Calcium concentration in seawater and exoskeletal calcification in the blue crab, Callinectes sapidus

The blue crab, Callinectes sapidus Rathbun, supports valuable commercial fisheries in the temperate areas of the coastal Atlantic states and US Gulf of Mexico (GOM). Soft crabs form a small part of the total United States blue crab landings, but have a higher market value on a per pound basis than do hard crabs. The soft crab industry depends upon the capture of premolt crabs which are held in open or closed seawater systems until they molt. A major factor limiting profitability is the labor-intensive nature of the shedding operation. Extending the length of time crabs remain in the softshell stage would significantly reduce labor requirements. Exoskeletal calcification in blue crabs is achieved predominantly with calcium absorbed from seawater. In the present study, seawater at a salinity of 12 per thousand, with calcium levels reduced to 60-80% of normal, decreased the calcification rate without increasing shedding mortality. Salinity did not influence calcification rates for crabs shed at 5 per thousand, 12 per thousand, and 25 per thousand, provided calcium concentrations were within the normal ranges for their respective salinities: average = 54, 139, and 281 mg/l. Crabs molting over the range of test salinities in waters with normal calcium levels remained soft for approximately 4 h. At test salinities with reduced calcium levels, the rate of exoskeletal calcification decreased as total ionic strength of the seawater increased. Crabs shed in low-calcium seawater at 25 per thousand remained longer in the softshell stage than crabs shed at 5 per thousand or 12 per thousand. Modifying existing closed system shedding facilities to incorporate low-calcium seawater technology would reduce labor requirements, increase profitability, and produce a higher quality soft crab.