Euryhaline tilapia Oreochromis mossambicus

regulated by, and suited to, the environment in which they reproduce. In many teleosts, which spawn in either freshwater or seawater, sperm motility is initiated by osmotic shock when sperm are ejaculated. These osmolality dependent regulatory systems are quite different in freshwater cyprinid teleosts, such as carp, gold fish and zebrafish, and marine teleosts, such as the pufferfish and flounder. Sperm of freshwater teleosts are quiescent at the osmolality of seminal plasma, referred to as isotonic conditions (approximately 300 mosmol kg–1). They begin to move when suspended in hypotonic water (<300 mosmol kg–1) and show high motility (as % motile sperm) in freshwater. By contrast, sperm of marine teleosts begin to move when released into hypertonic water (>300 mosmol kg–1), and show the highest motility at approximately 1000 mosmol kg–1, which is almost equivalent to seawater (Morisawa and Suzuki, 1980; Oda and Morisawa, 1993). It has therefore been assumed that the difference in osmolality between seminal plasma and the external water, corresponding to the spawning ground, is an important regulatory factor of sperm motility in many teleosts. If this is the case, the regulatory system of sperm motility seems to be reversed in freshwater and seawater teleosts, since the osmolality of seminal plasma is intermediate between that of freshwater and seawater. The tilapia Oreochromis mossambicus, a euryhaline teleost, has the can acclimate to wide range of salinities, from freshwater to seawater, by a mechanism that includes chloride cells (Sakamoto et al., 1997; Balm et al., 1994; Borski et al., 1994). A unique feature of tilapia is that they can reproduce in both freshwater and seawater, even though there is a large osmotic difference between those two environments (Brock, 1954). Thus, it has been assumed that tilapia sperm can swim independently of osmolality, or modulate their regulatory mechanism to suit fertilization in either high or low salinity. It is possible that the motility regulatory system of tilapia sperm is quite different from those reported for other teleosts. We have already reported that tilapia sperm acclimated to seawater were motile in hypertonic water, i.e. seawater, in the presence of Ca2+, whereas in tilapia acclimated to freshwater sperm could not swim in hypertonic conditions even in the presence of Ca2+ (Morita and Okuno, 1998). Linhart et al. (1999) 913 The Journal of Experimental Biology 206, 913-921 © 2003 The Company of Biologists Ltd doi:10.1242/jeb.00153