The euryhaline killifish Fundulus heteroclitus lives in estuaries and salt marshes on the eastern coast of North America. Killifish must

INTRODUCTION The euryhaline killifish Fundulus heteroclitus lives in estuaries and salt marshes on the eastern coast of North America. Killifish must dynamically regulate their ion balance because of routine fluctuations in salinity in their natural environment. This species can tolerate changes in salinity from freshwater to nearly four times seawater (Griffith, 1974), and have therefore been used extensively to understand fish osmoregulation (Wood and Marshall, 1994), the mechanisms of physiological plasticity (Marshall, 2003; Scott et al., 2004a; Scott et al., 2005b) and the evolution of salinity tolerance (Scott et al., 2004b; Scott and Schulte, 2005; Singer et al., 2008). When killifish move into seawater they rapidly increase active ion secretion by two secretory epithelia, the gills and opercular epithelium, to counteract passive salt loading (Marshall et al., 1999; Wood and Laurent, 2003; Prodocimo et al., 2007). The initial activation of ion secretion in opercular epithelium involves membrane trafficking and rapid activation of pre-existing ion transporters (Hoffmann et al., 2002; Marshall et al., 2002b), which may also be true for the gills (Towle et al., 1977; Mancera and McCormick, 2000). Longer-term modulation of ion secretion involves transcriptional increases in gill ion transporter abundance (Scott et al., 2004a) as well as changes in the morphology of both secretory epithelia (Hossler et al., 1985; Daborn et al., 2001; Katoh et al., 2001; Laurent et al., 2006). While a fair amount is known about transcriptional regulation of ion transport in the gills of killifish transferred to seawater (Singer et al., 1998; Scott et al., 2004a; Scott and Schulte, 2005; Choe et al., 2006; Shaw et al., 2007), much less is known about the opercular epithelium. Gene expression differs greatly between these tissues after transfer of intact killifish to freshwater (Scott et al., 2005a). These molecular differences are probably the basis for divergent physiological mechanisms of ion transport in freshwater between these two organs (Wood and Marshall, 1994; Marshall et al., 1997; Patrick et al., 1997; Burgess et al., 1998; Patrick and Wood, 1999). However, the gills and opercular epithelium are thought to function similarly in seawater (Burns and Copeland, 1950; Karnaky et al., 1977), which has prompted wide-spread in vitro use of the thin opercular membrane as a model for understanding the function and regulation of ion secretion in the gills of marine fish (reviewed by Zadunaisky, 1984; Karnaky, 1986; Péqueux et al., 1988; Wood and Marshall, 1994; Marshall, 1995; Marshall and Bryson, 1998; Marshall and Singer, 2002; Marshall, 2003). It is therefore of interest to determine whether the molecular responses of these two tissues to seawater transfer are similar. The intestine is essential for counteracting passive water loss in seawater fish (Potts and Evans, 1967). Water is ingested and absorbed across the intestine, following osmotic gradients that are created by transepithelial NaCl transport driven by the activity of basolateral Na/K-ATPase (Loretz, 1995; Schettino and Lionetto, 2003; Grosell et al., 2005). Some recent evidence suggests that this organ may dynamically regulate salt absorption in response to salinity change (Scott et al., 2006; Grosell et al., 2007), but little is The Journal of Experimental Biology 211, 2450-2459 Published by The Company of Biologists 2008 doi:10.1242/jeb.017947