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Evaluation of food sources assimilated by unionid mussels using fatty acid trophic markers in Japanese freshwater ecosystems.

ABSTRACT Freshwater unionid bivalves play an important role in nutrient cycling and organic matter transportation and provide hard substrates for other benthic animals in freshwater ecosystems. Therefore, the conservation of unionid bivalves is considered essential for maintaining aquatic biodiversity. In this study, the assimilated food sources of six species of freshwater bivalves, Unio douglasiae. Unto douglasiae nipponensis, Unio biwae, Anodonta japonica. Pronodularia japonensis, and Lanceolarici grayana, were investigated using fatty acid trophic markers. The contribution (percentage of total identified fatty acids) of the trophic markers showed a similar tendency in all the bivalve species, even though they were sampled from various habitats. Fatty acid trophic markers of diatoms (20:5n3) and green algae and/or cyanobacteria (18:2n6 + 18:3n3) were dominant in all bivalve samples, ranging from 4.8% to 10.2% and 7.3% to 10.6%, respectively. Bacterial fatty acids were also detected in large amounts, ranging from 1.7% to 5.4%. In this study, all unionid bivalves contained diatom, green algal/cyanobacterial, and bacterial markers in substantial proportions, and no individual depended on a single food source, stressing the importance of ingesting and assimilating various food types.

KEY WORDS: food chain, bivalves, diet, biomarker, Unio


The roles and functions of bivalves in freshwater and marine ecosystems are well documented; they include nutrient cycling (Medina et al. 1995, Nakamura & Kerciku 2000, Atkinson et al. 2010, 2013), organic matter transportation from the water column to the benthic community by filter feeding (Deslous-Paoli et al. 1992, Navarro & Thompson 1997, Gergs & Rothhaupt 2008), and the provision of hard substrates for other animals (Werner & Rothhaupt 2007). Furthermore, some species of freshwater bivalves aid the reproduction of the Japanese bitterling Rhodeus ocellatus, an endangered fish species in Japanese freshwater ecosystems, by allowing R. ocellatus to lay their eggs inside the bivalves (Nagata 1985). Thus, conservation of freshwater bivalves is crucial to maintaining the function and biodiversity of freshwater ecosystems.

Populations of most species of freshwater unionid bivalves have declined worldwide (Bogan 1996); therefore, the development of aquaculture methods to rear freshwater bivalves has gained attention. These attempts have failed largely owing to a lack of information concerning appropriate food sources for the optimal growth and survival of freshwater bivalves.

Several studies microscopically examined the stomach contents of wild freshwater bivalves and revealed that algae were ingested; however, this method only allows for the investigation of recently ingested food sources and provides less information about the assimilability of each component. Feeding preferences and assimilation have been widely investigated using fatty acid trophic markers by studies on trophic relationships in both freshwater (Makhutova et al. 2013, Newton et al. 2013) and marine bivalves (Napolitano et al. 1997, Latyshev et al. 2004). Some fatty acids are specific to taxonomic groups and can be identified by analyzing higher trophic level organisms; thus, these fatty acids are suitable for use as tracers in food web studies (Dalsgaard et al. 2003). Moreover, the fatty acid composition of animals reveals time-integrated dietary information (Napolitano 1999) and could be used to determine the sources of assimilated foods; however, these techniques have not yet been applied to Japanese unionid bivalves. In this study, the fatty acid composition of several species of freshwater bivalves from a wide range of habitats in Japan was analyzed to identify their assimilated food sources.



Samples were collected from six species of unionid bivalves from several habitats in Japan (Table 1). At an artificial pond located near Lake Izunuma, northern Japan, Unio douglasiae were sampled (38[degrees] 43' 20.91" N 141[degrees]05' 33.16" E)in September and December 2011, and Unio douglasiae nipponensis was sampled at the Mou River (36[degrees] 49' 25.54" N 136[degrees] 56' 50.28" E) in July 2013. Local government designated U. douglasiae nipponensis as an endangered species. In November 2013, Unio douglasiae biwae, an endemic species of Lake Biwa, was sampled at the east coast of the largest lake in Japan (35[degrees] 21' 45.31" N 136[degrees] 16' 16.01" E). In December 2012, Pronodularia japonensis was collected at an agricultural irrigation site in Kawajima City (35[degrees] 58' 33.84" N 139[degrees] 28' 17.16" E), and in November 2012, Lanceolaria grayana cuspidata and Anodonta japonica were sampled at the Asahi River (34[degrees] 50' 28.03" N 133[degrees] 55' 24.84" E). In the Ministry of Environment's Red Data list, P. japonensis and L. grayana cuspidata are designated as nearthreatened species.

The soft parts of the sampled bivalves were obtained from the shells and stored in a -30[degrees]C freezer for further analysis.

Fatty Acid Analysis

Contamination from the stomach contents was avoided by using only foot muscles for fatty acid analysis. Lipid extraction and esterification were performed using the one-step method developed by Abdulkadir and Tsuchiya (2008), with slight modifications. First, about 50-200 mg of freeze-dried muscle samples were placed in glass tubes containing 5 ml hexane, 2 ml 14% [BF.sub.3] methanol, and nitrogen gas to fill the head space. The glass tubes were heated to 100[degrees]C for 2 h in a water bath to extract lipids and obtain each fatty acid. After the tubes were cooled at room temperature, 1 ml hexane and 2 ml distilled water were added, and the tubes were shaken vigorously. The upper hexane layer, which included the fatty acid methyl esters, was transferred to a gas chromatography vial after 3 min of centrifugation at 2,500 rpm. Next, 1 [micro]l of the hexane was analyzed in a gas chromatograph (GC-2014; Shimadzu) equipped with a capillary column (Select FAME; 100 m x 0.25 mm id; Agilent Technologies). The column temperature was programmed to increase from 150[degrees]C (with a 5-min hold) to 230[degrees]C at a rate of 4[degrees]C/min, and a hold of 10 min. Subsequently, the temperature was increased to 250[degrees]C at 4[degrees]C/min, with a hold of 10 min. Helium gas was used as the carrier. The temperatures of the injector and the flame ionization detector were 270[degrees]C and 280[degrees]C, respectively.

Fatty acid peaks were identified by comparing with the retention times of commercial standard mixtures (Supelco Inc.). Peaks that were not found in the standards were ignored. These unknown peaks were less than 15% (maximum sample) of the total peak area. The contribution of each fatty acid is represented as a percentage of the total identified fatty acids.


Fatty acid trophic markers used in this and previous studies are summarized in Table 2. Although these fatty acids have been considered taxonomic group specific, some might be synthesized by other aquatic animals. For instance, fatty acid trophic markers of diatoms (20:5n3) can be synthesized in vivo if the animal possess specific enzymes and food sources that contain the fatty acid precursor, 18:3n3 (Bell et al. 1986). This synthesis could preclude the accurate estimations of diatom assimilation. Diatoms are, however, known to contain higher amounts of 16:ln7 than that in other algal species, which enables the use of the 16:1 n7/16:0 ratio as a biomarker for diatoms. When a diatom multiplies, the 16:1 n7/l6:0 ratio increases (Shin et al. 2008), thus, ratios of greater than 1 can be used as a marker for diatoms (Budge et al. 2001). Thus, the concentrations of 20:5n3 and the 16:1 n7/16:0 ratio in bivalves would show a positive relationship if biosynthesis does not occur. The relationship between the concentration of 20:5n3 and the 16:1 n7/16:0 ratio was investigated by using correlation analysis.


In all the species, the most abundant fatty acid was 16:0 or 20:4n6 (Appendix 1). Most fatty acids were detected in all bivalves, except fori-15:0, a-15:0, 17:1,and 18: ln9t, which were found only at low levels in one species (<0.6%). The ratio of n3/n6 ranged from 0.5 to 0.8, except in the Unio biwae obtained from Lake Biwa.

Trophic markers of the green algae/cyanobacteria and diatoms were dominant for all species (Fig. 1). Substantial levels of bacterial and dinoflagellate trophic markers were also detected in all the species; however, trophic markers of higher plants were only detected in the Unio douglasiae obtained from the Izu Pond (in September) and in smaller amount in Anodonta japonica (0.4% and 0.7%).

A significant positive relationship was found between 20:5n3 and 16:1 n7/16:0 in unionid bivalves (r = 0.82, P < 0.05; Fig. 2).


In this study, the available food sources at each study site were different because the trophic state, sediment characteristics, and sampling season varied. Nonetheless, the relative presence of fatty acid trophic markers was similar for all the bivalves, suggesting that different bivalve species access similar food sources under a wide range of food availability conditions. All unionid bivalves contained diatom, green algal/cyanobacterial, bacterial, and dinoflagellate markers in substantial proportions. Diatom and green/cyanobacterial biomarkers were especially dominant in freshwater unionid bivalves.

For marine bivalves, fatty acid studies revealed that diatoms were the primary assimilated food source, because the detection of their fatty acid biomarker were at levels ranging from 11.2% to 26.1% (Taylor & Savage 2006, Grahl-Nielsen et al. 2010). Indeed, diatoms have been used as feed for marine bivalves in aquaculture (Coutteau & Sorgeloos 1992). For freshwater unionid bivalves, 20:5n3 was observed in all species at relatively higher levels (4.8%-10.2%) than those of the other biomarker fatty acids. A significant positive relationship was detected between 20:5n3 and 16:1 n7/l6:0 for unionid bivalves, which indicated that the 20:5n3 detected in the bivalves was mainly derived from diatoms. The level of 20:5n3 in unionid bivalves was, however, considerably lower than that in marine bivalves, but the contribution of green algae/cyanobacteria biomarkers, namely 18:2n6 and 18:3n3, was considerably higher in unionid bivalves than in marine bivalves. Both 18:2n6 and 18:3n3 are found in the green algae and cyanobacteria of freshwater ecosystems; thus, unfortunately, their food contribution to these bivalves could not be separated. Liu et al. (2009) conducted feeding experiments and showed that Unio douglasiae preferred the cyanobacterium Microcystis aeruginosa to the green alga Scenedesmus obliquus. In a laboratory feeding experiment, Anodonta woodiana showed higher growth potential when fed cyanobacteria than when fed green algae (Liu et al. 2014). These studies suggest that cyanobacteria are more efficiently assimilated by bivalves than by green algae. Typically, green algae have a thick cell wall that can prevent them from being digested. Cyanobacteria are also, however, not ideal food sources for aquatic animals. First, cyanobacteria are known to produce a toxin, namely microcystin, which might be harmful to some aquatic organisms. A high concentration of microcystin was detected in U. douglasiae, which is abundant in the eutrophic Lake Suwa (Yokoyama & Park 2002). This might imply that microcystin is not toxic to the bivalve and that the organism can ingest and assimilate cyanobacteria. The absence of sterols and essential fatty acids such as 20:5n3 and 22:6n3 in cyanobacteria, however, seems to be another negative attribute for their use as a food source for bivalves (Basen et al. 2012). These nutrients are vital for appropriate somatic growth and reproduction (Hendriks et al. 2003). Thus, cyanobacteria cannot be a sole food source for unionids. As shown in the present study, several types of algal trophic markers were found in unionid bivalves, implying that diverse food sources are required for their survival.

In this study, the contribution of bacterial fatty acid biomarkers found in unionid bivalves ranged front 1.7% to 5%, which almost matched the data previously reported for other unionid bivalves (4% 6%; Newton et al. 2013), indicating that bacteria were the natural diet of unionid bivalves. As with cyanobacteria, the lack of some essential fatty acids and sterols in bacteria suggests that bacteria alone cannot support the appropriate growth of bivalves (Newton et al. 2013, Taipale et al. 2014). Bacteria could, however, supplement a good diet when combined with algae, because bacteria contain a high amount of protein (Brown et al. 1996). Unionid bivalves seemed to obtain their essential nutrition from various types of food sources in their natural habitat, indicating that providing a mixed diet might be the key to successful aquaculture of unionid bivalves.


We would like to thank Prof. D. Tanaka (Toyama University), Mr. D. Banba (Toray Techno Co., Ltd.), Dr. Y. Kimochi (Center for Environmental Science in Saitama), and Dr. M. Nishio (Hirni City) for their help with bivalve sampling. We also thank Drs. Ichise and Furuta (Lake Biwa Environmental Research Institute) for their help in organizing the survey. Part of this study was supported by JSPS KAK.ENHI Grant Numbers 25290084 and 25289151.


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Fatty acid composition of unionid bivalves (% of total fatty acids
[+ or -] 1 SD).

                              Unio douglasiae

                          Izunuma         Izunuma       douglasiae
                         September       December       nipponensis

Fatty acids             Mean     SD     Mean     SD     Mean     SD

14:0                     1.1    0.2      1.0    0.1      0.9    0.1
i-15:0                   0.0    0.0      0.3    0.1      0.0    0.0
a-15:0                   0.0    0.0      0.0    0.0      0.0    0.0
15:0                     1.7    0.1      1.8    0.2      3.0    0.2
i-16:0                   0.6    0.1      0.5    0.1      0.0    0.0
16:0                    18.0    1.9     15.8    1.1     23.7    0.6
16:1 [omega]7            5.2    2.6      6.4    0.9      4.1    0.8
i-17:0                   2.5    0.6      2.3    0.3      1.5    0.2
a-17:0                   0.6    0.2      0.6    0.1      0.0    0.0
17:0                     3.7    0.2      3.2    0.2      4.0    0.1
17:1                     0.3    0.2      0.0    0.0      0.0    0.0
18:0                    11.5    0.8      8.7    0.8     12.3    0.4
18:1 [omega]9t           0.0    0.0      0.0    0.0      0.0    0.0
18:1 [omega]9c           5.8    0.8      5.3    0.6      8.0    0.8
18:1 [omega]7            1.7    0.3      1.4    0.1      1.6    0.1
18:2 [omega]6c           4.0    0.7      4.4    0.5      4.7    0.1
18:3 [omega]6            0.0    0.0      0.3    0.1      0.0    0.0
18:3 [omega]3            3.5    0.8      3.6    0.4      2.6    0.1
18:4 [omega]3            0.0    0.0      0.7    0.1      0.0    0.0
20:0                     1.0    0.2      0.9    0.2      1.9    0.2
20:1 [omega]9            1.8    0.2      1.5    0.1      1.5    0.2
20:2 [omega]6            0.7    0.1      0.5    0.1      0.0    0.0
20:3 [omega]6            0.0    0.0      0.2    0.2      0.0    0.0
20:4 [omega]6           23.1    1.2     22.0    1.1     18.5    0.4
20:4 [omega]3            0.6    0.2      0.7    0.1      0.0    0.0
22:0                     0.8    0.5      1.1    0.2      2.6    0.5
20:5 [omega]3            6.5    0.5     10.0    0.4      4.8    0.4
24:0                     0.4    0.2      0.0    0.0      0.0    0.0
22:5 [omega]3            3.1    0.4      4.0    0.8      2.8    0.2
22:6 [omega]3            1.8    0.2      3.3    0.3      1.4    0.1
Total                  100.0    0.0    100.0    0.0    100.0    0.0
[summation][omega]3     15.4    1.3     22.3    1.6     11.7    0.5
[summation][omega]6     27.8    1.2     27.4    1.3     23.2    0.4
[omega]3/[omega]6        0.6    0.0      0.8    0.1      0.5    0.0

                                       Lanceolaria       Anodonta
                        Unio biwae       grayana         japonica

Fatty acids             Mean     SD     Mean     SD     Mean     SD

14:0                     1.2    0.1      0.7    0.2      0.9     --
i-15:0                   0.0    0.0      0.0    0.0      0.0     --
a-15:0                   0.0    0.0      0.1    0.1      0.0     --
15:0                     1.4    0.3      2.1    0.2      3.2     --
i-16:0                   0.0    0.0      0.1    0.2      0.5     --
16:0                    23.9    0.6     18.3    1.2     16.9     --
16:1 [omega]7            8.7    1.3      6.7    0.4      5.0     --
i-17:0                   0.4    0.4      1.8    0.3      1.7     --
a-17:0                   0.0    0.0      0.0    0.0      0.0     --
17:0                     2.2    0.2      3.7    0.2      3.9     --
17:1                     0.0    0.0      0.0    0.0      0.0     --
18:0                    11.0    0.3     10.3    0.3     11.0     --
18:1 [omega]9t           0.0    0.0      0.4    0.3      0.0     --
18:1 [omega]9c           4.8    0.4      6.8    0.7      6.6     --
18:1 [omega]7            1.3    0.1      1.3    0.8      1.3     --
18:2 [omega]6c           4.7    0.2      5.4    0.6      5.5     --
18:3 [omega]6            0.0    0.0      0.0    0.0      0.0     --
18:3 [omega]3            3.3    0.4      2.2    0.3      2.1     --
18:4 [omega]3            1.3    0.2      0.1    0.3      0.0     --
20:0                     1.1    0.1      1.8    0.3      1.1     --
20:1 [omega]9            1.7    0.3      1.6    0.2      1.2     --
20:2 [omega]6            0.7    0.1      0.5    0.3      0.6     --
20:3 [omega]6            0.0    0.0      0.0    0.0      0.0     --
20:4 [omega]6           12.2    0.6     21.2    1.1     21.5     --
20:4 [omega]3            0.0    0.0      0.0    0.0      0.0     --
22:0                     1.1    0.2      1.2    0.5      0.9     --
20:5 [omega]3           10.2    0.6      7.4    1.4      6.9     --
24:0                     0.0    0.0      0.0    0.0      0.7     --
22:5 [omega]3            4.8    0.7      5.1    0.6      6.0     --
22:6 [omega]3            3.8    0.3      1.2    0.2      2.6     --
Total                  100.0    0.0    100.0    0.0    100.0     --
[summation][omega]3     23.5    2.1     16.1    1.7     17.6     --
[summation][omega]6     17.7    0.8     27.1    1.3     27.6     --
[omega]3/[omega]6        1.3    0.2      0.6    0.1      0.6     --


Fatty acids             Mean     SD

14:0                     0.0    0.0
i-15:0                   0.0    0.0
a-15:0                   0.0    0.0
15:0                     2.5    0.2
i-16:0                   0.0    0.0
16:0                    20.0    0.8
16:1 [omega]7            7.1    0.6
i-17:0                   1.2    0.8
a-17:0                   0.0    0.0
17:0                     3.3    0.2
17:1                     0.0    0.0
18:0                     9.7    0.9
18:1 [omega]9t           0.0    0.0
18:1 [omega]9c          10.7    1.1
18:1 [omega]7            2.4    0.4
18:2 [omega]6c           7.4    0.6
18:3 [omega]6            0.0    0.0
18:3 [omega]3            2.2    0.5
18:4 [omega]3            0.0    0.0
20:0                     1.1    0.4
20:1 [omega]9            0.0    0.0
20:2 [omega]6            0.0    0.0
20:3 [omega]6            0.0    0.0
20:4 [omega]6           20.5    1.8
20:4 [omega]3            0.0    0.0
22:0                     0.0    0.0
20:5 [omega]3            6.6    0.7
24:0                     0.0    0.0
22:5 [omega]3            3.2    0.4
22:6 [omega]3            2.2    0.5
Total                  100.0    0.0
[summation][omega]3     14.2    1.5
[summation][omega]6     27.9    1.4
[omega]3/[omega]6        0.5    0.1


(1) Department oj Civil and Environmental Engineering, Tohoku University, 6-6-06 Aramaki-Aoba, Aoba, Sendai 980-8579, Japan; (2) Center for Environmental Science in Saitama, 914 Kamitanadare, Kazo, Saitama 347-0115, Japan

* Corresponding author. E-mail:

DOI: 10.2983/035.035.0125

Information about the bivalve sampling locations.

Species            Site          Date             Sample size

Unio douglasiae    Izunuma       September 2010        5
                                 December 2010         8

Unio douglasiae    Himi          July 2013             3

Unio biwae         Lake Biwa     November 2013         3

Lanceolaria        Asahi River   November 2012         5

Anodonta                         November 2012         1

Pronodularia       Kawajima      December 2012         5

Species            Ecosystem type            Sediment type

Unio douglasiae    Artificial pond           Mud

Unio douglasiae    River                     Mud

Unio biwae         Lake                      Sand

Lanceolaria        River                     Sand and mud

Anodonta           River                     Sand and mud

Pronodularia       Agricultural irrigation   Mud

Fatty acid trophic markers used in this study.

     Fatty acid             Marker for             Reference

i-15:0. a-15:0          Bacteria             Mfilinge et al. (2005)
i-17:0. a-17:0
18:2n6, 18:3n3          Green algae and      Mfilinge et al. (2005)
20:5n3                  Diatoms              Mfilinge et al. (2005)
18:4n3, 22:6n3          Dnioflagellates      Napolitano (1999)
Long chain fatty acid   Terrestrial plants   Meziane et al. (1997)
  (from C24 to C32:0)
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Article Details
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Author:Fujibayashi, Megumu; Nishimura, Osamu; Tanaka, Hitoshi
Publication:Journal of Shellfish Research
Article Type:Report
Geographic Code:9JAPA
Date:Apr 1, 2016
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