Habitat alteration and community-level effects of an exotic mussel, Musculista senhousia

The mussel Musculista senhousia is capable of marked habitat alteration through the construction of byssal mats on the surface of soft sed~ments Here 1 demonstrate the importance of this alteration on sedimentary properties and res~dent macrofaunal assemblages of a tidal flat in Mission Bay, San Diego, California, USA, where the mussel is exotic. In well-developed mats, percent fine sediments, percent combust~ble organic matter, and sediment shear strengths were increased relative to adjacent areas without mats. Comparisons of naturally occurring areas with and without mats of M. senhousia, as well as compansons of the same tidal flat when mussels were seasonally present and absent, revealed that assemblages w~thin mussel mats differ from those in sediments without mats. The primary effect of the mussel and its mats was facilitation of other organisms. Total densities of all macrofaunal ~ndiv~duals as well as specles richness were typically higher inside than outside mussel mats. Two species that exhibited large enhancements of densities within mussel mats were the tanaid Leptochelia dubia and the gastropod Barleeia subtenuis. Oligochaetes, a numerically important component of the tidal flat, appear least facilitated by the presence of mats. A shortterm, manipulative experiment that examined the effects of mussel mat mimics on the colonization of macrofauna suggested that the presence of physical structure alone can produce several of the patterns observed in naturally occurring mussel mats. These results highlight that alteration of habitats is an important effect of exotic species, and that these habitat alterations can have subsequent effects on resident biotic communities.