Salinity and substratum effects on condition index of the bivalve Rangia cuneata

The condition index (CI) of the clam Rangia cuneata was measured monthly at 8 stations in 2 substrate types and 3 salinity regimes in the James River estuary, Virginia, USA. CI was higher in clams from sand bottoms than from mud, and decreased in both substrata in fresher water. Values were lowest in early spring and at a maximum in early autumn, with a pronounced autumn peak in the sand substratum. This pattern coincided with the annual salinity cycle. A field experiment showed that some factor associated with the water overlying a sand or mud substratum, rather than the nature of the substratum itself, was important in determining CI. The hypothesis is presented that this factor may be the higher suspended solids concentrations measured immediately over mud bottoms.Virginia Institute of Marine Science Contribution No. 782.