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Asian clams: the most successful aquatic invaders

Some scientist considers Asian clams (Corbicula fluminea) to be the most successful aquatic invasive species, as evidenced by their being found on every continent except for Antarctica (1, 2, 3, 4, 5, 6). In addition, the species results in the loss of a billion dollars a year in the U.S. from clogging intact pipes (7). The clams are small, typically ranging from the size of a penny to a quarter, brownish yellowish, and triangular oval. They have distinctly raised growth groves. The species may be confused with native finger clams, but native species are more oval, found buried entirely, and have very fine growth ridges (8). While multiple species of Asian clams are invading aquatic systems, the most common species is C. fluminea (9, 10).

Biology

Asian clams are filter feeders but can also feed on the substrate called pedal feeding (11). The clam has one of the highest rates of filter-feeding of any bivalve (8). Unfortunately, the high filtration rate also means that they excrete many nutrients that can cause algal blooms (12,13).

The clams are found in the bottoms of water bodies, often buried in the substrate. Consequently, they are adapted to a wide range of substrates. Clams can also survive up to 36 days without being submerged in water (11).

The clams are 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 (12, 14). In these conditions, the clams will die and start to decompose, releasing compounds that may be harmful to other aquatic life, like native freshwater mussels (15).

Life Cycle

Asian Clam. Photo credit Lauren Schramm

Asian clams can reproduce in large numbers as they can start reproducing at around 3 to 6 months (8). The adult lives 3 to 4 years and can produce 100,000 juveniles per year (8). Adults can self fertilize, and the fertilized eggs develop into juveniles in the gills in 4 to 5 days. (8)

Range

In the United States, Asian clams can be found throughout 44 states but have the highest numbers in the southeast, Texas, and southwest. Significant populations globally can be found in South America, Europe, and Asia. Native populations also exist in Africa and Australia.

Researchers have found that various climate change models will increase the suitable habitat for Asain clams from 6.6 percent of aquatic habitats to 12.7 percent by 2050 in the U.S. (16).

Map of significant populations of Asain Clams (solid circles), sourced from Mackie & Brinsmead, 2017.

Introduction history

It is widely theorized that they were introduced as a food source on the west coast of the United States and were first discovered in the country in 1938 in California (17, 18). Another theory is that they were hitchhikers with imported Giant Pacific oysters (Crassostrea gigas) (8). Several factors have led to successful invasions of Asian clams, including its ability to produce a high number of offspring, self-fertilization, rapid sexual maturity, lack of a parasitic life stage (most other freshwater clams have this), filter-feeding, and the ability to disperse across long distances (11). In addition, juvenile clams can travel long distances using mucus “parachutes” (8).

Management options

Unfortunately, there are limited management options as once an invader enters an aquatic system, there are several different pathways it can travel. For example, Asian clams can be disturbed via fish eating them and then being excreted in waste (19). Other dispersal methods are not fully understood but likely include; bait buckets, bilge water, live wells, water currents, and transportation by wildlife (8).

Due to the rapid spread, there have been few peer-reviewed management techniques for the clam (8). However, a physical barrier can be installed to stop the spread, the clams can be dredged for and removed, and molluscicides (a chemical that specifically kills mussels) can be used (7). The latter two options should only be limited in use as they can potentially harm native mussels.

Interactions with native mussels

Regarding interactions with other species, an emphasis should be placed on Unionidae, or the family of freshwater pearly mussels. Freshwater mussels are considered the most imperiled group of organisms in North America, with only a quarter of species having stable populations (20, 21, 22, 23, 24, 25, 26, 27).

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 (21, 28, 29, 30, 31).

Historically, native mussels dominated the streams and rivers of eastern North America (32). In addition, mussels are long-lived, with some species living up to 100 years (33). On average, many do not reproduce until age 7, which increases their susceptibility to threats contributing to their decline (34). Conversely, Asian clams typically live 3 to 4 years and reproduce early (35, 36).

Asian clams and mussels occupy the same physical space in rivers, streams, and lakes. They also are both filter feeders. It was initially thought that Asian clams did not impact native mussels (37). Although some early researchers (38) did state there were declines in native mussels due to new populations of Asian clams; these researchers were in the minority. In addition to differing habitat preferences, their ranges did not appear to overlap (39, 37, 40, 41).

Current research points to several ways Asian clams can negatively impact native mussels, including direct competition for food, displacing of juvenile mussels, and ingestion of mussel sperm. Additionally, Asian clams are prone to die-offs that produce excess ammonia, which can be fatal to native mussels (42, 43).

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Sources:

  1. Leff, L. G., Burch, J. L., & McArthur, J. V. (1990). Spatial distribution, seston removal, and potential competitive interactions of the bivalves Corbicula fluminea and Elliptio complanata, in a coastal plain stream. Freshwater Biology, 24(2), 409.
  2. Hornbach, D. J. (1992). Life history traits of a riverine population of the Asian clam Corbicula fluminea. American Midland, 248- Naturalist 257.
  3. Karatayev, A. Y., Padilla, D. K., Minchin, D., Boltovskoy, D., & Burlakova, L. E. (2007). Changes in global economies and trade: the potential spread of exotic freshwater bivalves. Biological Invasions, 9(2), 161-180.
  4. Lucy, F. E., & Graczyk, T. K. (2008). Revision of the distribution of Corbicula fluminea (Müller, 1744) in the Iberian Peninsula. Aquatic Invasions, 3, 355-358.
  5. Sousa, R., Antunes, C., & Guilhermino, L. E. D. P. S. (2008a). Ecology of the invasive Asian clam Corbicula fluminea (Müller, 1774) in aquatic ecosystems: an overview. In Annales de Limnologie-International Journal of Limnology. EDP Sciences, (Vol. 44, No. 2, pp. 85-94).
  6. Crespo, D., Dolbeth, M., Leston, S., Sousa, R., & Pardal, M. Â. (2015). Distribution of Corbicula fluminea (Müller, 1774) in the invaded range: a geographic approach with notes on species traits variability. Biological Invasions, 17(7), 2087-2101.
  7. https://www.adkwatershed.org/asian-clam
  8. https://nyis.info/invasive_species/asian-clam/
  9. Renard, E., Bachmann, V., Cariou, M. L., & Moreteau, J. C. (2000). Morphological and molecular differentiation of invasive freshwater species of the genus Corbicula (Bivalvia, Corbiculidea) suggest the presence of three taxa in French rivers. Molecular Ecology, 9(12), 2009-2016.
  10. Siripattrawan, S., Park, J. K., & Foighil, D. Ó. (2000). Two lineages of the introduced Asian freshwater clam Corbicula occur in North America. Journal of Molluscan Studies, 66(3), 423-429.
  11. McMahon, R. F. (2002). Evolutionary and physiological adaptations of aquatic invasive animals: r selection versus resistance. Canadian Journal of Fisheries and Aquatic Sciences, 59(7), 1235-1244.
  12. Phelps, H. L. (1994). The Asiatic clam (Corbicula fluminea) invasion and system-level ecological change in the Potomac River estuary near Washington, DC. Estuaries, 17(3), 614-621.
  13. Wittman, M., Reuter, J., Schladow, G., Hackley, S., Allen, B., Chandra, S., & Caires, A. (2008, December). University of California Davis, Reseach, Aquatic Invasive Species, Asian Clam pdf. (2008). Retrieved from University of California Davis: http://terc.ucdavis.edu/research/AsianClam2009.pdf
  14. Sousa, R., Nogueira, A. J., Gaspar, M. B., Antunes, C., & Guilhermino, L. (2008b). Growth and extremely high production of the non-indigenous invasive species Corbicula fluminea (Müller, 1774): possible implications for ecosystem functioning. Estuarine, Coastal and Shelf Science, 80(2), 289-295.
  15. Cherry, D. S., Scheller, J. L., Cooper, N. L., & Bidwell, J. R. (2005). Potential effects of Asian clam (Corbicula fluminea) die-offs on native freshwater mussels (Unionidae) I: water-column ammonia levels and ammonia toxicity. Journal of the North American Benthological Society, 24(2), 369-380.
  16. Gama, M., Crespo, D., Dolbeth, M., & Anastácio, P. M. (2017). Ensemble forecasting of Corbicula fluminea worldwide distribution: Projections of the impact of climate change. Aquatic Conservation: Marine and Freshwater Ecosystems, 27(3), 675-684.
  17. Sinclair, R.M. and Isom, G.B. 1963. Further Studies on the Introduced Asiatic Clam (Corbicula) in Tennessee. Tennessee Stream Pollution Control Board, Tennessee Department of Public Health. 1-75.
  18. Counts, C. L. (1981). Corbicula fluminea (Bivalvia: Corbiculidea) in. British Columbia. Nautilus, 95, 12-13.
  19. Gatlin, M. R., Shoup, D. E., & Long, J. M. (2013). Invasive zebra mussels (Driessena polymorpha) and Asian clams (Corbicula fluminea) survive gut passage of migratory fish species: implications for dispersal. Biological Invasions, 15(6), 1195-1200.
  20. Allan, J. D., & Flecker, A. S. (1993). Biodiversity conservation in running waters. BioScience, 43(1), 32-43.
  21. Bogan, A. E. (1993). Freshwater bivalve extinctions (Mollusca: Unionoida): a search for causes. American Zoologist, 33(6), 599-609.
  22. Lydeard, C., & Mayden, R. L. (1995). A diverse and endangered aquatic ecosystem of the southeast United States. Conservation Biology, 9(4), 800-805.
  23. Strayer, D. L., Downing, J. A., Haag, W. R., King, T. L., Layzer, J. B., Newton, T. J., & Nichols, J. S. (2004). Changing perspectives on pearly mussels, North America’s most imperiled animals. BioScience, 54(5), 429-439.
  24. Dudgeon, D., Arthington, A. H., Gessner, M. O., Kawabata, Z. I., Knowler, D. J., Lévêque, C., & Sullivan, C. A. (2006). Freshwater biodiversity: importance, threats, status and conservation challenges. Biological reviews, 81(2), 163-182.
  25. Bogan, A. E. (2008). Global diversity of freshwater mussels (Mollusca, Bivavle) in freshwater. Hydrobiologia, 595, 139-147.
  26. Ford, D. F. (2013). Ground-truthing Maxent in East Texas rivers. (Masters dissertation, The University of Texas at Tyler).
  27. Lopes‐Lima, M., Sousa, R., Geist, J., Aldridge, D. C., Araujo, R., Bergengren, J., & Zogaris, S. (2017). Conservation status of freshwater mussels in Europe: state of the art and future challenges. Biological Reviews, 92(1), 572-607.
  28. Williams, J. D., Warren Jr, M. L., Cummings, K. S., Harris, J. L., & Neves, R. J. (1993). Conservation status of freshwater mussels of the United States and Canada. Fisheries, 18(9), 6-22.
  29. Howells, R. G., Neck, R. W., & Murray, H. D. (1996). Freshwater mussels of Texas. University of Texas Press.
  30. Lydeard, C., Cowie, R. H., Ponder, W. F., Bogan, A. E., Bouchet, P., Clark, S. A., & Thompson, F. G. (2004). The global decline of nonmarine mollusks. BioScience, 54(4), 321-330.
  31. Allen, D. C., & Vaughn, C. C. (2010). Complex hydraulic and substrate variables limit freshwater mussel species richness and abundance. Journal of the North American Benthological Society, 29(2), 383-394.
  32. Parmalee, P. W., & Bogan, A. E. (1998). Freshwater mussels of Tennessee. University of Tennessee Press.
  33. Anthony, J. L., Kesler, D. H., Downing, W. L., & Downing, J. A. (2001). Length‐specific growth rates in freshwater mussels (Bivalvia: Unionidae): extreme longevity or generalized growth cessation?. Freshwater Biology, 46(10), 1349-1359.
  34. Kacar, A. (2011). Some microbial characteristics of mussels (Mytilus galloprovincialis) in coastal city area. Environmental Science and Pollution Research, 18(8), 1384-1389.
  35. Prezant, R. S., & Chalermwat, K. (1984). Flotation of the bivalve Corbicula fluminea as a means of dispersal. Science, 225(4669), 1491-1493.
  36. McMahon, R. F., & Bogan, A. E. (2001). Bivalves. Ecology and Classification of North American Freshwater Invertebrates. 2nd ed. Edited by Thorp, JH, and AP Covich, Academic Press, New York.
  37. Kraemer, L. R. (1979). Corbicula (Bivalvia: Sphaeriacea) vs. indigenous mussels (Bivalvia: Unionacea) in US rivers: A hard case for interspecific competition?. American Zoologist, 19(4), 1085-1096.
  38. Gardner, J. A. (1976). The invasion of the Asiatic clam (Corbicula manilensis Philippi) in the Altamaha River, Georgia. Nautilus, 90 (3), 117-125.
  39. Sickel, J. B. (1973). A new record of Corbicula manilensis (Philippi) in the southern Atlantic slope region of Georgia. Nautilus, 87 (1), 11-12.
  40. Clarke, A. H. (1988). Aspects of corbiculid-unionid sympatry in the United States. Malacology Data Net, 2(3/4), 57-99.
  41. Belanger, S. E., Farris, J. L., Cherry, D. S., & Cairns Jr, J. (1990). Validation of Corbicula fluminea  growth reductions induced by copper in artificial streams and river systems. Canadian Journal of Fisheries and Aquatic Sciences, 47(5), 904-914.
  42. Scheller, J. L. (1997). The effect of dieoffs of Asian clams (Corbicula fluminea) on native freshwater mussels (Unionidae) (Doctoral dissertation, Virginia Tech).
  43. Ilarri, M. I., Antunes, C., Guilhermino, L., & Sousa, R. (2011). Massive mortality of the Asian clam Corbicula fluminea in a highly invaded area. Biological Invasions, 13(2), 277-280.

Map source: Mackie, G. L., & Brinsmead, J. K. (2017). A risk assessment of the golden mussel, Limnoperna fortunei (Dunker, 1857) for Ontario, Canada. Management of Biological Invasions, 8(3), 383.

Asian clam ID chart: https://adkinvasives.com/Invasive-Species/Detail/31

Using Conservation Techniques to Stop Poaching of Wildlife

The following was a term paper written for my conservation biology class in collaboration with Hallie Draegert

Abstract:

It is a well-known fact that the human population is increasing in size. The growth of one population, the humans, tends to impact other populations negativity. In this case, the concern is the impact it has on wildlife populations. As long as wildlife and human populations coexist problems will arise.  These problems can occur in response to tension between local human communities and the attempts of organizations working to conserve native populations of animals. Often these organizations seem to forget that the local’s livelihoods depend on exploiting the local animals. This has led to the creation of conservation models that do not work because they did not consider the local politics.

One of the most common crimes committed is poaching. Poaching has led directly to the extinction of many species and induces stress, on currently stable species. This could make future extinction of the population an issue that needs to be addressed. Since poaching is a serious problem for highly stressed species, it is critical to work to prevent the crime. There are many current techniques used to reduce and combat poaching. However, the issue has not been fully resolved. This is likely because only a single model is in use at a time in an area.

geograph-6097299-by-Mike-Pennington
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The rainforest is teeming with dynamic relationships!

There are many dynamic relationships in the rainforests of the world. Which is expected as they are home to up to 40 percent of known plant and animal species. Within the forest there are a widely variety of symbiotic relationships, which means there are interactions between two species. For some species the relationship is beneficial for both parties, this is call mutualism. For example the Brazil nut trees (Bertholletia excelsa) produces a nut that is an only be cracked by a few species, one of which is various agouti (Dasyprocta sp.) which is a small rat like animal. The tree benefits as it’s seeds are dispersed and the agouti receives a free meal.

1142px-central_american_agouti_bp

Description
English: A Central American Agouti (Dasyprocta punctata) in Panama experimenting with a western diet.
Source BirdPhotos.com
Author Tomfriedel

In some cases one species is unaffected and the other benefits, this is commensalism. A example of this would be a frog sheltering itself from a storm event under the leaves of a plant. The plant is un effected but the frog benefits. Lastly, parasitism is when one species is harmed and the other benefits. Bot flies are a common tropical parasite that lay their eggs on a mammal host. The eggs then hatch and the larva develop in the skin of the host animal.

frog in rain

A frog in the rain in Belize. Image by Lauren Schramm

Competition is a symbiotic relationship in while two species use the same resources. Because it requires an energy investment both species are harmed, but the harm may be unequal. Many rainforest plants are animal pollinated or their seeds are dispersed by animals. Resource partitioning occurs when a resource is divided among species in some way like time or place. One way plants reduce their competition against each other is by targeting different animal groups. For example flowers pollinated by bats are often white making them easier to see at night, while flower pollinated by other animals are red, orange, and yellow. Bat pollinated flowers also contain a musky smells, while flower pollinated by moths, bees, and other insects have a strong fragrance. Plants also reduce competition by flowering and fruiting at different times. This is one reason there are always flowers and fruits available in the rainforest at any given time.

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PCR and Gel Electrophoresis

I used to teach a genetics lab and thought I would share what I taught my students. Before running polymerase chain reactions (or PCR), to make copies of your sample DNA, it is important to run a gel with your DNA extraction to see if the extraction worked. To run a gel, the DNA sample is loaded into wells on one side of a gel. An electrical current is used to separate out the DNA to analyze it. If your DNA extraction did not work it is a waste of money to run PCR on the sample. This gel is run in buffer TBE. TBE contains tris which raises the pH to 9, a boric acid which lowers the pH to optimizes it for the target enzymes, and EDTA which chelates metal ions and those stops DNeasy activity, which is the enzyme used for DNA extraction. The molecules in the gel separate based on charge, size, and shape. Personally, I like to use 1% agarose gel with TBE at 120 volts for 30 minutes.

gel

But what is agarose? It’s a seaweed polysaccharide that allows the gel to thicken. The more agarose you use the thicker the gel and the longer it will take your DNA sample to run through the gel. If you need to separate out really small fragments you would need a really thick gel but since we are looking to see how well the DNA extraction worked we can use a thin gel. You should also stain the DNA sample before putting the sample in the gel. If you don’t stain the sample you wouldn’t be able to see where your sample goes in the gel and run the risk if running your sample off of the gel. Gels also often have glyercide in them that binds to the DNA and makes it heavier so when you load your DNA into the well on the gel it is less likely that the DNA floats away. It another well a ladder is also typically used. In this case, we used a 1000 kb base pair ladder. A ladder is DNA fragments that are cut at known lengths. This allows it to act as a ruler for your other samples.

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The genetics of viruses

Viruses are infectious particles made of nucleic acid encased in a protective protein coat and, sometimes, a membranous envelope. The genome of viruses may consist of double-stranded DNA, single-stranded DNA, double-stranded RNA, or single-stranded RNA, depending on the kind of virus. The viral genome is usually organized as a single linear or circular molecule of nucleic acid. The genome is encased in a protein shell called a capsid which is derived from the host cell. The most complex capsids are found in viruses that infect bacteria, called bacteriophages or phages. A virus has a genome but can reproduce only within a host cell. Dr. Beijerinck used the sap from one generation of infected plants to infect the second generation of plants that could, in turn, infect future generations. Dr. Beijerinck determined that the pathogen could reproduce only in the host, could not be cultivated on nutrient media, and was not killed by alcohol which generally kills bacteria.

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An isolated virus is unable to reproduce—or do anything else, except infect an appropriate host. This is because viruses lack the enzymes for metabolism and the ribosomes for protein synthesis. Each type of virus can infect and parasitize only a limited range of host cells, called its host range. Viruses identify host cells by a “lock and key” fit between proteins on the outside of the virus and specific receptor molecules on the host’s surface. Most viruses of eukaryotes attack specific tissues. Most DNA viruses use the DNA polymerases of the host cell to synthesize new genomes along the templates provided by the viral DNA.

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The Cell Cycle and it’s phases

In the cell cycle, cell division functions in reproduction, growth, and repair. The two forms of cell division are mitosis and meiosis. Mitosis requires the distribution of identical DNA to two daughter cells. Meiosis yields four nonidentical daughter cells, each with half the chromosomes of the parent. The purpose of meiosis is to produce cells for reproduction, and as you want the offspring to have the same number of chromosomes as the parents’ sex cells need to have half the number of chromosomes as the parents. A cell duplicates its DNA, moves the two copies to opposite ends of the cell, and then divides into two cells.  This genetic information that makes up the cell’s genetic information is called its genome.

The genome is made up chromosomes. Every eukaryotic species has a characteristic number of chromosomes (packaged DNA) in each cell nucleus. Eukaryotic chromosomes are made of chromatin which is a complex of DNA and protein. When a cell is not dividing chromosomes are chromatin fiber in the nucleus. Each duplicated chromosome consists of two sister chromatids, which appear when cell divisions are about to take place. The chromatids are attached at what is called the centromere. After the chromatids divide mitosis is followed by division of the cytoplasm or cytokinesis.

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R and K selected species

R-selected and K-selected species are terms that biologists use to describe animals’ reproductive strategies. Either an animal produces a large number of offspring, and it is a numbers game for those offspring, or the animals have few offspring and invest a significant amount of time in those offspring. R-selected species are those that favor a large number of offspring. R-selected species include insects, amphibians, many fish, and reptiles. They tend to be smaller organisms, so the energy used to make each individual is low, and they live in unstable environments. They also have shorter lifespans and reach sexual maturity quickly. They have a type III survivorship pattern which means that earlier in life, more organisms will die than later on in their life. In these species, the number of offspring is crucial because it directly impacts the population size. Continue reading →