Shell allometry and length-mass-density relationship for Mytilus edulis inan experimental food-regulated situation

We examined the influence of food availability and population density on th e morphometry and shell length body mass relationship of Mytilus edulis. Mu ssels were reared in the laboratory for 22 mo at 8 different density levels in 11 chambers supplied with natural seston at 2 different concentrations. This allowed us to assess separately the effects of food availability and mussel density. The shell length/width and shell height/width ratios were a ffected by food, density and time. Mussels tended to be narrower (flatter) at high density and at low food level. Therefore, narrow shells could resul t from reduced food concentration in high density situations without implyi ng physical interference. Shell mass was also influenced by both food and d ensity levels, but to a lesser extent than tissue dry mass. In contrast wit h soft tissue mass, shell mass increased significantly for all food and den sity levels between October 1995 and October 1996, The elevation of the she ll length-body mass-population density relationship was lower at low food a vailability, The slope of the tridimensional relationship, however, increas ed with decreasing food level, indicating apparent asymmetric competition f or all food-density treatments pooled together, This pattern, however, is m isleading because mussels obviously cannot interact among chambers. Since t he slopes of length mass relationships are used in predicting self-thinning exponents in space-regulated situations, it follows that self-thinning exp onents in mussels should be sensitive to background food level, thus limiti ng the use of self-thinning relationships for resolving factors regulating growth.