Critter Catalog

What do they look like?

Buckhorns have thick shells that are thin and long. They are green or light brown when buckhorns are younger, and turn darker brown or black over time. The shells are covered in bumps that are different sizes and shapes. The shells of males are short. The shells of females are shrunken and both the top and bottom shells are curved. The most pointy part of the shell, which is called a beak, has ridges going down the sides. (Cummins and Mayer, 1997)

• Other Physical Features
• ectothermic
• heterothermic
• bilateral symmetry

• Sexual Dimorphism
• sexes shaped differently

• Range mass

  1120 (high) g

  39.47 (high) oz

• Range length

  20 (high) cm

  7.87 (high) in

Where do they live?

Buckhorn mussles, which are also called pistol grips, live in the central and eastern United States. They are found in Texas, Pennsylvania, Tennessee, Mississippi, Kentucky, Georgia, Alabama, Minnesota, Missouri, Iowa, Arkansas, Louisiana, Maryland, Oklahoma, Delaware, North Carolina, Ohio, Indiana, Wisconsin, Illinois, West Virginia, and Virginia. (Minnesota Dept. of Natural Resources, 2012)

• Biogeographic Regions
• nearctic
  • native

What kind of habitat do they need?

Buckhorn live in medium-sized or large rivers where they burrow in sand and gravel on the bottom. They also live in rivers with mud or silt bottoms. (Minnesota Dept. of Natural Resources, 2012)

• These animals are found in the following types of habitat
• temperate
• freshwater

• Aquatic Biomes
• benthic
• rivers and streams

• Range depth

  1 to 20 m

  3.28 to 65.62 ft

• Average depth

  8 m

  26.25 ft

How do they grow?

Buckhorns reproduce in the spring. Male buckhorns release reproductive cells into the water and females take them in to fertilize their eggs. The eggs grow inside the gills of the mothers from May to August. After the eggs develop into larvae, they are released into the water. They attach to the gills of fish by holding on with their valves. While attached to the fish, they grow into to juvenile buckhorns. Later, the juveniles let go of the fish and settle on the bottom of the river or stream. (Minnesota Dept. of Natural Resources, 2012)

• Development - Life Cycle
• metamorphosis

How do they reproduce?

Male buckhorns release reproductive cells into the water in spring. Females take in the water with reproductive cells and fertilize their eggs. Buckhorn larvae are released in summer. (Cummins and Mayer, 1997)

• Mating System
• polygynandrous (promiscuous)

Females keep the eggs in their gills until they develop into larvae. Females release the larvae, and they attach onto fish. The larvae are around 0.122 mm long and 0.109 mm wide. While the larvae are attached to fish, they grow into juveniles. Then, they drop off and settle to the bottom of the stream or river. (Jirka and Neves, 1992; Minnesota Dept. of Natural Resources, 2012)

• Key Reproductive Features
• seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• simultaneous hermaphrodite
• sexual
• fertilization
  • internal
• broadcast (group) spawning
• ovoviviparous

• How often does reproduction occur?

  Buckhorn mussels breed once a year.

• Breeding season

  Buckhorn mussels breed in the spring.

• Range gestation period

  115 to 136 days

• Average gestation period

  122 days

• Average age at sexual or reproductive maturity (female)

  6 years

• Average age at sexual or reproductive maturity (male)

  6 years

The amount of parental investment in buckhorns is low. Male buckhorns produce reproductive cells. Females shelter and provide food to the eggs and larvae in their gills until they are released to attach to fish.

• Parental Investment
• pre-fertilization
  • provisioning
• pre-hatching/birth
  • provisioning
    • female
  • protecting
    • female

How long do they live?

The best way to tell the age of a mussel is by counting its growth rings, like a tree. Scientists haven't studied buckhorns in particular, but mussels can live as long as 70 years or more. (Comfort, 1957; Kidwell and Rothfus, 2010; Minnesota Dept. of Natural Resources, 2012)

How do they behave?

Adult buckhorn mussels don't move very far. They are able to move around the bottom using their foot. (Amyot and Downing, 1997; Jirka and Neves, 1992; Kidwell and Rothfus, 2010; Minnesota Dept. of Natural Resources, 2012)

• Key Behaviors
• parasite
• motile
• sedentary

Home Range

Scientists don't know the exact size of the area where buckhorns live. Most mussels don't move very much after they grow into adults.

How do they communicate with each other?

River mussels keep their balance using organs called statocysts and statoliths, which help them feel gravity. They have touch cells on the fleshy part of their body. They might be able to feel vibrations as well. Their larvae snap shut if they are touched, and this snapping motion is also used to attach to a fish host. Some female mussels release their larvae when a shadow crosses them that looks like a host fish. They might also understand chemical signals to reproduce. (Winhold, 2004)

• Communication Channels
• chemical

• Perception Channels
• visual
• tactile
• vibrations
• chemical

What do they eat?

Mussels are filter feeders, meaning they strain bits of food out of the water. They pull water through pores and into their gills using cilia, which look like tiny hairs. The bits of food get trapped in their gills and then mucous moves the food to their digestive system. They get nutrients and oxygen this way from rotting bits of dead plants and animals. (Missouri Dept. of Conservation, 2011)

• Primary Diet
• detritivore

• Plant Foods
• algae
• phytoplankton

• Other Foods
• detritus
• microbes

• Foraging Behavior
• filter-feeding

What eats them and how do they avoid being eaten?

Buckhorn mussels have a hard shell which protects them from predators. Their dark colors help camouflage them in their environment. They probably have similar predators to other related mussels. River mussels are eaten by ducks and geese, raccoons, river otters, and muskrats. (Cummins and Mayer, 1997)

• These animal colors help protect them
• cryptic

• Known Predators

  • muskrats (Ondotra zibethicus)
  • raccoons (Procyon lotor)
  • river otters (Lontra canadensis)
  • herons and egrets (Ardeinae)
  • ducks and geese (Anatidae)

What roles do they have in the ecosystem?

Where there are mussel beds, there are more other species and also more large animals. Buckhorn mussel larvae live attached to flathead catfish, brown bullheads and yellow bullheads. They are damaged by zebra mussels, which attach to their shells and cause them to starve to death. Buckhorn mussels can also become infected with a kind of ciliate parasite. (Antipa and Small, 1971; Borthagaray and Carranza, 2006)

• Ecosystem Impact
• biodegradation
• parasite

Do they cause problems?

There are no known negative impacts of buckhorn mussels on humans.

How do they interact with us?

Buckhorn mussels were once used to make pearl buttons, and now their shells are used to grow oysters for pearls. Mussels are also useful for for testing the quality of the water where they live because they live long lives and don't move around very much. (Missouri Dept. of Conservation, 2011; Smith and Jepsen, 2008)

• Ways that people benefit from these animals:
• body parts are source of valuable material
• research and education

Are they endangered?

Buckhorn mussels are threatened in Minnesota, Virginia, and Wisconsin. They have gone extinct in North Carolina. (Minnesota Dept. of Natural Resources, 2012; Wang and Bush, 2008)

• IUCN Red List

  Not Evaluated

• US Federal List

  No special status

• CITES

  No special status

• State of Michigan List

  No special status

Contributors

Jacob Pederson (author), Minnesota State University, Mankato, Robert Sorensen (editor), Minnesota State University, Mankato, Renee Mulcrone (editor), Special Projects, Catherine Kent (editor), Special Projects.

References

Amyot, J., J. Downing. 1997. Seasonal variation in vertical and horizontal movement of the freshwater bivalve Elliptio complanata. Freshwater Biology, 37: 345-354. Accessed February 27, 2012 at http://www.public.iastate.edu/~downing/tier%202/jadpdfs/1997%20FB%20Seasonal%20Variation%2037.%20345-354.pdf.

Antipa, G., E. Small. 1971. The occurrence of Thigmotrichous ciliated protozoa inhabiting the mantle cavity of unionid molluscs of Illinois. American Microscopical Society, 90 (4): 463-472. Accessed February 27, 2012 at http://www.jstor.org/stable/3225461?seq=3.

Borthagaray, A., A. Carranza. 2006. Mussels as ecosystem engineers: Their contribution to species richness in a rocky littoral community. Acta Oecologica, 31 (3): 243-250.

Comfort, A. 1957. The duration of life in molluscs. Journal of Molluscan Studies, 32 (6): 219-241.

Cummins, K., C. Mayer. 1997. "Tritogonia verrucosa (Rafinesque, 1820)" (On-line). Field guide to freshwater mussels of the midwest, Illinois Natural History Survey Manual 5. Accessed February 27, 2012 at http://www.inhs.illinois.edu/animals_plants/mollusk/musselmanual/page26_7.html.

Jirka, K., R. Neves. 1992. Reproductive biology of four species of freshwater mussels (Mollusca:Unionidae) in the New River, Virginia and West Virginia. Journal of Freshwater Ecology, 7 (1): 35-45. Accessed February 27, 2012 at http://www.fishwild.vt.edu/mussel/PDFfiles/reproduction_biology.pdf.

Kidwell, S., T. Rothfus. 2010. The living, the dead, and the expected dead: variation in life span. Paleobiology, 36(4): 615–640. Accessed February 27, 2012 at http://geosci.uchicago.edu/pdfs/kidwell/KidwellRothfus2010Paleobio.pdf.

Minnesota Dept. of Natural Resources, 2012. "Tritogonia verrucosa (Rafinesque, 1820)" (On-line). Minnesota Department of Natural Resources Rare Species Guide. Accessed February 27, 2012 at http://www.dnr.state.mn.us/rsg/profile.html?action=elementDetail&selectedElement=IMBIV44010.

Missouri Dept. of Conservation, 2011. "Pistolgrip (also Buckhorn)" (On-line). Field Guide, Aquatic Invertebrates. Accessed February 27, 2012 at http://mdc.mo.gov/discover-nature/field-guide/pistolgrip-also-buckhorn.

Smith, A., S. Jepsen. 2008. Overlooked gems: The benefits of freshwater mussels. WINGS, Fall: 14-19. Accessed February 27, 2012 at http://www.xerces.org/wp-content/uploads/2009/01/mussel_article.pdf.

Wang, S., A. Bush. 2008. Adjusting global extinction rates to account for taxonomic susceptibility. Paleobiology, 34(4): 435-455. Accessed February 27, 2012 at http://www.swarthmore.edu/NatSci/swang1/Publications/pbio2008b.pdf.

Winhold, L. 2004. "Unionidae" (On-line). Animal Diversity Web. Accessed February 27, 2012 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Unionidae.html.

Wisconsin Dept. of Nat. Resources, 2009. "Endangered Resources Program Species Information" (On-line). Buckhorn (Tritogonia verrucosa). Accessed February 27, 2012 at http://dnr.wi.gov/org/land/er/biodiversity/index.asp?mode=info&Grp=19&SpecCode=IMBIV44010.