Ecophysiological responses of invasive and indigenous mytilids in the Ría de Vigo (NW Spain)

The impact of an alien species is correlated with its abundance and potential to colonise new environments. Consequently, the crucial aspects that give a mytilid species invasive potential include its strength and capacity for adhesion to a wide variety of substrates and its ecological tolerance to environmental conditions. The alien black pygmy mussel Xenostrobus securis settles together with the indigenous and raft cultured mussel Mytilus galloprovincialis on hard substrates of the inner coastline of the Ría de Vigo (Atlantic Coast NW Spain). Key parameters for the ecological success of these species could include their byssus structure and ecological plasticity. In this study, we assessed their comparative ability to attach to different substrates under different environmental conditions, using both in situ and laboratory approaches. Byssus threads secreted by X. securis were found to be extremely thin and their thickness did not vary with mussel size. This thinness of byssus filaments in X. securis was compensated by the secretion of a huge number (up to thousand of byssus filaments). Although no increase in attachment strength was found with increasing mussel size in this species, multiple byssus thread secretion might be a key factor in the successful invasion of inner areas of the estuary. Furthermore, X. securis exposes a much lower shell area to lift and drag forces in mussel conglomerates. By contrast, the indigenous species M. galloprovincialis secretes thicker byssus threads, which provide stronger in situ attachment displaying a linear increase in strength relative to the size of mussels. The exposure of both mytilid species to different abiotic factors in the laboratory confirmed the weaker byssus secretion and limited variation in attachment strength in the alien species. On one hand, X. securis seemed to be better adapted to life on soft bottoms and not capable of much plasticity when attached to different substrates. However, X. securis might be also well adapted to variable environmental conditions, e.g., salinity fluctuations, with no need to modify its byssus properties to ensure secure attachment. Attachment strength of M. galloprovincialis was higher than X. securis in all abiotic conditions tested. Results are discussed in relation to the ecological tolerance and impact of these mytilid species. © EDP Sciences, IFREMER, IRD 2011