. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 100 - 80 - .2 60 -I o IB —I 1 1 1 30 60 90 120 Time (min) o J 60 90 120 Time (min) Libinia emarginata 150 180. 150 180 30 60 90 120 Time (min) 150 180 Figure 6. Percentage time of antennule retraction of 16 crabs (mean ± SEM) during 3 h in seawater concentrations ranging from 100%-25%: (a) Callinectes saniiliis. (b) Carcinus maenas, (c) Cancer magister. and (d) Libinia emarginata. 1964, 1966). Evidence suggests that the posterior gills have the highest Na-K-ATPase activity (Florkin and Schoffe- niels, 1969; Neufeld et ai. 19
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 100 - 80 - .2 60 -I o IB —I 1 1 1 30 60 90 120 Time (min) o J 60 90 120 Time (min) Libinia emarginata 150 180. 150 180 30 60 90 120 Time (min) 150 180 Figure 6. Percentage time of antennule retraction of 16 crabs (mean ± SEM) during 3 h in seawater concentrations ranging from 100%-25%: (a) Callinectes saniiliis. (b) Carcinus maenas, (c) Cancer magister. and (d) Libinia emarginata. 1964, 1966). Evidence suggests that the posterior gills have the highest Na-K-ATPase activity (Florkin and Schoffe- niels, 1969; Neufeld et ai. 1980: Siebers et al., 1982, 1983, 1985, 1986). and therefore a ventilatory reversal would bring water into contact with the pumps of the posterior gills, enhancing active ion uptake. In support of this con- cept. Callinectes sapidus, which is the most efficient osmo- regulator tested, exhibits the highest frequency of mouthpart movements (and hence ventilatory reversals: Fig. 3a), and this behavior decreases with the declining osmoregulatory ability of the species (Fig. 3), with the weak osmoregulator Cancer magister (Fig. 3c) and the stenohaline Libinia emar- ginata (Fig. 3d) showing substantially less ventilatory re- versals than the two efficient osmoregulators (Fig. 3a. b). In addition, both Callinectes sapidus (Fig. 3a) and Carcinus maenas (Fig. 3b) showed a stepwise increase in mouthpart movements with decreasing salinity; this increase was sus- tained in 507r and 257c SW. for the 3-h experimental period. In both Cancer magister and Libinia emarginata (Fig. 3c, d: Table I), mouthpart movements increased only during the first 30 min. Thereafter, the crabs kept the mouth- parts sealed, isolating the branchial chamber. This sealing of the branchial chambers has been reported previously for Cancer magister (Sugarman et 1980). Isolation of the branchial chambers, in conjunction with a decrease in gill blood flow via a reduced cardiac output (McGaw and Mc- Mahon, 1996: Co
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