Effects of suspended inorganic matter on filtration and grazing rates of the invasive musselLimnoperna fortunei(Bivalvia: Mytiloidea)

Abstract

The Asian mytiloid Limnoperna fortunei (Dunker, 1857) was first recorded in South America along the coast of the Rio de la Plata estuary in 1991 (Pastorino et al., 1993). Since then the mussel has expanded its range, colonizing most of the Rio de la Plata basin, as well as several minor watersheds. Because of its wide distribution, high densities and significant ecosystem engineering capabilities, L. fortunei has had sizeable impacts on the waterbodies colonized, including modification of nutrient concentrations and ratios, enhancement of water transparency, macrophyte growth and effects on cyanobacterial blooms, the abundance and diversity of benthic invertebrates, sedimentation rates and food availability for fishes (Sylvester, Boltovskoy & Cataldo, 2007; Boltovskoy et al., 2009, 2013; Cataldo et al., 2012a, b; Boltovskoy & Correa, 2015; Paolucci & Thuesen, 2015). Impacts on human activities have been especially marked: L. fortunei larvae enter raw water conduits of open cooling systems and develop large beds in pipes and other components, clogging them and causing pressure loss, overheating and corrosion (Boltovskoy, Xu & Nakano, 2015). Northward expansion of L. fortunei is expected to continue beyond South America and into Central and North America (Karatayev et al., 2015). Available data indicate that potentially colonizable areas include all continents except Antarctica (Kluza & McNyset, 2005; Karatayev et al., 2015), but the fact that water bodies lacking mussels exist in watersheds where L. fortunei has been present for decades suggests that some environmental conditions may limit the expansion of this invader (Darrigran et al., 2011). Among these, the concentration of suspended solids is of particular interest. Suspended matter can affect respiration, growth, parasite infestation and reproduction of the organisms (Robinson, Wehling & Morse, 1984; Alexander, Thorp & Fell, 1994; Rosewarne et al., 2013), thereby restricting their geographic spread, but this constraint has not been explicitly addressed in models of the potential distribution of L. fortunei (Kluza & McNyset, 2005;Oliveira, Hamilton & Jacobi, 2010). In order to estimate the tolerance of L. fortunei of inorganic suspended solids, thus providing data for analysis of its potential distribution worldwide, we assessed the species’ capability of filtering water and retaining phytoplankton at different clay concentrations. Individuals of L. fortunei were collected from Buenos Aires (348320S; 588250W) and stored in aerated aquaria filled with dechlorinated tap water at 23–25 8C. They were fed ad libitum on cultured algae (.95% Scenedesmus spp., mean biovolume 1300–1600 mm) known to be actively consumed by the mussel (Cataldo et al., 2012a). Individuals 15–20 mm (mean 17.3 mm) in shell length were isolated from the clumps and placed in flat trays in order to verify their vitality. Actively filtering individuals were transferred from the trays to acclimation vessels at 27 8C for 48 h. All individuals were starved for 24 h and then stocked in cylindrical plastic netting cages (10 cm high, 6 cm in diameter) placed at mid-depth in the experimental 2-l containers (Fig. 1). In the latter, an air hose was attached laterally to a tube located vertically on the bottom, thereby suctioning settling sediment particles and returning them to the water column (Fig. 1). Algal concentrations in the experimental containers ranged from 30 to 60 (mean 43.5+6.3 SE) mg Chl a l, mimicking usual values for eutrophic water bodies (Jones & Lee, 1982). Bentonite clay with a mean particle diameter of 9.45 mm, within the range of .90% of the inorganic suspended solids in the South American water bodies colonized by L. fortunei (Carignan, 1999; Sarubbi, Pittau & Menendez, 2004), was used at different concentrations: 0 g l (controls), 0.1, 0.5, 1, 2, 4, 6, and 8 g l. All experiments were performed at 27 8C (typical of the lower Parana River and Rio de la Plata estuary during the summer) in a controlled temperature chamber. For each sediment concentration, three replicates without (controls) and three with 60 individuals of L. fortunei were used. Upon termination of each experiment (120 min) all individuals were measured to the nearest 0.01 mm with digital calipers, and their tissues extracted and dried at 60 8C to constant weight (DTW). Immediately before start and after termination, each experimental container was sampled (40–150 ml) to estimate chlorophyll a concentrations. Samples were filtered through fibreglass filters (Whatman GF/F) and pigment extraction was performed with hot (60–70 8C) ethanol in darkness two or three times in order to avoid underestimations due to chlorophyll adsorption upon sediment particles (Koyama, Shimomura & Yanagi, 1968). The extracts were clarified by centrifugation, their volume adjusted and the absorbance at 665 and 750 nm measured with a spectrophotometer before and after acidification with HCl (1 N). Pigment concentrations were calculated according to Marker et al. (1980).