Impact of Asian Clams (Corbicula fluminea) on on Unionidae, or the family of pearly freshwater mussels


Corbicula fluminea is a species of freshwater clam that is native to eastern Asia and has become an extraordinarily successful invader of freshwater ecosystems and is found on every continent except for Antarctica (Leff et al., 1990; Hornbach, 1992; Karatayev et al., 2007; Lucy & Graczyk, 2008; Sousa et al., 2008a; Crespo et al., 2015). It is widely theorized they were introduced as a food source on the west coast of the United States and were first discovered in the country in 1938 (Sinclair & Isom, 1963; Counts, 1981). Impressivly, C. fluminea have the highest secondary production values ever measured for a species colonizing a freshwater ecosystem and the highest record net productivity for any bivalve (McMahon, 2002; Sousa et al., 2008b). While there are multiple species of Corbicula that are invading aquatic systems, the most common species is C. fluminea (Renard et al., 2000; Siripattrawan et al., 2000). There are several factors that have led to successful invasions of C. fluminea including high fecundity, functional hermaphroditism, self-fertilization, rapid sexual maturity, lack of a parasitic life stage, pedal feeding (that is feeding via the cilia on the foot) and filter feeding, and the ability to disperse across long distances (McMahon, 2002).

Due to the volume of successful invasions researchers have sought to investigate the impact of C. fluminea on aquatic systems. In terms of interactions with other species, an emphasis should be placed on Unionidae, or the family of freshwater pearly mussels, of which there is the greatest species diversity in North America. Unionidae are considered the most imperiled group of organisms in North America with only a quarter of species having stable populations (Allan & Flecker, 1993; Bogan, 1993; Lydeard & Mayden, 1995; Vaughn & Taylor, 1999; Strayer, 2004; Dugeon et al., 2006; Bogan, 2008; Ford, 2013; Lopes-Lima et al., 2017). In fact, it is estimated that over 40 percent of the mussel species in the Tennessee River are no longer reproducing (Ricciardi & Rasmussen, 1999). Their declines are due to previous commercial harvesting, invasive species, sedimentation, declines in fish host species, urban development, disease, predation, poor water quality/eutrophication, dams resulting in habitat alterations, habitat destruction and channelization (Bogan, 1993; Williams et al., 1993; Howells, 1996; Lydeard et al., 2004; Allen & Vaughn, 2010). Historically, Unionidae dominated the benthos of eastern North America (Parmalee & Bogan, 1998). Mussels are long lived, with some species living up to 100 years (Anthony et al., 2001). Many do not reproduce until age 7, on average, which increases their susceptibility to threats contributing to their decline (Kacar, 2011). Conversely, C. fluminea typically live 1 to 5 years and reproduce at an early age (Prezant & Charlermwat, 1984; McMahon & Bogan, 2001). C. fluminea and Unionidae occupy the same physical space in rivers, streams, and lakes- the benthos. They also are both filter feeders but C. fluminea will also exercise pedal feeding (Reid et al., 1992; Hakenkamp & Palmer, 1999).

It was originally thought that C. fluminea and Unionidae occupied different ecological niches and therefore there would be no negative effect on Unionidae (Kraemer, 1979). Although some early researchers such as Gardner (1976) did state there were declines in Unionidae due to new populations of C. fluminea, these researchers were in the minority. In addition to differing ecological niches, spatial distribution did not appear to overlap (Sickel, 1973; Kraemer, 1979; Clarke, 1988; Belanger et al., 1990). However, this is likely due to a difference in the preferred sediment type (Counts, 1986; Belanger et al., 1990; McMahon, 1991). Current research points to several pathways through which C. fluminea can negatively impact Unionidae including direct competition for food, displacing of juvenile mussels, physically harming the glochidia, or larvae, of the mussels, and ingestion of mussel sperm. Additionally, C. fluminea are prone to die-offs which produce excess ammonia which can be fatal to Unionidae (Scheller, 1997; Ilarri et al., 2011). Laboratory experiments have confirmed that C. fluminea have a negative impact on Unionidae. Ferreira-Rodriguez et al. (2018) placed Unio Delphinus (a freshwater mussel with no common name native to Europe)in cages with different treatments of C. fluminea and determined that there was a decline in carbohydrate concentration, a measure of biological condition, as a response to higher C. fluminea density. The results were able to explain the loss of up to 30 percent of the U. delphinus populations in the Iberian Peninsula that occurred in the last decade since the introduction of C. fluminea.

The direct competition of C. fluminea and Unionidae has been studied in limited capacities. It is clear from studies C. fluminea can filter out vast quantities of phytoplankton and other materials (Cohen et al., 1984; Lauritsen, 1986; Leff et al., 1990; Phelps, 1994), a vital food source for native mussels. Cohen et al. (1984) discovered that C. fluminea can filter out 30 percent of the phytoplankton from a 100 milliliter (mL) river water sample in 2 hours. Researchers attributed a depression in the phytoplankton abundance to C. fluminea. C. fluminea has been shown in laboratory experiments to have a filtration rate of 11 to 1370 mL per hour per clam (Buttner & Hiedenger, 1981; Lauritsen, 1986) whereas representative species of Unionidae can filter between 60 to 490 mL per hour per clam (Stanczyowska et al., 1973).

 Strayer (1999) has suggested that the “winner” in the competition for food resources between C. fluminea and various species of Unionidae is not the species that is able filter out the most food but the species that is able to survive the longest with limited resources. In most cases this will be Unionidae, but there is limited data to support how much food intake Unionidae species require for maintenance. Juveniles may be the most suspectable to influences of competition as they have higher energetic requirements than other live stages (Yeager et al., 1994). Seston (a mix of bacteria, detritus, plankton, and other materials) is reduced by C. fluminea in other pathways besides food intake. Rejected material, that is material that is ingested and then discarded as it is realized to not be food, and excreted wastes are removed from the water column and deposited in the sediment (Tenore & Dunstan, 1973), therefore further limiting the food available to Unionidae.

Atkinson et al. (2011) looked at the feeding preferences of both C. fluminea and Elliptio crassidens, or the Elephant ear. They looked at the rate at which each consumed detritus, heterotrophic bacteria, picoautotrophs (organism capable of being autotrophic or heterotrophic), nano autotrophs (small phytoplankton with sizes ranging from 2 to 20 micrometers (µm)) and heterotrophic nano eukaryotes (small zooplankton with sizes ranging from 2 to 20 µm).  The goal was to determine if there was competition for one group of plankton. Both species preferred organic and living materials. E. crassidens preferred to consume heterotrophic nano eukaryotes, whereas C. fluminea was less selective and preferred small materials such as picoautotrophs. This study suggests there is a functionally different niche of C. fluminea and E. crassidens. However, there were two limitations in applying this laboratory experiment on the ecosystem scale. One limitation is that while there was not much direct competition in the laboratory experiments in the flied due to the sheer number of C. fluminea, there is potential for high competition as mussels are typically found in lower densities. Per individual, C. fluminea filtered and excreted more material. In addition, this study only looked at the feeding preference of one mussel species, so it is hard to generalize for all species. However, there is additional evidence for competition between C. fluminea and Unionidae, Leff et al. (1990) found that C. fluminea can filter at a faster rate than Elliptio complanata, or the Eastern Elliptio.  It is worth noting that the larger filtration rates by C. fluminea can damage Unionidae in other ways, besides direct competition.

C. fluminea filters particles ranging in size from 5 to 30,000 µm2 (Wallace et al., 1977), a size range of particles that happens to include glochidia (Waller et al., 1988; Wei & Wang, 1995; Şereflişan et al., 2009). Modesto et al. (2019) formally investigated the survival rate of Anodonta anatina, or the duck mussel,glochidia with varying amount C. fluminea over 6, 12, 24 and 48 hours in a laboratory setting. There was an increase in mortality of A. anatina glochidia with increasing amounts of C. fluminea after a 48-hour period of exposure. There are two possible mechanisms of mortality: one is that the filtration of C. fluminea causes mechanical damage that results in fatalities and the other is that ammonia produced by C. fluminea induced fatalities in the glochidia. Damage to glochidia could cause huge effects on mussel populations as it limits their ability to colonize new locations and maintain current populations that are already struggling.

Gama et al. (2017) found that various climate change models will increase the suitable habitat for C. fluminea from 6.6 percent of aquatic habitats to 12.7 percent by 2050. Ferreira-Rodriquez et al. (2018) evaluated competition between Unio delphinus and C. fluminea during simulated heat waves as more frequent and intense heat waves are anticipated with climate change, as well. The study sought to measure competition during heat stress as these types of events have not been well documented regarding C. fluminea and U. delphinus. Through laboratory experiments, C. fluminea was found to be competitive superior to Unionidae species. However, it is worth noting that C. fluminea populations have been documented to undergo die off events in extreme temperatures whereas these events are less common in Unionidae (Phelps, 1994; Sousa et al., 2008b).

Vaughn and Spooner (2006) evaluated the spatial scales at which C. fluminea and Unionidae are currently found. Researchers looked at stream reaches and 0.25 square meter (m2) patches in the Ouachita Highlands, a region with high mussel richness, in the summers of 1999 to 2001 within 30 reaches of 8 rivers. Water quality was also measured in the form of temperature, dissolved oxygen, pH, and conductivity. These factors were not significant. C. fluminea was found in significantly higher concentrations in quadrants without mussels but when the patch scale data was pooled to represent the whole stream reach, the relationship was no longer statistically significant. However, the relationship did remain significant when looking at data for only juvenile mussels. There are two possible explanations: competition for food with juvenile mussels and the sediment disturbing habits of C. fluminea may displace juvenile mussels.

Haag et al. (2020) followed the survival and growth of 4 species of Unionidae in the field over 84 days at 17 sites within the same watershed in Kentucky. It was widely hypothesized that Unionidae declines in this drainage basin were due to water pollution from coal mining. However, researchers determined that the declines where attributed to C. fluminea. Growth in the juvenile mussels was best explained by temperature, which had a positive effect, and C. fluminea abundance. The most logical pathway for this was C. fluminea causing a reduction in food resources.Yeager et al. (2000) also found that abundance of C. fluminea had a negative impact on the growth of Unionidae juveniles. There was a 78 percent reduction in growth at only 625 C. fluminea per m2 and up to a 93 percent reduction when there were 5,000 individuals per m2. In contrast Haag et al. determined there was a 69 percent decrease in juvenile mussel growth with 625 C. fluminea per m2 and 84 percent decrease in growth with 5,000 individuals per m2. Both studies show there is a significant reduction in the growth of juveniles which may led to low recruitment in already struggling populations.

Pedal feeding and disturbing of the sediments may also limit the amount of preferred habitat for Unionidae. Because benthic macroinvertebrates, such as Unionidae, are highly impacted by the hydraulic environment (Statzner et al., 1988), hydraulic data can be used to predict mussel distribution. The more stable a category of sediment, the more likely it is that mussels will be found there (Hammontree, 2012). Stability of sediments is commonly cited as the best predictor of Unionidae distribution (Holland-Bartels, 1990; Strayer & Ralley, 1993; Layzer & Madison, 1995; Johnson & Brown, 2000; Box et al., 2002; McRae et al., 2004; Hammontree, 2012; Ford, 2013). Therefore, decreased stability of sediments by C. fluminea can limit preferred habitats of mussels. Pedal feeding and increased turbidity reduce levels of bacteria in the sediments (Hakenkamp et al., 2001) which are known to benefit Unionidae (Atkinson et al., 2013; Atkinson et al., 2014). Yeager (1994) also determined that juvenile mussels can be trapped inside C. fluminea during pedal feeding.

C. fluminea is prone to die offs due to silt load from spring runoff, temperatures extremes, and low dissolved oxygen, which is usually associated with low water flows (Phelps, 1994; Sousa et al., 2008b). As deceased C. fluminea decompose, ammonia, both ionized and nonionized, is produced. Cherry et al. (2005), in laboratory simulated die offs of C. fluminea in flow through systems, found the concentration of NH3 (also referred to as unionized ammonia) reached 5.04 per milligram per liter (mg/L) at 26°C. A lethal dose of NH3 was also determined for both the glochidia and adults of Villosa iris, or the rainbow mussel which is found in the St. Lawrence, upper Mississippi, Ohio, Tennessee, and Cumberland River Basins. Over 96 hours the lethal dose was determined to be 0.11 mg/L for glochidia and 0.8 mg/L for adult mussels, which is significantly smaller than the NH3 concentrations in the simulated die offs. The observed concentration of NH3 also generally exceeded the LD50, the lethal dose that kills 50 percent of organism over 96 hours, by 40 times. Here again, the juvenile mussels and glochidia were the most sensitive to the potential impact of C. fluminea. Likely this can be attributed to their lack of thick shells which can protect adult mussels from ammonia.

Small impacts to early life stages can have large impacts on the reproductive success of populations. Scheller (1997) determined that the LD50 for Pyganodon grandis, or the giant floater, have a LD50 that is 5 times higher than V.iris. Scheller also determined that unionized ammonia during simulated C. fluminea die offs peaked at 4.06 to 5.04 mg/L, which is still above lethal levels. A study of 10 species of mussel determined the LD50 of total ammonia, which includes both ionized and unionized, for adult mussels ranged from 2.56 to 8.97 mg/L (Augspurger et al., 2003) which would be obtainable during C. fluminea die offs. The timing of these events may further strain Unionidae as the tend to occur in low flow during the summer. Not only does the low flow led to the persistence of ammonia, Scheller determined that dissolved oxygen was 2.5 mg/L during the simulated die offs. However, the significant effects of die offs were limited to streams that had a density of 10,000 individuals per square meter (Cherry et al., 2005). This is a density that has been observed in streams (Gardner, 1976; Cherry et al., 1986; McMahon & Williams, 1986; Cataldo & Boltovskoy, 1998). In fact, in the New River in Virginia over the course of two months the C. fluminea population increase from 2,529 individuals per m2 to 269,105 m2 (Cherry et al., 1986).

Die offs may not be the only pathway for C. fluminea to induce ammonium toxicity in Unionidae. Cooper et al. (2008) also determined that the concentration of NH3 was positively correlated with temperature and the density of C. fluminea. Juvenile mussels were determined to be particularly susceptible due to their incapacities to avoid areas with elevated NH3.  Cooper et al. also determined that glochidia in areas of the stream with elevated NH3 were no longer viable. Zhang et al. (2011) determined that with increased disturbance of sediments, C. fluminea increased the release of nitrates and ammonium into the overlying water. Lauritsen and Mozley (1989) also calculated the amount of total ammonia released by living C. fluminea and found it to be an appreciable amount ranging from 357 to 8,642 µmoles per square meter per day. Williams and McMahon (1985) observed a 20 to 40 times increase in the amount of ammonia excretion during spawning in the spring.

C. fluminea can have a negative effect on some Unionidae species through direct competition for food resources. The pedal feeding of C. fluminea reduces the quality of habitat for Unionidae and causes mechanical damage of glochidia. The effects are likely not limited to those outlined in this paper. Phelps (1994) determined C. fluminea were responsible for a resurgence in submerged aquatic vegetation due to decreased turbidity which led to trophic cascades in the Potomac River. It has been suggested that C. fluminea may be a disease vector for Unionidae (Sickel, 1986; Sousa, 2008b; Haag et al., 2020). Most of the research has been conducted on lotic systems therefore more research is needed on lentic systems. It has yet to be seen how climate change will impact C. fluminea and Unionidae populations, but based on limited research, it is possible that C. fluminea will thrive while Unionidae will face further declines (Ferreira-Rodríguez & Padro, 2017). There may also be profound effects on mussel reproduction as mussels are dioecious, that is, fertilization occurs when the female ingest sperm that had been broadcast in the water column. It has not been studied if C. fluminea has the capability to remove that sperm and therefore limit reproductive success, nor the potential toxicity of ammonia on mussel sperm. Reproduction of Unionidae typically occurs in the summer when C. fluminea die offs are more likely to occur. While more research is needed on the how the effect may be mitigated it is abundantly clear C. fluminea has a pronounced negative impact on the most imperiled group of animals in North America- Unionidae.

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