Characterization of swim bladder non-inflation (SBN) in angelfish, Pterophyllum scalare (Schultz), and the effect of exposure to methylene blue S T Perlberg 1 , A Diamant 2 , R Ofir 3 and D Zilberg 1 1 The Albert Katz Department of Dryland Biotechnologies, The Jacob Blaustein Institutes for Desert Research, Ben- Gurion University of the Negev, Israel 2 National Center for Mariculture, Israel Oceanographic and Limnological Research, Eilat, Israel 3 Hazeva Research and Development Center, The Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Israel Abstract Failure to inflate the swim bladder is regarded a major obstacle in the rearing of many fish species. We present a study of swim bladder non-inflation (SBN) in angelfish, Pterophyllum scalare. A normal developing primordial swim bladder was first dis- cernable at the end of the first day post-hatch (p.h.) as a cluster of epithelial cells with a central lumen, surrounded by presumably mesenchymal cells. Ini- tial inflation occurred on the fourth day p.h. Prior to inflation the swim bladder epithelium consisted of an outer squamous and inner columnar layer. Cells of the inner layer were filled at their basal region with an amorphous material, which disap- peared upon inflation. A pneumatic duct was ab- sent, and larvae presented no need to reach the water surface for inflation, suggesting that angelfish are pure physoclists. A model for the role of the amorphous material in normal initial inflation is proposed. Abnormal swim bladders were apparent from the fourth day p.h., and methylene blue (MB) at a concentration of 5 ppm significantly increased the prevalence of SBN. Histologically, abnormal swim bladders in larvae hatched in 5 ppm MB could not be distinguished from those in fish raised under routine conditions (0.5 ppm MB). We sug- gest that MB may have a teratogenic effect in angelfish. Keywords: angelfish, histopathology, inflation, methylene blue, swim bladder. Introduction Initial inflation of the swim bladder is considered one of the most critical stages of larval development (Blaxter 1988). Most larvae inflate their swim bladder early on, at a developmental stage coincid- ing with the depletion of yolk reserves and the initiation of external feeding (Tait 1960; Doroshev & Cornacchia 1979; Battaglene & Talbot 1990). In most studied species, the swim bladder develops as a simple evagination of the alimentary canal (McE- wen 1940; Johnston 1953; Steen 1970; Boulhic & Gabaudan 1992; Govoni & Hoss 2001; Trotter, Pankhurst & Battaglene 2004). Physostomes retain to maturity a connection between the swim bladder and the digestive tract by means of a pneumatic duct, while physoclists have no such connection (Steen 1970). However, most physoclists are in reality Ôtransient-physostomesÕ, possessing a patent pneumatic duct as larvae and losing it as adults (Bailey & Doroshev 1995; Trotter, Pankhurst & Hart 2001). ÔPure physoclistsÕ that have no patent pneumatic duct as larvae or as adults such as Oreochromis mossambicus (Peters) (Tilapia mossamb- ica), are rare (Doroshev & Cornacchia 1979; Doroshev, Cornacchia & Hogan 1981). Failure to inflate the swim bladder is regarded a major obstacle in the rearing of many fish species, mainly in transient physostomes. In Australian bass, Journal of Fish Diseases 2008, 31, 215–228 Correspondence D Zilberg, The Jacob Blaustein Institutes for Desert Research, Midreshet Ben Gurion, 84990, Israel (e-mail: dzilberg@bgu.ac.il) 215 Ó 2008 The Authors. Journal compilation Ó 2008 Blackwell Publishing Ltd