When crabs acclimated to full-strength seawater are transferred to dilute seawater below 27 ppt, a number of acute and chronic changes occur (reviewed in Lucu and Towle, 2003).
It is possible that no significant change in [Na.sup.+],[K.sup.+]-ATPase activity was observed because transporters other than [Na.sup.+],[K.sup.+]-ATPase may be more important in short-term control of ion uptake, as proposed by Lucu and Towle (2003).
Reversible phosphorylation of the [alpha]-subunit modulates [Na.sup.+],[K.sup.+]-ATPase activity in vertebrates (Ewart and Klip, 1995; Chibalin et al., 1999), and evidence has been presented to suggest that this mechanism also may modulate activity of existing [Na.sup.+],[K.sup.+]-ATPase enzyme in crab gills (Pequeux, 1995; Lucu and Flik, 1999; Towle et al., 2001).
This regulation is facilitated by the enzyme [Na.sup.+],[K.sup.+]-ATPase located in epithelial cells of the gills, the primary osmoregulatory organ in these crabs (reviewed by Mantel and Farmer, 1983; Pequeux, 1995; Lucu and Towle, 2003).
When crabs acclimated to full-strength seawater are subsequently transferred to low salinity, both the size of the osmoregulatory patch and the specific activity of [Na.sup.+],[K.sup.+]-ATPase increase in the posterior gills (reviewed in Mantel and Farmer, 1983; Pequeux, 1995; Lucu and Towle, 2003).
A similar time course for the response has been reported previously (Neufeld et al., 1980; Lucu and Flik, 1999; Lovett et al., 2003a,b).
In general, animal cells routinely expend 20%-30% of their total metabolic energy on the activity of a single protein complex, the sodium pump ([Na.sup.+], [K.sup.+]-ATPase; Siems et al., 1982, 1992), and for adult marine invertebrates, the sodium pump can potentially account for 30%-70% of tissue metabolism (Baker and Connelly, 1966; Lucu
and Pavicic, 1995).
Carcinus maenas, which is classed as an efficient osmoregulator, had hemolymph osmolality levels [ILLUSTRATION FOR FIGURE 1 OMITTED] similar to previous reports (Lucu
et al., 1973; Rankin and Davenport, 1981).
The latter uptake route involves Ca entry by simple diffusion (Sorenson et al., 1980; Coimbra et al., 1988), or through channels (Beirao et al., 1989; Lucu
, 1994), and subsequent intracellular transport by Ca-binding proteins (Bawden, 1989; Feher et al., 1992), Ca vesicles (Jones and Davis, 1982; Akins and Tuan, 1993), or as calcareous concretions (Graf and Meyran, 1983; Ziegler, 1996).