Ionic Requirements of Blue Crab, Callinectes sapidus, in Environments Containing Low Concentrations of Total Dissolved Solids
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The goal of this study was to develop an ionic environment containing 1 g/L total dissolved solids (TDS) that would support the survival and growth of juvenile blue crabs (designated a “mixed-ion solution”). The 1 g/L TDS level was selected in order to minimize the cost of preparing ponds and to potentially reduce the need for effluent permitting in inland areas. The general approach was to provide selected ions in approximate concentrations found in 1 g/L dilute seawater. In a series of three-week exposures beginning with ~20 mm carapace-width crabs, we found: 1) Growth in artificial sea-salt treatments of 1, 2, 4, and 8 g/L TDS averaged 76% ± 20.2% (mean ± SD) and was not significantly affected by treatments. The number of molts, feed intake and modified feed conversion ratio were not significantly affected by treatments. 2) Growth during exposure to 1 g/L sea-salt, 0.5 g/L sea-salt + 0.5 g/L mixed- ions or 1 g/L mixed-ions averaged 41% ± 0.49% and was not significantly affected by treatment. Although not quantified, some exoskeletons in the mixed-ion treatment appeared soft and off-colored, leading us to investigate the need for environmental strontium in the next experiment. 3) Average survival during the 21-d exposure of 1 g/L mixed-ions with 0, 1 and 2 mg/L strontium was 89% ± 12.7% and did not differ significantly across treatments. Growth across treatments averaged 40% ± 25.3% and was not significantly affected by treatment. In this experiment and in two failed experiments (not reported), environmental pH was considered lower than optimal for both molting (Perry et al., 2001; Cameron and Wood, 1985; Cameron, 1985) and nitrification by the bio-filters (Spotte, 1979). Consequently, sodium bicarbonate was substituted in the next experiment for approximately 17.2% of the sodium chloride in order to affectively buffer against pH changes. 4) All crabs in the 1 g/L mixed-ion environment (with 1 mg/L strontium) survived the 21-d exposure to temperatures of 26, 29, and 32oC. Growth in all treatments averaged 71% ± 12.1% and was not significantly affected by treatments. Due to two previous, failed experiments, 133 mg/L of sodium bicarbonate was substituted for 133 mg/L of sodium chloride to maintain pH levels above 7.5. These results indicate that blue crabs can survive and grow in 1 g/L mixed-ion solution. This work was supported with funding by the Mississippi Department of Marine Resources.