Impaired shell gaping and food depletion as mechanisms of asymmetric competition in mussels1

Abstract

We studied the effect of two different mechanisms in generating asymmetric competition in mussels. In the control group, unrestricted growth was assessed from mussels growing individually. In the second and third groups, the setup was adjusted such that mussels were grouped in pairs, but with shells either not touching (testing for a closeness effect: exploitation competition), or touching so that total space for gaping was reduced by 50% (testing for constraint effect: interference competition). Taken together, shell constraint and closeness resulted in growth reduction only in the two smallest size classes tested. Therefore competition was asymmetric. Closeness was the only significant factor. The asymmetric nature of exploitation competition was probably due to more severe small-scale depletion in seston around small mussels, since numerical simulations suggested that without small-scale patterns in seston depletion, exploitation competition is symmetric. Our simulations also suggest i) that had it been significant, interference would have resulted in asymmetric competition, and ii) behavioral periodic shell closure of large mussels may have provided a mechanism for alleviating the effect of shell constraint on small mussels, but that the required amplitude of behavioral gap reduction is about 50%. Asymmetric competition in mussel populations appears to be attributable to small-scale spatial structure in seston depletion, unless crowding is so severe that other mechanisms come into play.