Villosa fabalis

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Geographic Range

The rayed-bean is found in sporadic in the Ohio River drainage north to Lake Erie drainages in Michigan and Ontario, and in the Duck and the upper Tennessee Rivers.

In Michigan V. fabalis is found in the Detroit River, Lake Erie and its tributaries. (Burch, 1975; Carman, 2001)

Habitat

The rayed bean is found in lakes and small to large streams. Substrates in inhabits include mud, sand and gravel. This species may also be associated with water willow stands.

In the Clinton River headwaters the rayed bean was found buried in sand among aquatic vegetation roots in about four inches of flowing water. It seems more common in lake plain areas of the Lake Erie basin where substrates are finer. (Cummings and Mayer, 1992; van der Schalie, 1938; Watters, 1995)

  • Aquatic Biomes
  • rivers and streams

Physical Description

The rayed bean is up to 3.8 cm (1.5 inches) long , and is elongate in shape. The shell is usually fairly thick, solid and moderately inflated. The anterior end is rounded, the posterior end bluntly pointed. The dorsal margin is straight and the ventral margin is straight to slightly curved.

Umbos are low, being raised only slightly above the hinge line. The beak sculpture has two or three heavy ridges, knobbed posteriorly.

The periostracum (outer shell layer) is smooth except for growth lines. The shell is green, yellowish-green or brown with numerous dark green wavy rays. Older specimens tend to be more brown.

On the inner shell, the left valve has two pseudocardinal teeth, which are triangular, heavy, large and rough. The two lateral teeth are short, low and heavy. The right valve has one triangular, heavy, large pseudocardinal tooth. Sometimes small tubercular teeth are on either side of this psuedocardinal tooth. The one lateral tooth is also short, low, and heavy.

The beak cavity is shallow. The nacre is white and is iridescent at the posterior end.

In Michigan, this species can be confused with the lilliput and the rainbow. Both the lilliput and rainbow are thinner and have more delicate hinge and teeth. The lilliput is slightly more cylindrical. The rainbow is larger, has more yellowish coloring and more prominent green rays. (Cummings and Mayer, 1992; Parmalee, 1967; Watters, 1995)

  • Sexual Dimorphism
  • sexes shaped differently
  • Range length
    3.8 (high) cm
    1.50 (high) in

Development

Fertilized eggs are brooded in the marsupia (water tubes) up to 11 months, where they develop into larvae, called glochidia. The glochidia are then released into the water where they must attach to the gill filaments and/or general body surface of the host fish. After attachment, epithelial tissue from the host fish grows over and encapsulates a glochidium, usually within a few hours. The glochidia then metamorphoses into a juvenile mussel within a few days or weeks. After metamorphosis, the juvenile is sloughed off as a free-living organism. Juveniles are found in the substrate where they develop into adults. (Arey, 1921; Lefevre and Curtis, 1910)

Reproduction

Age to sexual maturity for this species is unknown. Unionids are gonochoristic (sexes are separate) and viviparous. The glochidia, which are the larval stage of the mussels, are released live from the female after they are fully developed.

In general, gametogenesis in unionids is initiated by increasing water temperatures. The general life cycle of a unionid, includes open fertilization. Males release sperm into the water, which is taken in by the females through their respiratory current. The eggs are internally fertilized in the suprabranchial chambers, then pass into water tubes of the gills, where they develop into glochidia.

Villosa fabalis is a long-term brooder, and was gravid in the Huron River from mid-August to the following spring in late May. This species probably spawns from June to July in Michigan. (Lefevre and Curtis, 1912; van der Schalie, 1938; Watters, 1995)

  • Breeding interval
    The snuffbox mussel breeds once in the warmer months of the year.
  • Breeding season
    In Michigan, the breeding season is mid-July to August.
  • Range gestation period
    10 (high) months

Females brood fertilized eggs in their marsupial pouch. The fertilized eggs develop into glochidia. There is no parental investment after the female releases the glochidia.

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

Lifespan/Longevity

The age of mussels can be determined by looking at annual rings on the shell. However, no demographic data on this species has been recorded.

Behavior

Mussels in general are rather sedentary, although they may move in response to changing water levels and conditions. Although not thoroughly documented, the mussels may vertically migrate to release glochidia and spawn. Often they are found buried under the substrate. (Oesch, 1984; )

Communication and Perception

The middle lobe of the mantle edge has most of a bivalve's sensory organs. Paired statocysts, which are fluid filled chambers with a solid granule or pellet (a statolity) are in the mussel's foot. The statocysts help the mussel with georeception, or orientation.

Mussels are heterothermic, and therefore are sensitive and responsive to temperature.

Unionids in general may have some form of chemical reception to recognize fish hosts. Mantle flaps in the lampsilines are modified to attract potential fish hosts. How the rayed bean attracts or if it recognizes its fish host is unknown.

Glochidia respond to both touch, light and some chemical cues. In general, when touched or a fluid is introduced, they will respond by clamping shut. (Arey, 1921; Brusca and Brusca, 2003; Watters, 1995)

Food Habits

In general, unionids are filter feeders. The mussels use cilia to pump water into the incurrent siphon where food is caught in a mucus lining in the demibranchs. Particles are sorted by the labial palps and then directed to the mouth. Mussels have been cultured on algae, but they may also ingest bacteria, protozoans and other organic particles.

The parasitic glochidial stage absorbs blood and nutrients from hosts after attachment. Mantle cells within the glochidia feed off of the host’s tissue through phagocytocis. (Arey, 1921; Meglitsch and Schram, 1991; Watters, 1995)

Predation

Unionids in general are preyed upon by muskrats, raccoons, minks, otters, and some birds. Juveniles are probably also fed upon by freshwater drum, sheepshead, lake sturgeon, spotted suckers, redhorses, and pumpkinseeds.

Unionid mortality and reproduction is affected by unionicolid mites and monogenic trematodes feeding on gill and mantle tissue. Parasitic chironomid larvae may destroy up to half the mussel gill. (Cummings and Mayer, 1992; Watters, 1995)

Ecosystem Roles

While freshwater mussels require a host fish for metamorphosis, the host fish for V. fabalis has not been determined.

Economic Importance for Humans: Positive

Mussels are ecological indicators. Their presence in a water body usually indicates good water quality.

Economic Importance for Humans: Negative

There are no significant negative impacts of mussels on humans.

Conservation Status

Villosa fabalis is a federal candidate species being considered for listing in the United States. Currently it is listed as endangered in Michigan, Ohio, and New York. Indiana lists the rayed bean as Special Concern.

In Canada, V. fabalis is endangered under the Species At Risk Act. (Environment Canada, 2003; Hove, 2004)

Contributors

Renee Sherman Mulcrone (author).

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

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.

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.

parasite

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

phytoplankton

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

planktivore

an animal that mainly eats plankton

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

vibrations

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

visual

uses sight to communicate

viviparous

reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

References

Arey, L. 1921. An experimental study on glochidia and the factors underlying encystment. J. Exp. Zool., 33: 463-499.

Brusca, R., G. Brusca. 2003. Invertebrates. Sunderland, Massachusetts: Sinauer Associates, Inc..

Burch, J. 1975. Freshwater unionacean clams (Mollusca: Pelecypoda) of North America. Hamburg, Michigan: Malacological Publications.

Carman, S. 2001. Special animal abstract for Villosa fabalis (rayed bean mussel). Lansing, Michigan: Michigan Natural Features Inventory. Accessed October 10, 2005 at http://web4.msue.msu.edu/mnfi/abstracts/aquatics/Villosa_fabalis.pdf.

Cummings, K., C. Mayer. 1992. Field guide to freshwater mussels of the Midwest. Champaign, Illinois: Illinois Natural History Survey Manual 5. Accessed August 25, 2005 at http://www.inhs.uiuc.edu/cbd/collections/mollusk/fieldguide.html.

Environment Canada, 2003. "Species At Risk" (On-line). Accessed September 21, 2005 at http://www.speciesatrisk.gc.ca/default_e.cfm.

Hoeh, W., R. Trdan. 1985. Freshwater mussels (Pelecypoda: Unionidae) of the major tributaries of the St. Clair River, Michigan. Malacological Review, 18: 115-116.

Hove, M. 2004. "Links to each state's listed freshwater mussels, invertebrates, or fauna" (On-line). Accessed September 21, 2005 at http://www.fw.umn.edu/Personnel/staff/Hove/State.TE.mussels.

Lefevre, G., W. Curtis. 1912. Experiments in the artificial propagation of fresh-water mussels. Proc. Internat. Fishery Congress, Washington. Bull. Bur. Fisheries, 28: 617-626.

Lefevre, G., W. Curtis. 1910. Reproduction and parasitism in the Unionidae. J. Expt. Biol., 9: 79-115.

Meglitsch, P., F. Schram. 1991. Invertebrate Zoology, Third Edition. New York, NY: Oxford University Press, Inc.

Oesch, R. 1984. Missouri naiades, a guide to the mussels of Missouri. Jefferson City, Missouri: Missouri Department of Conservation.

Parmalee, P. 1967. The fresh-water mussels of Illinois. Springfield, Illinois: Illinois State Museum popular science series 8. 108 pp.

Watters, G. 1995. A guide to the freshwater mussels of Ohio. Columbus, Ohio: Ohio Department of Natural Resources.

van der Schalie, H. 1938. The naiad fauna of the Huron River, in southeastern Michigan. Miscellaneous Publications of the Museum of Zoology, University of Michigan, 40: 1-83.