Introduction
Most freshwater mussels are endobenthic and use their muscular foot and shell to burrow into the sediment (Allen & Vaughn, 2009; Christian et al., 2020). This behavior may promote substrate destabilization through bioturbation, which modifies the benthic habitat (Vaughn & Hakenkamp, 2001). For example, mussels may mix different sediment layers, altering their physical, chemical and microbial properties (Boeker et al., 2016; Simeone, Tagliaro & Beasley, 2021a; Sansom, Bennett & Atkinson, 2022), and thus the abundance of co-occurring species (Simeone, Tagliaro & Beasley, 2021b; Martinski & Woolnough, 2023).
The burrowing behavior of individual mussel species may vary with season and reproductive cycle (Amyot & Downing, 1998); flow regime (Schwalb & Pusch, 2007); substrate composition (Eissenhauer et al., 2023); and mussel density and shell shape (Allen & Vaughn, 2009). Of particular interest for this study, mussel shell shape is strongly associated with habitat features in temperate rivers (e.g. hydrodynamics and sediment composition) (Watters, 1994; Hornbach, Kurth & Hove, 2010). These associations are linked with horizontal and vertical movements of mussels (Schwalb & Pusch, 2007). Nonetheless, information about these relationships is scarce in the Neotropics. The association between hydrodynamics and shell shape of Castalia ambigua Lamarck, 1819 in the eastern Amazon was demonstrated by Simeone et al. (2022) but there is no information on how morphological differences may influence C. ambigua behavior in the sediment.
South America has approximately 168 native species of freshwater mussels, with species of the genus Castalia Lamarck, 1819 (Unionida: Hyriidae) distributed across the main river basins (Olivera-Hyde et al., 2020). In particular, the species C. ambigua is found in the Amazon, Paraguay and Paraná River basins, and has a restricted distribution in the Uruguay River basin (Pereira et al., 2014). In addition, this species is mainly associated with areas of low hydrodynamic energy in river margins with poorly compacted sandy substrates and low silt deposition (Simeone, Tagliaro & Beasley, 2021c).
In the Amazon, recent studies have described the effect of C. ambigua density (Simeone et al., 2021b), and their filtration and biodeposition activities on associated macroinvertebrates and water quality (Simeone et al., 2021a). However, the lack of information on horizontal and vertical movements of mussels leads us to ask: How does mussel burrowing behavior vary according to density and shell shape in the Amazon? Thus, the present study, using laboratory experiments, aimed to measure the responses of C. ambigua in relation to combinations of two different shell morphotypes and three different densities.
Methods
Mussel sampling
We collected the specimens of C. ambigua used in the laboratory experiment in November 2019 from the Caeté River, a morphologically unaltered alluvial lowland river, approximately 150 km long with a sixth-order basin, located in northeastern Pará state, in the eastern Brazilian Amazon (Figure 1). Prior to mussel sampling, we collected sediment and water from the Caeté River, that were used to regularly replace the water during the experiments. At a single site (Figure 1), we collected specimens of C. ambigua excavating the sediment by hand to a depth of 15 cm. In the Caeté river, there are two distinct morphotypes of C. ambigua representing ecophenotypic plasticity (Olivera-Hyde et al., 2020) and shaped by hydrodynamics and/or sexual dimorphism (Simeone et al., 2022); the first, Morphotype I, has an elongate posterior margin with laterally compressed shells, and is frequently found in high hydrodynamic energy habitats. The second, Morphotype II, has rounded anterior-posterior margins with laterally inflated shells, and mainly occurs in low hydrodynamic energy habitats (Simeone et al., 2022). Fifty individuals of C. ambigua were collected: 26 individuals from Morphotype I (mean length of 43.9 ± 2.2 mm) and 24 individuals from Morphotype II (mean length of 38.1 ± 1.5 mm). All mussels were placed in a container with a sediment layer and water from their habitat in the Caeté River, transported to the laboratory and maintained in aerated aquarium.