Density-dependent effects on morphological plasticity of Mytilus gallloprovincialis in suspended culture

Abstract

Bivalve molluscs are characterized by high morphological plasticity in response to variations in local environmental conditions. In the present study, we evaluate this capacity in the mussel Mytilus galloprovincialis with regard to intra-specific competition caused by cultivation density. Suspended cultivation ropes at different initial densities (220, 370, 500, 570, 800 and 1150 individuals per meter of rope) were placed on a raft in the Ría Ares-Betanzos, following standard cultivation techniques. From May to October, covering the period from thinning out to harvest, various morphological indicators (length/width, length/height and height/width ratios) and allometric relationships (Volume-Length and Projected area-Length) in addition to energy distribution (dry tissue weight/dry valve weight) were analyzed. Differences in morphological indicators due to cultivation density were observed while no influence on the energy distribution was detected. From the second cultivation month (June), a decrease was recorded in the length/height ratio of individuals due to cultivation density. Similarly, there was also a decrease in the length/width ratio, although this decrease only became statistically significant from September. In addition, the allometric relationships studied (Volume-Length and Projected area-Length) suggested asymmetric competition processes. Differences in volume or projected area between densities were only detected at the end of the experimental culture (October) and only in the smaller individuals. These morphological adaptations can be understood as a strategy to mitigate the effects of intra-specific competition, palliating the consequences of physical interference at high densities.