Olfactory sensitivity to bile fluid and bile salts in the European eel (Anguilla anguilla), goldfish (Carassius auratus) and Mozambique tilapia

INTRODUCTION Bile is a multi-functional fluid that acts both as a detergent to aid lipid digestion, and as an excretory fluid for substances that cannot be eliminated by urine (Hofmann, 1999). Bile salts, which are a key constituent of bile, are steroidal compounds with a characteristic fused four-carbon-ring skeleton often found conjugated with glycine, taurine, cysteine or sulphate groups (Goto et al., 2003; Hofmann, 1999; Moschetta et al., 2005). In modern mammals, bile salts are mostly C24 carboxylic acids, whereas in primitive mammals, cartilaginous fish and reptiles, C27 acids and alcohols are found (Haslewood, 1967; Hofmann and Hagey, 2008; Moschetta et al., 2005). Many species of fish have been shown to have an acute olfactory sensitivity to bile salts (Døving et al., 1980; Siefkes and Li, 2004; Zhang et al., 2001). Although the biological function of this phenomenon is unclear, with the notable exception of the sea lamprey Petromyzon marinus (Li et al., 2002; Polkinghorne et al., 2001; Sorensen et al., 2005), the stability and water solubility of the bile salt molecule makes it an ideal candidate for a role in chemical communication (Rosenthal and Lobel, 2006). We have recently shown that the odour of bile from the European eel (Anguilla anguilla) depends not only on the sex, but also changes with sexual maturity (Huertas et al., 2007), suggesting a possible role for bile salts as sex pheromones in teleosts. Bile salts are unique to vertebrates (Haslewood, 1967), and most bile salts found in the aquatic environment (apart from shorelines) are probably from the faeces of fish (Velez et al., 2009; Zhang et al., 2001). Although most fish have the C24 bile acid taurocholic acid, there is sufficient structural variability in the bile salt structures among different fish groups (Haslewood, 1967) to serve as a means of detecting bile salts released by congeners and by other species. For example, behavioural evidence has shown that prey fish are able to recognise potential predators by olfaction (Kristensen and Closs, 2004; Rosenthal and Lobel, 2006). Furthermore, migration could involve olfactory detection of bile salts (Siefkes and Li, 2004). To test inter-specific sensitivity to bile acids, we used a comparative approach with three teleost species; the European eel (Anguilla anguilla Linnaeus 1758, Anguilliformes; hereafter referred to as ‘eel’), goldfish (Carassius auratus Linnaeus 1758, Cypriniformes) and Mozambique tilapia (Oreochromis mossambicus Peters 1852, Perciformes; hereafter ‘tilapia’). Not only are these three species phylogenetically diverse, belonging to three different orders ranging from a member of an early-diverging group (the eel) to a more recently diverging group (the tilapia) but also their diets differ, ranging from algae, plants and detritus (tilapia), aquatic invertebrates (tilapia, goldfish and eel) to small fish and other aquatic vertebrates (eel). Thus, potential odour-inducing chemicals present in the bile of these species are likely to be different. The Journal of Experimental Biology 213, 308-317 Published by The Company of Biologists 2010 doi:10.1242/jeb.033142