecules. These histological differences are a critical step in the determination of the functional relationship between feeding and absorption of nutrients (Tanaka, 1971, 1972; Sarasquete et al., 1995). The purpose of this study was to describe the ontogenetic development of the digestive tract on the light microscopic level, to provide a basis for future nutritional studies of catfish larvae. Materials and Methods Larvae of S. galnis were obtained from the freshwater hatchery ÔPoljanaÕ, Croatia. Three-day-old larvae were stocked in a flow-through plastic tank of 50-l volume. Starting at day 4 post-hatching (DPH), larvae were fed twice a day during the daylight period with zooplankton and starter for catfish. Zooplanktons were caught in ponds and starter ÔCatCo crumble excellent exÕ was obtained from ÔCOPENSÕ Netherlands. Pellet size was appropriate to larval size (0.2–0.8 mm). The chemical composition of the feed is given in Table.1. Ó 2007 The Authors. Journal compilation Ó 2007 Blackwell Verlag www.blackwell-synergy.com Anat. Histol. Embryol. 37, 141–146 (2008) doi: 10.1111/j.1439-0264.2007.00812.x ISSN 0340–2096