## 1 Introduction  
Widely distributed in East Asia (China, Japan, and Korea), Unionoida, which includes the genera _Anodonta_ and _Ulrio_, is a group of freshwater mussels, representing a guild of benthic, burrowing, and filter-feeding bivalves [1]. Bivalves are efficient filter feeders of particles suspended in the water column, including bacterioplankton, protists, phytoplankton, zooplankton, and other abiotic particles [2, 3, 4]. Thus, in rivers and lakes where they are abundant, unionoids are expected to play pivotal roles in driving ecosystem functions, such as carbon flow and nitrogen recycling.  
The bivalve species _Ulrio douglasiae_ is widespread in Asia and has been consumed as food for 500 years in Korea [5, 6]. At present, this species is neither threatened nor endangered [7]. Despite the lack of extensive research (i.e., taxonomic and ecological studies) on unionid bivalves, including _Ulrio douglasiae_ in Korea, their biomass and diversity are anticipated to decline due to repetitive cyanobacterial blooms. These are usually triggered by the destruction of habitat, pollution, loss of host-fish for larval development, and outbreaks of invasive species [8]. Algal blooms represent a serious threat during every dry season or early summer (between May and July). Unfortunately, whether _Unio douglasiae_ populations in Korea can persist through repetitive cyanobacterial blooms, primarily those of _Microystis_, _Dolichospermum_, _Oscillatoria_, and _Aphaizomenon_, remains unclear.  
Cyanobacterial blooms are common in eutrophic or hypertrophic freshwater ecosystems worldwide [9, 10]. These blooms negatively affect the nutritional quality of food sources in such ecosystems, and toxins secreted by these cyanobacteria (e.g., microccystn or anatoxin) are harmful to aquatic organisms and humans. Unionid mussels are known to filter feed on cyanobacteria, such as colony-forming _Microcytsi aeruginosa_ and filamentous _Planktothrix agardii_[11; 12]. In addition, _Unio douglasiae_ and _Corbicula fluminea_ actively graze on both toxic and non-toxic _Microcytsi aeruginosa_, while _Anodonta anatina_ shows a slight preference for green algal species, such as _Scenedesmus_, although it can still filter feed and ingest both _Microcytsi_ and _Planktothrix_[13]. Ultimately, these unionoids can reduce the abundance of cyanobacteria in the water column, although cyanobacteria bloom globally reduces the diversity and health of other species [10].  
In the present study, stable isotope tracers (\({}^{13}\)C and \({}^{15}\)N) were used to evaluate the accumulation of cyanobacterial (_Microcytsi aeruginosa_) carbon and nitrogen in unionid bivalves (_Unio douglasiae_) through laboratory experiments. Isotope labeling techniques based on the enrichment of \({}^{13}\)C and \({}^{15}\)N stable isotope ratios have been commonly applied to determine the uptake rate of carbon and nitrogen and their allocation to various animal tissues or organs. These techniques can also be used to trace the fate of algal-derived organic matter in natural biota and elucidate energy sources and pathways [14; 15; 16]. Using \({}^{13}\)C- and \({}^{15}\)N-labeled phytoplankton, the assimilation pathways of carbon and nitrogen in aquatic organisms can be directly traced. The inorganic forms of \({}^{13}\)C(HCO\({}_{3}\)\({}^{-}\)) and \({}^{15}\)N(NO\({}_{3}\)\({}^{-}\)) tracers were assimilated into the _Microcytsi_ as organic forms through photosynthesis, and synthesized newly organic cells.  
The objective of the present study was to evaluate the feeding effect of a unionid bivalve (_Unio douglasiae_) on a toxic cyanobacterium _Microcytsi aeruginosa_ as a nutrient resource and to determine whether the unionid assimilates _Microcytsi_ carbon and nitrogen using \({}^{13}\)C and \({}^{15}\)N dual tracers in a short period of time with no current.