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Microhabitat use by the invasive Asian clam (Corbicula fluminea) in the Guadalupe River, Texas.

Abstract.--Corbicula fluminea (the Asian Clam) is an ecologically important invasive species in North America and worldwide. In Texas, C. fluminea is present in all major drainage basins. This study evaluated the relationship between selected microhabitat features and C. fluminea density in the Guadalupe River, Texas to determine which factors provide favorable conditions for C. fluminea. Microhabitat features and C. fluminea density were sampled at 10 stations spanning the length of the Guadalupe River. Total dissolved solids (TDS) and water depth were negatively correlated with C. fluminea density. Given the high C. fluminea densities found in this study (e.g., up to 629 per [m.sup.2] along with a high filtration capability, a reduction of organic matter in the invaded waters may be a possible explanation. The microhabitat features measured in this study suggest that the Guadalupe River is well suited for C. fluminea and given the observed conditions persist; further invasions along the river are likely. The distribution and density of C. fluminea in the Guadalupe River depend on multiple factors and understanding the influence of microhabitat features will become important for management activities in the future.


The Asian Clam [Corbicula fluminea, (Muller)] is considered to be one of the most aggressive invasive species found in American and European freshwater ecosystems. Studies in the last twenty years have shown widespread geographic dispersion and invasion (reviewed in Sousa et al. 2008a). Native to Asia, Africa and Australia, C. fluminea was first documented in North America in the 1920s (Counts 1981; 1986). Within forty years, C. fluminea spread from the Pacific Northwest to the Atlantic coast of the United States (Sousa et al. 2008a). This species is currently found in 39 continental states, Hawaii, and Mexico. C. fluminea are naturally dispersed as juveniles and are passively transported by water currents, in the guts of fish, or on the feet and feathers of aquatic birds (McMahon 1999). In addition, several types of human-mediated activities such as water transportation, recreation, and food use are thought to have contributed to the long distance dispersal of C. fluminea across natural geographical barriers (McMahon 1999).

Many life cycle traits of C. fluminea also contribute to successful invasion and dispersion. C. fluminea is hermaphroditic with typically two reproductive periods per year: spring and fall (McMahon 1999). Fertilization occurs internally and the incubation period of 5-6 days allows for a single clam to produce several broods per reproductive period (McMahon 1999). The average number of young produced for each hermaphroditic individual is over 68,000 juveniles per year (Aldridge & McMahon 1978). C. fluminea exhibits rapid growth as a result of its high filtration and assimilation rates, reaches maturity in three to nine months, and has a life span of one to five years (McMahon 1999). The short life span, early maturity, rapid growth, high fecundity, and juvenile dispersion capacity of C. fluminea make this species highly invasive and able to colonize new environments; potentially contributing to the worldwide spread over the last several decades (Araujo et al. 1993; Cataldo & Boltovskoy 1999; McMahon 1999; 2002; Darrigran 2002; Sousa et al. 2008b). McMahon (2002) concluded that the invasive success of C. fluminea relies more on these lifecycle characteristics rather than its physiological tolerance to environmental extremes.

The introduction of C. fluminea has consequences for other ecosystem elements within invaded waters (reviewed in Sousa et al. 2008a). In large abundances, C. fluminea can alter sediments by burrowing and bioturbation activity (Vaughn & Hakenkamp 2001) and may serve as vectors for new parasites and diseases (Sousa et al. 2008a). With a high filtration rate and pedal feeding, C. fluminea consume large amounts of primary producers in the water and sediments, and may reduce food availability to other species (McMahon 1991; Strayer 1999). Studies also suggest that C. fluminea has negatively impacted the already rapidly declining native bivalves by depleting food resources, reducing habitat space and reducing bivalve reproduction by ingesting large numbers of unionid sperm, glochidia, and newly formed juveniles (Strayer 1999). C. fluminea also impacts the human economy by biofouling water channels, irrigation canals, and the water systems of power stations, treatment plants, and factories (McMahon 1999; Darrigran 2002).

The first record of C.fluminea in Texas was in the Neches River in 1958 (Howells 2004). By the mid to late 1970s, the invasive clam was found throughout central, eastern, and northern Texas and within all of the major drainage basins of the state, including the Guadalupe River Basin (Karatayev et al. 2005a). By 2005, sampling confirmed that C. fluminea was present in 180 of the 257 Texas counties with the probability of more occurrences that have not yet been documented (Karatayev et al. 2005a).

Previous investigations of C. fluminea in Texas have focused on the biogeography and population dynamics of the exotic bivalve within state waters (Britton and Murphy 1977; Aldridge and McMahon 1978; Fontanier 1982; Neck 1986; 1987; Howells 2004; Karatayev et al. 2003; 2005a). However, there is little discussion on the microhabitat features within Texas waterways that potentially provide the most favorable conditions for continued expansion of C. fluminea. The objective of this study was to evaluate the potential relationship between selected microhabitat variables of the Guadalupe River and measured C. fluminea density. An increased understanding of how C. fluminea population characteristics vary with abiotic factors in this river is important for future invasion control, prevention, and the reduction of negative impacts.


Study Area.--The Guadalupe River is approximately 370 kilometers in length and contains one major reservoir (Canyon Lake) near the city of New Braunfels (Fig. 1). Water releases from Canyon Dam largely control the flow for the middle reach of the river, making this area one of the most popular recreational sites in the state. Ten sampling stations were selected using several criteria: geographical locations spanning the entire length of the river (upper, middle, and lower reaches), accessibility to the water, and presence of a United States Geological Survey (USGS) real-time water data gage. The stations were numbered sequentially from 1-10 starting from the upper reach to the confluence with the San Antonio Bay on the Gulf of Mexico (Fig. 1). Each sampling station was associated with a USGS water gage identified by GPS coordinates and the USGS gage identification number. At each station, three transects were selected perpendicular to the flow of the water and spaced 1015 m apart. Substrate sampling occurred three meters from the shoreline as C. fluminea are usually restricted to shallower waters (Aldridge & McMahon 1978; McMahon 1999).

Sampling.-Water chemistry was sampled during October 2010 at each transect using a Horiba U-22 water quality meter including temperature ([degrees]C), pH, dissolved oxygen (DO, mg/L), total dissolved solids (g/L), turbidity (NTU), conductivity (S/m), salinity (percent) and water depth at the sample station (cm). Water quality parameters were averaged across the three transects to achieve a sampling mean for each of the 10 stations. Discharge ([m.sup.3]/s) and gage height (m) were recorded from the USGS Real-Time Water Data website ( The mean water flow and gage height measurements were taken from three consecutive 15-minute readings for the sampling period. The dominant substrate type was classified by particle size using the Texas Commission on Environmental Quality (TCEQ) water monitoring guide (2007).

C. fluminea density was determined at each transect using a 0.25 [m.sup.2] quadrat made of PVC piping and right-angle connectors. The quadrat was secured to the river bottom using U-shaped ground anchors. A clear-bottom bucket was used to examine the quadrat and visible clams were removed and set aside in a collection bucket. The river bottom was excavated by hand to a depth of 10-15 cm and the removed substrate was run through a 1.0 mm mesh sieve. Clams present on the sieve were also placed in the collection bucket. All removed live clams were identified to species, enumerated, and then placed back into the substrate within the marked quadrat. The C. fluminea density was averaged across the three subsamples to achieve a sampling mean for each of the 10 stations. Multiple linear regression was used to elucidate which microhabitat features explain variability in mean clam density ([alpha] = 0.05).


Water quality parameters and dominant substrate type varied among the sampling stations along the river (Table 1). Gage heights were slightly lower than mean heights reported in the USGS historical data for the months of October 2000-2009 (USGS 2010). Discharge values were approximately 35% lower than reported for the same history but were similar to values recorded during 2009. All measured water quality parameters were consistent with ranges reported by the Texas State University River Systems Institute water quality monitoring 2008 and 2009 data summary reports.

C. fluminea distribution and density also varied throughout the length of the river (Table 1). Live C. fluminea were found in seven of the ten sampling sites with Station 2 having the greatest measured density (629 individuals per [m.sup.2]) followed by Station 9 (225 per [m.sup.2]). The other sites had relatively low measured densities (mean of 4.25 individuals per [m.sup.2] excluding Stations 2 and 9). Live C. fluminea were not present at Stations 5, 8, and 10, however, Station 5 did have dead C. fluminea shells found within the sampled substrate and along the shoreline. Sampling depth and TDS were negatively correlated to C. fluminea density (F = 12.444, df = 2, 9, P = 0.005) and explained 71.8% of the variability in C. fluminea density in the Guadalupe River.


Our results showed that density of C. fluminea were greater in shallower waters of the Guadalupe River. C. fluminea may favor the shallow microhabitats due to food and oxygen availability as C. fluminea have been previously described as being intolerant of even moderate levels of hypoxia (Matthews & McMahon 1999). Belanger (1991) suggests that the lower threshold of DO tolerated by C. fluminea is between 1-3 mg/L. In this study, DO was not observed to occur below 9 mg/L suggesting that the Guadalupe River provides adequate levels of oxygen concentration throughout all reaches of the river. The density of C. fluminea was also found to be greater in areas with lower measured total dissolved solids (TDS). Previous studies have shown that the fdtering activities of C. fluminea increased water transparency (Buttner 1986; Leff et al. 1990; Phelps 1994; McMahon 1999). Suspended particles in the Guadalupe River were not specifically identified in this study but we hypothesize C. fluminea may be reducing organic matter through filter and pedal feeding activities.

Although significant, negative relationships were identified among water depth, total dissolved solids, and C. fluminea density; our results were likely influenced by the high densities measured at Stations 2 and 9. Given the observed microhabitat variables, our data suggests that all reaches of the Guadalupe River provide favorable conditions for C. fluminea life stages. Tolerable limits for C. fluminea include water temperatures from 2-37[degrees]C, upper limit salinity of 4-6% (Karatayev et al. 2005b), pH range of approximately 5.5-8.3 (Araujo et al. 1993), and DO lower limits of 1-3 mg/L (Belanger 1991). The microhabitat variables measured in this study fell within the published ranges of C. fluminea habitable conditions. Sand and gravel are reported as the preferred substrate however, C. fluminea has the ability to inhabit several types including mud, silt, sand, gravel, cobble, rock, and boulders (Belanger et al. 1985; Neck 1986; McMahon 1999). We found C. fluminea present on all substrate types in the Guadalupe River, except sand (e.g., Station 10).

With the exception of Stations 2 and 9, C. fluminea were found in relatively small abundances along the Guadalupe River and were absent in three of the sample sites. As discussed, the microhabitat features of all sites sampled appear capable of supporting the C. fluminea. Given that favorable conditions persist throughout the Guadalupe River, we hypothesize that 1) C. fluminea is likely to colonize the three 0 density sites 2) C. fluminea density is likely to increase in lower density sampled sites and 3) invasion may occur in new areas of the river. Therefore, we recommend this research effort be repeated, but also expanded, to include additional sampling sites and collections during different seasonal periods for several consecutive years. This would allow for a more robust evaluation of spatial and temporal patterns of microhabitat parameters and C. fluminea density. In addition, the proposed sampling would provide an opportunity to monitor C. fluminea proliferation, document new colonization, and aid in determining invasion control and management needs.

The results of this study should be considered preliminary with relationships between microhabitat variables and C. fluminea density requiring further investigation. Data provided by this investigation provide necessary baseline information not only to compare C. fluminea density in the Guadalupe River through time but also for comparison to other rivers. A more in depth understanding is imperative for reducing negative impact and prevention of further invasion within the Guadalupe River and other Texas waterways.


We thank Jonathan Sedlacek and Richard Stark for equipment and field assistance. We would like to thank the University of Nebraska at Kearney for their technical support.


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CES at:

Carey E. Sedlacek and Casey W. Schoenebeck

Department of Biology, University of Nebraska at Kearney, Kearney, NE 68849

Table 1. Mean microhabitat parameters (SE), dominant substrate type,
and mean C. fluminea density ([+ or -]SE) measured during October 2010
in the Guadalupe River, Texas. Conductivity (COND), dissolved oxygen
(DO), total dissolved solids (TDS), dominant substrate type (DST).
Salinity = 0.0% at all sites, therefore it was not included in the

         Discharge     Height (1)                   COND
Site   ([m.sup.3]/s)      (m)           pH         (mS/cm)

S1      0.68(0.00)        7.75      8.00(0.06)   46.00(0.00)
S2      1.98(0.02)        3.96      8.17(0.09)   51.33(0.33)
S3      2.07(0.00)        1.10      8.07(0.07)   55.67(0.33)
S4      3.82(0.00)        2.57      8.10(0.00)   54.67(0.33)
S5      3.11(0.00)        4.47      7.40(0.15)   52.33(1.76)
S6      5.92(0.00)        2.15      7.83(0.12)   56.00(0.58)
S7      27.86(0.06)      12.51      8.07(0.03)   59.67(0.33)
S8      20.01(0.05)       8.17      7.00(0.64)   57.00(0.58)
S9      22.33(0.04)       6.89      8.13(0.03)   54.33(1.67)
S10     33.98(1.99)       3.54      8.17(0.03)   77.33(0.33)

        Turbidity         DO           Temp        TDS (1)
Site      (NTU)         (mg/L)      ([degrees]C)    (g/L)

S1      9.67(2.73)    9.10(0.10)    20.00(0.06)     0.30
S2     39.33(2.60)    9.10(0.10)    21.13(0.03)     0.32
S3      7.67(3.48)    9.67(0.18)    20.27(0.03)     0.36
S4      0.00(0.00)    9.83(0.09)    23.27(0.03)     0.35
S5     14.00(10.15)   9.90(0.15)    20.47(0.12)     0.33
S6      3.00(0.58)    9.93(0.07)    22.40(0.00)     0.36
S7     15.00(2.08)    9.17(0.19)    23.90(0.00)     0.38
S8     23.67(4.67)    9.90(0.06)    21.53(0.03)     0.37
S9     14.33(2.03)    10.0(0.06)    22.83(0.03)     0.37
S10    20.67(1.45)    10.53(0.09)   25.27(0.03)     0.50

        Sampling                 Clam Density
Site   Depth (cm)      DST       (#/[m.sup.2])

S1     45.67(5.28)    Cobble    14.00 (2.65)
S2     16.50(0.75)   Silt/Mud   629.33 (125.74)
S3     36.40(2.23)    Cobble    8.67 (2.96)
S4     42.07(2.81)    Cobble    5.33 (2.67)
S5     43.20(4.39)   Silt/Mud   0.00 (0.00)
S6     46.57(3.37)    Gravel    2.33 (0.88)
S7     25.40(2.94)    Cobble    3.67 (0.67)
S8     40.63(2.92)    Cobble    0.00 (0.00)
S9     26.27(2.24)    Cobble    225.33 (17.07)
S10    22.00(1.70)     Sand     0.00 (0.00)

(1) SE = 0.00 for all Gage Height means, except SE = 0.01 for Site 2
Gage Height mean.

(2) SE = 0.00 for all TDS means.
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Author:Sedlacek, Carey E.; Schoenebeck, Casey W.
Publication:The Texas Journal of Science
Date:Feb 1, 2012
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