Tritogonia verrucosa

Geographic Range

Tritogonia verrucosa, the buckhorn (also called pistol grip), is found in the Neartic region. Buckhorns have been documented in the mid and eastern parts of the United States, including: 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)


Buckhorn are usually found in medium or large sized rivers where they burrow in sand and gravel substrates. Buckhorn have also been recorded in mud or silt substrates. (Minnesota Dept. of Natural Resources, 2012; Wisconsin Dept. of Nat. Resources, 2009)

  • 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

Physical Description

Buckhorn have a thick shell that is normally elongate. The entire shell (except the rounded posterior ridge of the female) has pustules of atypical sizes and shapes. Male shells are truncated, and females are compressed with a dorsal margin that is straight to slightly curved, with the ventral margin usually arched. The sculpture of the beak (umbo) is a series of pronounced ridges continuing down the sides. Shell color is green or light brown with green coloration, but with age, becoming dark brown to black in older shells. (Cummins and Mayer, 1997)

  • Sexual Dimorphism
  • sexes shaped differently
  • Range mass
    1120 (high) g
    39.47 (high) oz
  • Range length
    20 (high) cm
    7.87 (high) in


Buckhorn males release sperm into the water, which is then taken in by females through their incurrent siphon. The buckhorn is tachytictic, which means it spawns in spring and its bivalved larvae (glochidia) are released in summer. Females brood the fertilized eggs in their gills from May to August. After the eggs develop into glochidia they are released. Glochidia then attach to fish gills by clamping with their valves in a vise like fashion. While attached to the fish, they transform to juveniles. The juveniles then release from the gills of the fish and fall to the substrate of the aquatic system where they are found. (Minnesota Dept. of Natural Resources, 2012)


Male buckhorns release sperm into the water in spring, and the sperm are taken in indiscriminately by the females' incurrent siphons. The buckhorn is tachytictic, which means it spawns in spring and its bivalved larvae (glochidia) are released in summer. (Cummins and Mayer, 1997)

Once eggs are fertilized, the female broods them until they develop to glochidia. The glochidia released from the female are obligate parasites on fish. They will attach to the fish and metamorphose to juveniles, and then become independent living mussels. (Jirka and Neves, 1992; Minnesota Dept. of Natural Resources, 2012)

Glochidia length for buckhorns is on average 0.122 mm with a deviation of 0.003 mm. Width is a mean of 0.109 mm with a deviation of 0.003 mm. Hinge length is a mean of 0.049 mm with a deviation of 0.003 mm. The number of glochidia detected in one study was 1.8 offspring per 100 cubic meters. (Jirka and Neves, 1992; Minnesota Dept. of Natural Resources, 2012)

  • Breeding interval
    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 extent of parental investment for buckhorn females is to brood glochidia in their gills supplying nutrients until they are released into the environment. The only investment of buckhorn males is their gamete donation.

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


The most effective method to age bivalves is to use internal growth rings as indicators, however, no ageing studies have been done for Tritogonia verrucosa. Unionid mussels in general are long lived, sometimes living up to seven decades. (Comfort, 1957; Kidwell and Rothfus, 2010; Minnesota Dept. of Natural Resources, 2012)


Buckhorn mussels are sedentary for most of their lives, but can move within the sediments by use of its foot. (Amyot and Downing, 1997; Jirka and Neves, 1992; Kidwell and Rothfus, 2010; Minnesota Dept. of Natural Resources, 2012)

Home Range

The home range for buckhorns is unknown. Adult mussels move only a few meters from where they settle as juveniles.

Communication and Perception

Unionid mussels in general maintain equilibrium with statocysts and the statoliths, which help them sense gravity. Mussels respond to touch through tactile cells on the mantle. Mussels may detect vibrations, and some females will release glochidia when a shadow passing over them, as a potential host fish would. Spawning and potentially other behaviors such as glochidial release, may be a response to chemical cues. Buckhorn glochidia will snap shut in response to touch, which is a response needed to successfully attach to a fish host. (Winhold, 2004)

Food Habits

Buckhorn mussels extract nutrients and oxygen from detritus and water through their incurrent siphon, and discharge waste products through their excurrent siphon. (Missouri Dept. of Conservation, 2011)


Buckhorn mussels have a hard shell which protects them from predators. Their shell and mantle coloration helps them blend into thier habitat. In general, unionid mussels are preyed upon by ducks and geese (Anatidae), raccoons (Procyon lotor), river otters (Lontra canadensis), and muskrats (Ondotra zibethicus). No documentation of any specific species has been observed to actively prey on buckhorn mussels. (Cummins and Mayer, 1997)

  • Anti-predator Adaptations
  • cryptic

Ecosystem Roles

Within mussel beds there has been a positive correlation with species richness, and an increase in all macro-faunal groups. Host fish for buckhorn mussels include flathead catfish (Pylodictis olivaris), brown bullhead (Ameiurus nebulosus) and yellow bullhead (Ameiurus natalis). The introduction of zebra mussels (Dreissena polymorpha) has had a detrimental effect on native mussels as they attach themselves in large numbers to the shells, eventually causing death by starvation. Conchophthirus curtis, a kind of ciliate parasite, is found in buckhorn mussels. (Antipa and Small, 1971; Borthagaray and Carranza, 2006)

Species Used as Host
Commensal/Parasitic Species
  • ciliate protozoans (Conchophthirus curtis)

Economic Importance for Humans: Positive

At one time buckhorns were used in large numbers for making pearl buttons for clothing. The shells have also been used to seed marine oysters to stimulate growth of valuable pearls. Mussels in general are indicators of water quality since they are long-lived and sedentary. (Missouri Dept. of Conservation, 2011; Smith and Jepsen, 2008)

  • Positive Impacts
  • body parts are source of valuable material
  • research and education

Economic Importance for Humans: Negative

There are no known adverse effects of Tritogonia verrucosa on humans.

Conservation Status

Tritogonia verrucosa is listed as threatened by the states of Minnesota, Virginia, and Wisconsin through their individual state conservation programs. In North Carolina, this species is listed as extirpated. (Minnesota Dept. of Natural Resources, 2012; Wang and Bush, 2008)


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



living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map


Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.


helps break down and decompose dead plants and/or animals


uses smells or other chemicals to communicate


having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.


an animal that mainly eats decomposed plants and/or animals


particles of organic material from dead and decomposing organisms. Detritus is the result of the activity of decomposers (organisms that decompose organic material).


animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature


union of egg and spermatozoan


a method of feeding where small food particles are filtered from the surrounding water by various mechanisms. Used mainly by aquatic invertebrates, especially plankton, but also by baleen whales.


mainly lives in water that is not salty.


having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

internal fertilization

fertilization takes place within the female's body


A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.


having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.


reproduction in which eggs develop within the maternal body without additional nourishment from the parent and hatch within the parent or immediately after laying.


an organism that obtains nutrients from other organisms in a harmful way that doesn't cause immediate death


photosynthetic or plant constituent of plankton; mainly unicellular algae. (Compare to zooplankton.)


the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

seasonal breeding

breeding is confined to a particular season


remains in the same area


reproduction that includes combining the genetic contribution of two individuals, a male and a female


uses touch to communicate


that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).


movements of a hard surface that are produced by animals as signals to others


uses sight to communicate


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

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

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

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

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

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

Missouri Dept. of Conservation, 2011. "Pistolgrip (also Buckhorn)" (On-line). Field Guide, Aquatic Invertebrates. Accessed February 27, 2012 at

Smith, A., S. Jepsen. 2008. Overlooked gems: The benefits of freshwater mussels. WINGS, Fall: 14-19. Accessed February 27, 2012 at

Wang, S., A. Bush. 2008. Adjusting global extinction rates to account for taxonomic susceptibility. Paleobiology, 34(4): 435-455. Accessed February 27, 2012 at

Winhold, L. 2004. "Unionidae" (On-line). Animal Diversity Web. Accessed February 27, 2012 at

Wisconsin Dept. of Nat. Resources, 2009. "Endangered Resources Program Species Information" (On-line). Buckhorn (Tritogonia verrucosa). Accessed February 27, 2012 at