Cloning, tissue and ontogenetic expression of the taurine transporter Contrary to what was observed for the body fraction, TauT expression in the digestive tract was lower in the remaining fractions throughout the experimental period. Although these results indicate that in comparison to the remaining fractions, Senegalese sole larvae may have a low ability to transport taurine in the digestive tract, the capacity to absorb dietary taurine at gut level seems to be already established, at least, from the onset of metamor- phosis. The current data do not allow us to verify if TauT was expressed in the digestive tract earlier in the devel- opment, but it is observed that TauT expression in the digestive tract did not increase as much as in remaining fractions during larval development, suggesting that the capacity to absorb dietary taurine in the digestive tract is likely to be sufﬁciently high in the early larval stages. The ubiquitous nature of TauT reported for juveniles of other ﬁsh species (Takeuchi et al. 2000, 2001; Zarate and Bradley 2007) was also found in Senegalese sole juveniles, since TauT was expressed in all examined tissues. Among these, TauT expression was particularly high in brain, heart and eye. Not surprisingly, these are organs where taurine can be found in high concentrations and has been suggested to play important roles (Huxtable 1992). For instance, taurine has been suggested to modulate neurotransmitter and synaptic activity, acting as a membrane stabiliser or playing a key role in cell volume regulation, a crucial factor for the normal function of the central nervous system (Huxtable 1989). In heart, taurine functions are mainly related to calcium modulation for cardiac muscle contrac- tion, while in eye taurine has been indicated to prevent retinal degeneration by protecting photoreceptors, acting as a biological mirror behind the retina and having a shielding effect on rod outer segments on protection of high levels of light (Pasantes-Morales et al. 1981). In the digestive tract of juveniles, TauT was more expressed in stomach and hindgut. Regarding stomach, this high expression of TauT indicates that dietary taurine is readily absorbed when the digestive process begins. It is not known when absorption of dietary taurine in the stomach region may start. Most studies conclude that altricial marine ﬁsh larvae do not have a functional stom- ach at the onset of ﬁrst feeding (Rønnestad et al. 1999). However, in many ﬁsh the differentiation of stomach into a separate compartment with some short-term storage func- tion appears to start prior to the onset of gastric acid and pepsin production as shown in Japanese ﬂounder (Rønnestad et al. 2000). In Senegalese sole it has been suggested that the incipient stomach can already be observed at ﬁrst feeding (Ribeiro et al. 1999), but the functional characterisation of this structure, including any taurine absorbing capacity, remains to be established. The high expression of TauT found in the hindgut of juvenile Senegalese sole also indicates that this area may be an 1325 important site of taurine absorption during the larval stage. The high expression of TauT found in the hindgut also suggests that taurine endogenously used for bile salt con- jugation may be reabsorbed at the posterior end of the digestive tract. Therefore, these results suggest an enter- ohepatic recycling pathway for taurine in Senegalese sole, present at least from the juvenile stage. The enterohepatic recycling pathway of taurine is likely important for con- trolling taurine losses and maintaining taurine body pool in ﬂatﬁsh species. As observed in cats, the enterohepatic circulation of taurine can be disrupted by anaerobic bac- teria in the digestive tract, leading to a deconjugation of taurine from bile salts (Ikeda et al. 1963). Taurine released during this process is reabsorbed or undergoes further bacterial degradation, being lost from the taurine body pool. These losses may be substantial (between 30 and 50%) upon dietary factors that enhance bile secretion or increase taurine degradation by changes in gut ﬂora (Cooke et al. 1988; Hickman et al. 1992). With this in mind, the results obtained in the current study suggest that future research should address understanding the mechanisms underlying the enterohepatic recycling pathway of taurine in ﬂatﬁsh species, as well as controlling losses from the taurine body pool resulting from dietary changes. In conclusion, this study described the cloning and ontogenetic expression of TauT in the ﬂatﬁsh Senegalese sole. This transporter was highly similar to other vertebrate TauT, but changes in position 322 (S4) indicate that taurine transport may be different in mammals than in ﬁsh, reptiles or birds. Results also showed that Senegalese sole meta- morphosis is an important developmental trigger to pro- mote taurine transport, especially in larval muscle tissues. Moreover, results indicated that the larval capacity to absorb dietary taurine in the digestive tract seems to be established, at least from the onset of metamorphosis. In juveniles, taurine transport was especially high in brain, heart and eye, organs where taurine has been suggested to play important biological roles. Moreover, results from this study indicate that dietary taurine is quickly absorbed when the digestive process begins and taurine endogenously used for bile salt conjugation may be recycled at the posterior end of the digestive tract. Therefore, these results suggest an enterohepatic recycling pathway for taurine in Sene- galese sole, at least in the juvenile stage, a process that may be important for maintenance of the taurine body levels in ﬂatﬁsh species. Acknowledgments The authors would like to acknowledge A. Coelho & Castro, Lda (Po´voa de Varzim, Portugal) for supplying Senegalese sole eggs and to Laura Ribeiro for suggestions on sam- pling procedures. W. Pinto is thankful to Anabela Bensimon-Brito for advice and support on molecular biology procedures and bioinfor- matics. W. Pinto is also grateful to the YGGDRASIL mobility pro- gram funded by the Research Council of Norway that granted him a 123