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

The blue crab, Callinectes sapidus Rathbun, supports valuable commercial fi sheries in the temperate areas of the coastal Atlantic states and US Gulf o f Mexico (GOM). Soft crabs form a small parr of the total United States blu e crab landings, but have a higher market value on a per pound basis than d o hard crabs. The soft crab industry depends upon the capture of premolt cr abs which are held in open or closed seawater systems until they molt. A ma jor factor limiting profitability is the labor-intensive nature of the shed ding operation. Extending the length of time crabs remain in the softshell stage would significantly reduce labor requirements. Exoskeletal calcificat ion in blue crabs is achieved predominantly with calcium absorbed from seaw ater. In the present study, seawater at a salinity of 12 parts per thousand , with calcium levels reduced to 60-80% of normal, decreased the calcificat ion rate without increasing shedding mortality. Salinity did not influence calcification rates for crabs shed at 5 parts per thousand. 12 parts per th ousand, and 25 parts per thousand, provided calcium concentrations were wit hin the normal ranges for their respective salinities:(x) over bar = 54, 13 9, and 281 mg/l. Crabs molting over the range of test salinities in waters with normal calcium levels remained soft for - 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 parts per thousand remained longer in the softshell stage th an crabs shed at 5 parts per thousand or 12 parts per thousand. Modifying e xisting closed system shedding facilities to incorporate low-calcium seawat er technology would reduce labor requirements, increase profitability, and produce a higher quality soft crab. (C) 2001 Elsevier Science B.V. All righ ts reserved.