. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 48 P. S. NAIR AND W. E. ROBINSON 100. 100 mM protein Figure 2. Percentage distribution of free Ca (ionic Ca:+) in the blood plasma of Mercenaria mercenaria in the presence of a hypotheti- cal Ca-binding protein with 15 Ca-binding sites. The lines represent a range of binding affinity constants tested (log,,, Ka = 2-6 A/~'). Data were generated using the equilibrium speciation model MINTEQA2 (Allison et a/.. 1991). snail Biomphalaria gluhrutii impermeable to injected horseradish peroxidase (MW = 40 kDa). An important factor g
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 48 P. S. NAIR AND W. E. ROBINSON 100. 100 mM protein Figure 2. Percentage distribution of free Ca (ionic Ca:+) in the blood plasma of Mercenaria mercenaria in the presence of a hypotheti- cal Ca-binding protein with 15 Ca-binding sites. The lines represent a range of binding affinity constants tested (log,,, Ka = 2-6 A/~'). Data were generated using the equilibrium speciation model MINTEQA2 (Allison et a/.. 1991). snail Biomphalaria gluhrutii impermeable to injected horseradish peroxidase (MW = 40 kDa). An important factor governing the direction of Ca flux across the outer mantle epithelium is anaerobiosis. An- aerobiosis induced upon shell closure is accompanied by redissolution of CaCO, from the shell to maintain a more uniform plasma pH through the bicarbonate buffering sys- tem (Crenshaw and Neff, 1969; Hudson, 1992; Littlewood and Young, 1994). This leads to a net Ca flux away from the central zone EPF. On the other hand, injury to the shell causes remobilization of Ca into the EPF. Calcium for shell repair is derived predominantly from the calcium cells of the mantle and foot (see Watabe, 1983), by disso- lution of CaCOj and Ca,(PO4): spherules. Because the quahogs in our experiments had damaged shells and were kept out of water for as long as h, the 3-h timeframe for Ca exchange in this study probably reflected opposing Ca fluxes resulting from anaerobiosis and attempts at shell repair. Other studies have found similar timeframes for Ca exchange in bivalves. 4SCa introduced in the seawater took 2 h to reach a steady state with the mantle in Argopecten irmdians (Wheeler el <//.. 1975). and Crenshaw and Neff (1969) detected radioactivity in the EPF of M. mercenaria h after 45Ca was added to the seawater. Scrohicnlaria plana showed rapid shell dissolution about 2 h after the onset of salinity stress (Akberali. 1980). These results indicate that the timeframes involved in both inwa
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