Bioengineering by the alien false mussel Mytilopsis sallei (Bivalvia: Dreissenidae) affects small-scale distribution pattern of benthic macroinvertebrates in an estuarine system

ABSTRACT The false mussel Mytilopsis sallei (Récluz, 1849) invaded the Songkhla Lagoon System in southern Thailand more than two decades ago. The false mussel forms dense beds, creating three-dimensional habitat structures on soft substrata which harbour a myriad of organisms. We compare a community of benthic macroinvertebrates in a habitat modified by M. sallei with a community in an unmodified habitat. The study focused on a canal feeding into the southern end of the lagoon system and covered three seasons (north-east monsoon season, dry season and south-west monsoon season) in a 12-month period. A total of 26 taxa were present in the modified habitat and 22 in the unmodified habitat. In the modified habitat, abundances of other byssally attached bivalves, flatworms and some species of amphipods were more than an order of magnitude greater than abundances in the unmodified habitat. Differences in the community composition between habitats were observed but were dependent on sampling location and time. Although the modified habitat hosted greater numbers of species and individuals in most comparisons, the Shannon–Wiener index and Pielou's evenness index were lower for this habitat as it tended to be dominated by a few taxa. The results highlight the role of M. sallei as a habitat engineer that promotes species richness and abundance of some benthic macroinvertebrates. Nonetheless, temporal variation in environmental drivers, especially salinity, might also be important in determining biodiversity in the studied estuarine system.