Recruitment facilitation and spatial pattern formation in soft‐bottom mussel beds

Abstract

Mussels (Mytilus edulis) build massive, spatially complex, biogenic structures that alter the biotic and abiotic environment and provide a variety of ecosystem services. Unlike rocky shores, where mussels can attach to the primary substrate, soft sediments are unsuitable for mussel attachment. We used a simple lattice model, field sampling, and field and laboratory experiments to examine facilitation of recruitment (i.e., preferential larval, juvenile, and adult attachment to mussel biogenic structure) and its role in the development of power‐law spatial patterns observed in Maine, USA, soft‐bottom mussel beds. The model demonstrated that recruitment facilitation produces power‐law spatial structure similar to that in natural beds. Field results provided strong evidence for facilitation of recruitment to other mussels—they do not simply map onto a hard‐substrate template of gravel and shell hash. Mussels were spatially decoupled from non‐mussel hard substrates to which they can potentially recruit. Recent larval recruits were positively correlated with adult mussels, but not with other hard substrates. Mussels made byssal thread attachments to other mussels in much higher proportions than to other hard substrates. In a field experiment, mussel recruitment was highest to live mussels, followed by mussel shell hash and gravel, with almost no recruitment to muddy sand. In a laboratory experiment, evenly dispersed mussels rapidly self‐organized into power‐law clusters similar to those observed in nature. Collectively, the results indicate that facilitation of recruitment to existing mussels plays a major role in soft‐bottom spatial pattern development. The interaction between large‐scale resource availability (hard substrate) and local‐scale recruitment facilitation may be responsible for creating complex power‐law spatial structure in soft‐bottom mussel beds.