The euryhaline killifish Fundulus heteroclitus inhabits brackish water estuaries and salt marshes along the eastern coast of North America. These habitats undergo routine fluctuations in salinity, so killifish in their natural environment

The euryhaline killifish Fundulus heteroclitus inhabits brackish water estuaries and salt marshes along the eastern coast of North America. These habitats undergo routine fluctuations in salinity, so killifish in their natural environment must dynamically regulate ion balance. This species is known for its exceptional euryhalinity, as it can tolerate salinities ranging from freshwater to nearly four times seawater (Griffith, 1974). For these reasons, killifish have been used extensively to understand the physiological mechanisms of osmoregulation in aquatic animals, and have revealed many details that are fundamental to our current understanding (Wood and Marshall, 1994). For example, studies on fundulids have been essential for appreciating the role of the gills in modulating ion transport (Motais et al., 1966; Potts and Evans, 1967; Pic, 1978; Marshall, 2003), the importance of mitochondria-rich cells within the gill epithelium (Copeland, 1948; Philpott, 1965), the importance of the intestine for water absorption in seawater (Potts and Evans, 1967), the role of the kidney in water excretion in freshwater (Stanley and Fleming, 1964; Fleming and Stanley, 1965), and various aspects of the hormonal regulation of ion transport (Potts and Evans, 1966; Pickford et al., 1970; Scott et al., 2005). In addition to many studies on killifish in vivo, an early finding of great importance to understanding branchial ion transport was the discovery that skin lining the opercular bone of this species was similar to the gills with regards to many morphological and functional characteristics (Burns and Copeland, 1950; Karnaky et al., 1977). The flat opercular epithelium has since been the subject of numerous studies, many of which have established the mechanisms of ion secretion in seawater teleosts (Wood and Marshall, 1994; Marshall and Bryson, 1998). Although this preparation appears to be an excellent surrogate model for understanding the physiology of gills in seawater fish, recent findings suggest that The Journal of Experimental Biology 208, 2719-2729 Published by The Company of Biologists 2005 doi:10.1242/jeb.01688