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Fusconaia ebena

By Ryan Bohn

Geographic Range

The range for Fusconaia ebena, or ebonyshells, is the Mississippi River basin, including the Mississippi River, the Minnesota River, the St. Croix River, the Illinois River, the Ohio River, and other bodies of water in the basin that offer suitable living conditions for this mussel. Originally, the native range extended from around the Twin Cities in Minnesota and went all the way to the Gulf of Mexico. In 1913, a dam was built on the Mississippi River in Keokuk, Iowa, effectively cutting off the northern home range for breeding ebonyshells. ("United States Fish and Wildlife Service: Endangered Species: America's Mussels: Silent Sentinels", 2011; Cummings and Mayer, 1992; Illinois State Museum, et al., 2011; Wisconsin Dept. of Natural Resources, 2009)

Habitat

The preferred habitat of ebonyshells is in deep (6 ft/1.8 m), strong currents of large rivers, but they can be found in deeper or shallower waters with weaker currents. Ebonyshells are found in areas with rocks, gravel, or sand on the bottoms of these rivers and will sometimes burrow down underneath the sand or mud. (Wisconsin Dept. of Natural Resources, 2009)

  • Aquatic Biomes
  • rivers and streams
  • Average depth
    1.8 m
    5.91 ft

Physical Description

Ebonyshells can be characterized primarily by their size, shape, and color patterns. Adults are typically 7.4 cm in length but can grow up to 10.2 cm or more. They are circular, round or sub-elliptical. Their shell is thick and inflated with a pronounced beak which fits in with overall roundness. Their shell is smooth and lacks pustules. The dark color of mature adults is what gives ebonyshells their name. As mature adults, the exterior of the shell displays concentric bands that eminate from their beak. The color of these bands ranges from light and dark brown to black. As juveniles, these bands are more yellow, with bright green and brown bands. A distinguishing feature of ebonyshells is that there are no rays (perpendicular to the bands) on the shell. On the inner surface of the shell, they are pearly white and glossy, and bands are absent. They have four well developed pseudocardinal teeth (two in both the right and left valve), and three lateral teeth that are serrated and curved (two in the left valve and one in the right valve). (Cummings and Mayer, 1992; Illinois State Museum, et al., 2011; Wisconsin Dept. of Natural Resources, 2009)

  • Sexual Dimorphism
  • sexes alike
  • Range length
    10.2 (high) cm
    4.02 (high) in
  • Average length
    7.4 cm
    2.91 in

Development

After the ebonyshell eggs are fertilized, they develop into larvae (called glochidia) in the female's gills. Then, the larvae are released into the water to attach to a host fish. After the glochidia develop to juveniles, they detach from the host fish and drift in the water until settling at the river bottom. Here they will stay and continue to grow and repeat the cycle. ("United States Fish and Wildlife Service: Endangered Species: America's Mussels: Silent Sentinels", 2011)

Their primary host fish are skipjack herring (Alosa chrysochloris) which are migratory fish. Other suspected host species include black crappie (Pomoxis nigromaculatus), white crappie (Pomoxis annularis), and largemouth bass (Micropterus salmoides). The glochidia are packaged together to be transferred from female ebonyshells to skipjack herring or other host fish, creating a lure which attracts the fish. This lure is released into the water when the fish come close to the female ebonyshells. When the glochidial packet is released, it drifts up in the current, looking like a worm. The fish tear open and eat the glochidial packet, freeing the bivalved larvae that are filtered out through the gills as the fish breathes. The larvae attach to the gills or to an exterior surface of the fish, and the fish tissue forms a protective cyst around glochidia. The cyst is very small, and does not effect the host fish. The glochidia develop into juveniles while attached to the fish. After metamorphosing to juveniles, each glochidium is sloughed off and the juvenile becomes a free living organism. ("United States Fish and Wildlife Service: Endangered Species: America's Mussels: Silent Sentinels", 2011; Minnesota Dept of Natural Resources, 2011; Wisconsin Dept. of Natural Resources, 2009)

Once juveniles are free of the host fish, they eventually rest on the river bottom. They continue to grow during the warmer months of the year, adding shell material until the following winter. ("United States Fish and Wildlife Service: Endangered Species: America's Mussels: Silent Sentinels", 2011; Wisconsin Dept. of Natural Resources, 2009)

Reproduction

Freshwater unionid mussels typically live within close proximity to one another on the river bottom, referred to as beds. While there is no known signal or other type of communication between males and females, males release sperm into the water and the eggs are fertilized as females siphons in sperm as it siphons for food. The eggs are brooded in the gills of the female ebonyshells. Sperm that do not fertilze eggs quickly die off and drift down river. Because of this random ejection of sperm, it is not possible to know how many males or females are mating with each other. ("United States Fish and Wildlife Service: Endangered Species: America's Mussels: Silent Sentinels", 2011; Minnesota Dept of Natural Resources, 2011)

The breeding season of ebonyshells is from June until September. A single female ebonyshell is capable of producing and fertilizing tens of thousands of eggs each year. Once the eggs have hatched into larvae, they are passed on to the host fish where they continue to grow into juveniles. The time of development from larva to juvenile on the host fish, can take anywhere from a few days to a couple months. (Minnesota Dept of Natural Resources, 2011; Wisconsin Dept. of Natural Resources, 2009)

  • Breeding interval
    Ebonyshells spawn once every year.
  • Breeding season
    Ebonyshells breed from June to September.
  • Range number of offspring
    1000 to 50,000
  • Average number of offspring
    10,000
  • Range time to independence
    1 to 8 weeks
  • Average time to independence
    4 weeks

Parental investment is very minimal in ebonyshells and in other mussels. The main investment of either parent is to produce gametes. The female broods the fertilized eggs until they hatch into larvae. After release of the eggs, all association to the parent is gone. ("United States Fish and Wildlife Service: Endangered Species: America's Mussels: Silent Sentinels", 2011; Minnesota Dept of Natural Resources, 2011)

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

Lifespan/Longevity

The age of ebonyshells can be determined by examining the number concentric rings on the exterior of the shell (similar to aging a tree). The oldest ebonyshells found have been well over 100 years old, but the typical lifespan for this species is 10 to 40 years. ("United States Fish and Wildlife Service: Endangered Species: America's Mussels: Silent Sentinels", 2011; Minnesota Dept of Natural Resources, 2011)

  • Range lifespan
    Status: wild
    100 (high) years
  • Average lifespan
    Status: wild
    75 years
  • Typical lifespan
    Status: wild
    10 to 40 years

Behavior

Ebonyshells are mostly solitary species. They may live in close proximity to other ebonyshells but do not interact with each other. One of the only observed social interactions is the luring of the host fish by the female. When encountered by a predator, ebonyshells seal themselves shut, try to burrow into the muck, or shoot jets of water either for propulsion or to distract the predator. Ebonyshells move around by drifting in the current, water jet propulsion, or by using their pseudopod (foot). ("United States Fish and Wildlife Service: Endangered Species: America's Mussels: Silent Sentinels", 2011; Minnesota Dept of Natural Resources, 2011)

Home Range

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

Communication and Perception

Communication in not observed in ebonyshells except in the luring of the host fish and reacting to touch (most likely from a predator). ("United States Fish and Wildlife Service: Endangered Species: America's Mussels: Silent Sentinels", 2011)

Food Habits

Ebonyshells are filter feeders. First, they siphon water and the gills filter for food. Then, cilia draws the food out of gills and into the digestive system. Foods known to be extracted from this filtering includes: algae, plankton, small aquatic invertebrates (insects and worms), detritus, and decaying organic material. (Cummings and Mayer, 1992; Minnesota Dept of Natural Resources, 2011)

Predation

Common predators of mussels include raccoons (Procyon lotor), river otters (Lontra canadensis), muskrats (Ondotra zibethicus), herons and egrets (Ardeinae), and some larger fish (Actinopterygii). To combat predation, ebonyshells darkly colored on the outside of their shells to provide camouflage. Also, they have a hard thick shell and strong adductors (which pull and keep the shell closed) that help in protecting themselves from predators. ("United States Fish and Wildlife Service: Endangered Species: America's Mussels: Silent Sentinels", 2011; Minnesota Dept of Natural Resources, 2011)

  • Anti-predator Adaptations
  • cryptic

Ecosystem Roles

Besides their role in the food web, ebonyshells play an important role in the ecosystem. One thing that ebonyshells do is improve the quality of the water by filtering it. Their presence in the water is also good sign of water quality for use by other animals, like fish and birds. Their larvae use Skipjack herrings (Alosa chrysochloris), black crappies (Poxomis nigromaculatus), white crappies (Pomoxis annularis), and largemouth bass (Micropteris salmoides) as hosts during their development. ("United States Fish and Wildlife Service: Endangered Species: America's Mussels: Silent Sentinels", 2011)

Species Used as Host

Economic Importance for Humans: Positive

When settlers were coming into the Mississippi River area, ebonyshells are harvested in great quantities for their shells (thick and high in quality) which were used in the pearl button industry. This mass harvest pushed the species close to extinction in some areas. They were also once harvested for food, but they taste rather bitter and now marine mussels, clams and oysters are available which taste better. Today, mussels are still harvested primarily to be exported and then used in cultured pearl farms (some of which are in Japan). Mussels are valuable in scientific and medical research because mussels cannot get cancer, and the reason for this is unknown. ("United States Fish and Wildlife Service: Endangered Species: America's Mussels: Silent Sentinels", 2011; Wisconsin Dept. of Natural Resources, 2009)

Mussels are good indicators of water quality, and they also help to improve the quality of water by filtration. Presence of ebonyshells is also a good indicator for the health of the different host species, because they cannot live in an area absent of their host fish. ("United States Fish and Wildlife Service: Endangered Species: America's Mussels: Silent Sentinels", 2011; Wisconsin Dept. of Natural Resources, 2009)

People are now taking more action to help ebonyshells by enforcing habitat protection, dredging restrictions, impoundment restrictions, sand and gravel mining restrictions, and by developing of pathways (fish runaways) for host species to migrate unimpeded up river (bypassing the locks and dams). ("United States Fish and Wildlife Service: Endangered Species: America's Mussels: Silent Sentinels", 2011; Wisconsin Dept. of Natural Resources, 2009)

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

Economic Importance for Humans: Negative

There are no known adverse effects of ebonyshells on humans.

Conservation Status

There is no federal or national status ranking provided for ebonyshells. However, they are on some conservation lists in some states. They are endangered in Wisconsin and Missouri, threatened in Illinois and Ohio, and considered special interest in Illinois and Minnesota. ("United States Fish and Wildlife Service: Endangered Species: America's Mussels: Silent Sentinels", 2011; Cummings and Mayer, 1992; Illinois State Museum, et al., 2011; Minnesota Dept of Natural Resources, 2011; Wisconsin Dept. of Natural Resources, 2009)

Contributors

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

Glossary

Nearctic

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

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.

chemical

uses smells or other chemicals to communicate

cryptic

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.

detritivore

an animal that mainly eats decomposed plants and/or animals

detritus

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

ectothermic

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

fertilization

union of egg and spermatozoan

filter-feeding

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.

freshwater

mainly lives in water that is not salty.

herbivore

An animal that eats mainly plants or parts of plants.

heterothermic

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

metamorphosis

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.

motile

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.

omnivore

an animal that mainly eats all kinds of things, including plants and animals

ovoviviparous

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

phytoplankton

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

planktivore

an animal that mainly eats plankton

polygynandrous

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

riparian

Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).

seasonal breeding

breeding is confined to a particular season

sedentary

remains in the same area

sexual

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

tactile

uses touch to communicate

temperate

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).

References

United States Fish and Wildlife Service. United States Fish and Wildlife Service: Endangered Species: America's Mussels: Silent Sentinels. Unknown. Online: United States Fish and Wildlife Service. 2011. Accessed April 30, 2011 at http://www.fws.gov/midwest/endangered/clams/mussels.html.

Cummings, K., C. Mayer. 1992. "Fusconaia ebena" (On-line). Field guide to freshwater mussels of the Midwest. Illinois Natural History Survey Manual 5. Accessed July 21, 2011 at http://www.inhs.illinois.edu/animals_plants/mollusk/musselmanual/page42_3.html.

Illinois State Museum, , Havana Public Library District, Meredosia River Museum. 2011. "Ebony Shell" (On-line). Harvesting the River. Accessed July 11, 2011 at http://www.museum.state.il.us/RiverWeb/harvesting/harvest/mussels/species/ebonyshell.html.

Minnesota Dept of Natural Resources, 2011. "Fusconaia ebena (I. Lea, 1831)" (On-line). Minnesota Dept. of Natural Resources Rare Species Guide. Accessed July 21, 2011 at http://www.dnr.state.mn.us/rsg/profile.html?action=elementDetail&selectedElement=IMBIV17060.

Wisconsin Dept. of Natural Resources, 2009. "Ebony Shell (Fusconaia ebena)" (On-line). Endangered Resources Program Species Information. Accessed July 07, 2011 at http://dnr.wi.gov/org/land/er/biodiversity/index.asp?mode=info&Grp=19&SpecCode=IMBIV17060.

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