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

Abstract

-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.

Narrow valve gaping in the invasive mussel Limnoperna securis: implications for competition with the indigenous mussel Mytilus galloprovincialis in NW Spain

“…These shell characteristics provide insight into metabolic requirements given that gill tissues are distributed along the internal cavity of the shells. We calculated the shell surface area for our experimental mussels based on allometry relationships provided in Babarro and Lassudrie (2011). We found that while the two experimental groups (L. securis and M. galloprovincialis) had similar shell lengths (~ 35 mm, Mann-Whitney, P = 0.30), external shell surface area was on average 28% lower in the L. securis group compared to the M. galloprovincialis group (Mann-Whitney, P = 0.002).…”

Interspecies comparison of the mechanical properties and biochemical composition of byssal threads

“…Mytilus californianus fibres having diameter values of 200 µm had the highest strength, stiffness and ultimate force, whereas P. nobilis threads with diameters of only 50 µm were characterized by the lowest values among the pool. The byssus thread diameter depends on the anatomy of the pedal groove (Babarro and Lassudrie, 2011;Price, 1981), which is in turn dependent on the species and on the body size (Bell and Gosline, 1997). Mechanical properties of byssus threads may also be influenced by other factors that could not be evaluated in the field, such as the age and history of the threads as demonstrated by Carrington and Gosline (2004) in M. californianus, or by differential byssal thread degradation considering the large temperature differences between the coastal sites sampled (Moeser and Carrington, 2006).…”

“…2). These observations suggest that P. nobilis is probably relying on thread quantity over thread quality -a behaviour similar to X. secures, which secretes up to 1000 thin and weak threads (Babarro and Lassudrie, 2011). For P. nobilis, this byssal thread production strategy may be an advantageous means to achieve attachment in quiescent sandy habitats of the Mediterranean Sea, where this species is commonly found (García-March et al, 2007).…”

Effects of species and sites on metal concentrations in byssal threads of two mytilids

“…The third site is Sampaio (SP), located at the mouth of the Oitavén-Verdugo river flowing into the Ria. In addition, following recent investigations of the coexistence of two mytilid species [21,22], the elemental analysis of byssus in our work involved threads secreted by the indigenous and raft cultured mussel M. galloprovincialis and by the invasive black pygmy mussel Xenostrobus securis. Both species are present in SS, whereas only X. securis is found in SP and M. galloprovincialis only in CE.…”

“…However, the number of threads secreted by X. securis is even 20 times greater than those secreted by M. galloprovincialis. Threads of X. securis byssus were easier to remove in wet conditions using a magnifying glass, but the adhesive plaques, clearly visible only under a microscope [21], were not separated from them.…”