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.