Elliptio crassidens

Geographic Range

Elliptio crassidens has a wide range in the U.S. in midwestern, eastern and some southern states and a portion of Canada. In the U.S., it inhabits the Escambia and Apalachicola River drainages in the Florida panhandle, Alabama, Florida, Georgia, Illinois, Indiana, Kentucky, Louisiana, Minnesota, Mississippi, Missouri, Ohio, Tennessee, Virginia, West Virginia and Wisconsin. In Canada, it is found in Quebec and Ontario. This species is thought to be completely lost from Iowa, Oklahoma and Pennsylvania.

• Biogeographic Regions
• nearctic nearctic
  • native native

Habitat

Elephant-ear mussels live in mud, sand, gravel and rocky substrates (commonly limestone). They inhabit waters with moderate to swift currents in large creeks, rivers, or sometimes channels.

• Habitat Regions
• temperate temperate
• freshwater freshwater

• Aquatic Biomes
• benthic benthic
• rivers and streams

• Other Habitat Features
• riparian riparian

Physical Description

Elephant-ear mussels have heavy, thick, triangular shells. The shell is smooth. When they are younger, their outer shells are a reddish-brown with light green rays and vary from brown to black as adults with no rays. The inner shell is purple, although a few have been seen with pink or white. The shape of the shell can be compressed to somewhat inflated with a rounded anterior and more pointed posterior. The dorsal margin has a gentle curve. The ventral margin, although curved in younger mussels, becomes straight as the mussels age.

Other physical features include a low umbo (the oldest part of the shell), beak sculpture with two or three loops parallel to the growth lines (which may or may not be visible as the mussels age), a shallow beak cavity, pseudocardinal teeth, lateral teeth and sharp, prominent posterior ridge. Three well developed pseudocardinal teeth in total are present, two in the left valve and one in the right valve. The lateral teeth are also well developed, but are short, rough and straight.

It is difficult to distinguish between males and females of the species. However, at times a female will have an outer gill that is much thicker than the inner gill, and the outer gill is her brood chamber for developing embryos.

• Other Physical Features
• ectothermic ectothermic
• heterothermic heterothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• sexes alike

Development

The development of mussels like elephant-ears is fairly complex. Once an egg has been fertilized inside a female, it transforms into a larval stage, called a glochidium. The glochidia develop into juveniles while attached to host fish. Juveniles are a small form of adults and begin to grow when they break free from the host and fall to the substrate. The mantle of their shells works to produce new shell material that is continually added to the last. The umbo is the oldest part of the shell, and layers of shell are added to it.

• Development - Life Cycle
• metamorphosis metamorphosis

Reproduction

Elephant ear mussels are "tachytictic", so their breeding season is short. Males fertilize the female eggs by releasing their sperm into the water from a structure called the suprabranchial chamber. If the sperm reaches a nearby female, it is pulled through her incurrent siphon to her outer gills, or brood pouch.

• Mating System
• polygynandrous (promiscuous) polygynandrous (promiscuous)

Once the eggs are fertilized, they develop into the larval, or glochidial stage in the female's outer gills. In this stage, the larvae have only grown to about the size of dust particles. The female may react to the presence of a fish or other external cue and then release the glochidia through an excurrent siphon. The offspring attach to a host's gills or fins with their valves, which develops a protective cyst. After living on fish for several weeks as parasites, the juvenile clams break free and fall to bottom of the river. The juveniles then burrow into the substrate and develop into adult mussels.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• fertilization fertilization
  • internal internal

Male elephant-ear mussels only release sperm for fertilization and do not have any involvement in their growth or development. After eggs have been fertilized in the female and have grown into the larval glochidia form, the larvae receive nutrients from the mother and may stay in her gills that way for a few weeks to a few months. However, the mother provides no further parental care once they are released to host fish.

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

Lifespan/Longevity

Mussels in general can be very long-lived, from 20 to even 100 years. There are no specific longevity data for elephant-ear mussels.

Behavior

Like most native, freshwater mussels, this species usually disperses and is fairly sessile. Mussels may aggregate in "beds" but it isn't known how or why. The mussels also have a muscular foot which they fill with blood to extend and anchor themselves or inch their bodies forward.

• Key Behaviors
• sedentary sedentary

Home Range

Being mostly sessile and remaining very close to the place they first settled as a juvenile, elephant ear mussels are unlikely to move more than a few meters from this site during their adult life.

Communication and Perception

Organs of elephant-ear mussels and other mussels are usually tentacle mechanoreceptors or chemoreceptors. The osphradium, a clump of cells below the posterior adductor muscle, may also help with chemical sensitivity or in measuring the water's turbidity. Other sense organs, located mainly on the edge of their mantle, are involved with touch, chemical sense, and balance (statocysts). However, they are poorly developed.

In a way, females communicate with host fish during the development of their offspring. Some may cluster their larvae together in a way that attracts fish searching for food in order to give the offspring a better chance to attach to the fins or gills.

• Communication Channels
• visual visual
• tactile tactile
• chemical chemical

• Perception Channels
• polarized light polarized light
• tactile tactile
• chemical chemical

Food Habits

Like other mussels, elephant-ear mussels are filter feeders. Water and food particles are collected by hair-like cilia pulling water through ostia (pores) and into their gills. Food is trapped in a structure called the gill filament and motion from cilia move the food in a string of mucous toward the digestive system. They may feed on bacteria, protozoans, algae, plankton.

• Primary Diet
• planktivore planktivore

• Animal Foods
• zooplankton zooplankton

• Plant Foods
• algae
• phytoplankton phytoplankton

• Other Foods
• detritus detritus
• microbes

• Foraging Behavior
• filter-feeding filter-feeding

Predation

Like other freshwater mussels, elephant-ears have muscular foot and adductor muscles as anti-predator adaptations. Their foot can help anchor them into the substrate to prevent them from being taken away by the current or tugged free by an animal attempting to eat it. The adductor muscles help keep their shell closed tightly. Their hard shell also helps them avoid predation by providing a sort of camouflage with the rocks and ground around them and a hard-to-crack outer covering to protect their delicate inner tissues. Common predators include otters ( Lontra canadensis ), raccoons ( Procyon lotor ), muskrats ( Ondatra zibethicus ), herons and egrets ( Ardeidae ), and various fish ( Actinoptergygii ).

• Anti-predator Adaptations
• cryptic cryptic

Ecosystem Roles

When a number of elephant-ear mussels live close together, they can form mussel beds which provide important habitat for some fish and aquatic worms or insects. They also clean the water around them while filter feeding, giving other organisms more a more habitable environment. During reproduction, elephant-ear mussels use skipjack herring ( Alosa chrysochloris ) as a host for their larvae. Although the mussel larvae live on herring gills for nutrients until they mature to the next stage of development, they do not harm the fish in the process.

The introduction of zebra mussels ( Dreissena polymorpha ) brought an exotic parasite to elephant-ear mussels Elliptio crassidens and their habitat. Zebra mussels reproduce very quickly, much faster than native species, and in greater numbers. Along with taking up native mussel habitat, zebra mussels also attach hard surfaces, such as the shells of elephant-ear or other mussel species, hindering their natural behaviors and physiology.

• Ecosystem Impact
• creates habitat
• parasite parasite

Economic Importance for Humans: Positive

Mussels in general may remove unwanted toxins from the water as they filter feed. Their presence or absence can provide an indication of the health of the water system for their own species and others. Studying mussels like elephant-ears provides a better understanding of aquatic life systems and how they adapt. Mussels do not get cancer, and studies may be beneficial to combat this disease. Mussels were used to make tools by native Americans and were likely a food source as well. Thicker-shelled unionids were harvested for the button industry in the 20th century, and more recently are harvested for the pearl industry.

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

Economic Importance for Humans: Negative

There are no known negative economic effects of elephant-ear mussels.

Conservation Status

Elephant-ear mussels have endangered status in Minnesota, Missouri and Wisconsin, and considered threatened in Illinois.

Additional Links

Encyclopedia of Life

BioKIDS Critter Catalog

Contributors

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

References

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Brim Box, J., R. Dorazio, W. Liddell. 2002. Relationships between streambed substrate characteristics and freshwater mussels (Bivalvia:Unionidae) in coastal plain streams. Journal of the North American Benthological Society , 21/2: 253-260.

Cummings, K. 2012. " Elliptio crassidens " (On-line). IUCN Red List of Threatened Species. Accessed July 06, 2012 at http://www.iucnredlist.org/apps/redlist/details/188905/0 .

Dawley, C. 1947. Distribution of aquatic mollusks in Minnesota. American Midland Naturalist , 38/3: 671-697.

Gagnon, P., W. Michener, M. Freeman, J. Brim Box. 2006. Unionid habitat and assemblage composition in coastal plain tributaries of Flint River (Georgia). Southeastern Naturalist , 5/1: 31-52.

Grier, N. 1922. Final report on the study and appraisal of mussel resources in selected areas of the upper Mississippi River. American Midland Naturalist , 8/1: 1-33.

Hickman, Jr., C., L. Kats, S. Keen, B. Ober. 2011. Integrated Principles of Zoology: Fifteenth Edition . New York, NY: McGraw-Hill.

Illinois Natural History Survey, 2011. " Elliptio crassidens (Lamarck, 1819) Elephant-ear" (On-line). Illinois Natural History Survey Prairie Research Institute. Accessed July 06, 2012 at http://www.inhs.illinois.edu/animals_plants/mollusk/musselmanual/page66_7.html .

Jirka, K., R. Neves. 1992. Reproductive biology of four freshwater mussels (Mollusca: Unionidae) in the New River, Virginia and West Virginia. Journal of Freshwater Ecology , 7/1: 35-44.

Minnesota Dept. of Nat. Resources, 2012. " Elliptio crassidens (Lamarck, 1819) Elephant-ear" (On-line). Minnesota Department of Natural Resources. Accessed July 06, 2012 at http://www.dnr.state.mn.us/rsg/profile.html?action=elementDetail&selectedElement=IMBIV14080 .

NatureServe, 2012. " Elliptio crassidens - (Lamarck, 1819) Elephantear" (On-line). NatureServe Explorer. Accessed July 06, 2012 at http://www.natureserve.org/explorer/servlet/NatureServe?searchName=Elliptio+crassidens+ .

Van Der Schalie, H., A. Van Der Schalie. 1950. The Mussels of the Mississippi River. American Midland Naturalist , 44/2: 448-466.

U.S. Fish & Wildlife Service. 2012. "America's Mussels: Silent Sentinels (Midwest Region)" (On-line). U.S. Fish & Wildlife Service: Endangered Species. Accessed April 01, 2012 at http://www.fws.gov/midwest/endangered/clams/mussels.html .

Wisconsin Department of Natural Resources. Elephant Ear (Elliptio crassidens) . PUB-ER-085-99. Madison, Wisconsin: Wisconsin Department of Natural Resources. 1999. Accessed July 06, 2012 at http://dnr.wi.gov/topic/EndangeredResources/Animals.asp?mode=detail&SpecCode=IMBIV14080 .

Missouri Department of Conservation. 2011. "Elephant-ear Mussel: Elliptio crassidens " (On-line). Missouri Department of Conservation: Best Management Practices. Accessed July 06, 2012 at http://mdc.mo.gov/sites/default/files/resources/2010/08/9467_6413.pdf .

Virginia Department of Game and Inland Fisheries. 2012. "Freshwater Mussels" (On-line). Virginia.gov. Accessed April 02, 2012 at http://www.dgif.virginia.gov/wildlife/freshwater-mussels.asp .

U.S. Fish & Wildlife Service. 2006. "Threatened and Endangered Mussels" (On-line). U.S. Fish & Wildlife Service: Freshwater Mussels of the Upper Mississippi River System. Accessed July 06, 2012 at http://www.fws.gov/midwest/mussel/threatened.html .

U.S. Army Corps of Engineers. 2005. "What is a Freshwater Mussel?" (On-line). Engineer Research and Development Center. Accessed April 02, 2012 at http://el.erdc.usace.army.mil/mussels/freshwater.html .