INTERDEPENDENCE OF FOOD AND SPACE LIMITATION OF A BENTHIC SUSPENSION FEEDER - CONSEQUENCES FOR SELF-THINNING RELATIONSHIPS

We test the idea that competition for space and food in benthic suspension feeders is interdependent. Experimental evidence in support of this concept has been presented only for bryozoans. We present an experimental rejection of the hypothesis of the independence of food and space as factors limiting individual growth in small groups of blue mussel Mytilus edulis (L.). Mussels were grown in the laboratory, using natural seawater. In the present experiment, physical interference (crowding) and exploitative competition for food did co-occur. Growth of mussels changed from positive to negative along an assumed food gradient in series of the chambers employed in this experiment. In addition, growth was severely impeded by interference in small individuals, but not in large individuals: competition was asymmetrical with respect to size. We present a model of this interaction based on a biomechanical analysis of the effect of reduction of shell gape in neighbouring mussels interacting together. The model predicts that the effect of interference on individual growth is size-dependent and can be determined in terms of the shell-length / body-mass relationship. In the context of predicting self-thinning slopes in bivalves, our data strongly suggest that simultaneous occurrence of physical interference and exploitative competition results in a steepening of the expected slope (beta) of space-limited self-thinning relationships, when log biomass (B) is expressed as a function of log density (N) in the equation log B = log k + beta-log N.