Factors influencing shell size and shape in Donax incarnatus (Gmelin) inhabiting Panambur sandy beach, Mangalore, India

Abstract

The samples of wedge clam <em>Donax incarnatus</em> (Gmelin), sediment and seawater were collected for a period of 12 months from Panambur sandy beach, near Mangalore. A total of 2258 individuals of<em> D. incarnatus </em>were subjected to analysis. The air (27.5-31.2 ^oC), sand (28.5-36 ^oC) and seawater (25-32.2 ^oC) temperatures showed seasonal fluctuations. The dissolved oxygen (3.29-4.55 ml L^-1) and salinity (30.24-34.84 x 10^-3) of seawaters showed variations. Fine sand dominated (75.09 to 94.2%) throughout the period of study, followed by medium sand (1.51 to 20.65%). The density (89 to 39446 no. m^-2) and biomass (124 to 1892 g. m^-2) of wedge clams showed marked variations during breeding season. Principal component analysis of biological variables and environmental parameters showed six components which together accounted for 90.39% of total variance. The component 1 (dissolved oxygen, density, biomass, wet weight, dry) accounted for 24.02% of total variance followed by component 2 (air, water and sediment temperatures, salinity and <em>b</em> values of length-breadth and length-dry weight relationships) for 20.64% of total variance and component 3 (mean shell length, mean shell breadth, mean shell width) for 17.27% of total variance. The rest of the variance of 28.46% was accounted for component 4 (medium sand, fine sand, very fine sand), component 5 (coarse sand, <em>b</em> value of length-wet weight relationship, <em>b</em> value of length-width relationship) and component 6 (silt & clay). The hierarchical cluster analysis produced a dendrogram with six sub-groups of clusters. The temporal variability of six factor scores was delineated using factor analysis.