Geukensia demissa

Geographic Range

The ribbed mussel is native to the Atlantic coast of North America, from the southern Gulf of St. Lawrence, Canada to northeastern Florida and along the Gulf of Mexico from Florida to Yucatan.

In the mid 1800s the ribbed mussel was introduced to San Francisco Bay, California, apparently by accident, included with live oysters shipped by trans-continental rail for cultivation in the Bay. Since then it has been found in other locations on the Pacific coast, from Alamitos Bay south to Anaheim Bay, Newport Bay, Bolsa Chica Lagoon and Estero de Punta Banda, Baja California Norte, Mexico. The locations may have been sites of unrecorded oyster transplants, or the mussels may have arrived after attaching to hulls or other mobile objects. (Chesapeake Bay Program, 2004; Abbott, 1974; Blackwell, et al., 1977; Brousseau, 1984; Carlton, 1979; Carlton, 1992; Cohen and Carlton, 1995; Cohen, 2005; Torchin, et al., 2005)


The larvae of ribbed mussels settle on subtidal oyster reefs, in intertidal salt marshes and on man-made structures in these habitats. Sometimes they attach to one another in aggregations or to clumps of hollow grass stems (Spartina alterniflora) in low marshes. They are most abundant at the lowest shore levels within salt marshes and occur in small numbers in the high marsh zone above the average high water mark.

These mussels can tolerate water temperatures up to 133 degrees F (56°C) and and wide range of salinities, from near fresh water up to 70 ppt (twice the concentration of seawater). (Chesapeake Bay Program, 2004; Bertness and Grosholz, 1985; Coen and Luckenbach, 2000; Coen and Walters, 2005; Cohen, 2005; Franz and Tanacredi, 1993; Franz, 1993; Franz, 1997; Kuenzler, 1961a; Lin, 1989; Luckenbach, et al., 2005; Stiven and Kuenzler, 1979; Waite, et al., 1989)

Physical Description

Ribbed mussel are relatively large mussels. They range from 5 - 10 cm (4 inches) in length. The largest specimen recorded reached 13 cm. The shell is moderately thin and oblong or fan shaped. The upper margin is straight or slightly convex. The dorsal and ventral margins are parallel.

The periostracum (thin, glossy outer shell layer) is glossy, brownish black with some yellow to a bleached white color. It is grooved with pronounced, unbranched, radiating ribs, largest on the upper part of the hind end above a broad umbonal ridge, fine along lower margin. These give the species its common name. The inside of the shell is pearlescent, sometimes white or bluish-gray, tinged with purple/blue or purple/red at hind margin. There are no teeth at the hinge. At the head end of the shell there is no shelf on the inside.

In the summer, the color of the mantle varies between the sexes. In females the mantle tends to be a medium chocolate brown, in males is is lighter, a yellowish cream white color.

The broad umbo (hump at the center of the concentric growth lines) is a short distance behind the narrowed, rounded front end. The periostracum is often worn away around the umbo. There is no external siphon.

Like most bivalves, the species has a muscular "foot", capable of moving he animal slowly through sediment, . The foot can also secrete byssal threads -- hair-like adhesive filaments that help the mussel attach to grasses, nearby shells, or other solid objects.

One subspecies of Geukensia demissa is recognized. G. d. granosissima (Sowerby, 1914) ranges from the east and west coast of Florida to the Gulf of Mexico (Yucatan). It differs in the morphology of the shell (rib number) and ultrastructure. (Chesapeake Bay Program, 2004;, 2005; Blackwell, et al., 1977; Brousseau, 1984; Coen and Walters, 2005; Cohen, 2005)

  • Sexual Dimorphism
  • sexes colored or patterned differently
  • Range length
    13 (high) cm
    5.12 (high) in


This species has a planktonic larval stage that allows for dispersal to distant locations. The larvae settle out of the water column and attach to oyster reefs, saltmarsh plants, and other solid objects in shallow or intertidal waters. They then transform into the sedentary shelled form. Subsequent growth rate and time to maturity is strongly determined by environmental conditions, including tidal exposure, temperature, and available food. Time to ma (Cohen, 2005)


Ribbed mussels are broadcast spawners. At a particular time of year, individuals release eggs and sperm into the water, and fertilization occurs there.

Sexual maturation in this species is primarily determined by body weight, and this in turn is strongly influenced by the environment of individual mussels. Along the edge of the marsh, mussels usually become sexually mature during their second growing season. The minimum size for gametogenesis is around 12 mm. The average size for the sexually mature ribbed mussel is greater than 20 mm. A few meters from the edge of the marsh, the minimum size increases to about 17 mm. Higher up on shore, it is not uncommon to see mussels greater than 35 mm that do not show any external characteristics of gametogenesis. Mussels that are farther from the marsh edge tend to grow slower as a result of shorter submergence and feeding time, which can delay maturation an additional year compared to the mussels along the edge of the marsh.

The ribbed mussel spawns by external fertilization, sperm and eggs are released into the water column.

Gametogenesis begins in early spring and peaks in June and July. Maximum reproduction occurs between June and August, depending on location, and larvae can be found into early fall. (Chesapeake Bay Program, 2004; Borrero and Hilbish, 1988; Borrero, 1987; Cohen, 2005; Franz, 1997)

  • Breeding interval
    It is unknown how many times they spawn during one summer, but it is thought to be only one time.
  • Breeding season
    in the summer months

There is practically no parental investment in this species -- eggs do not receive substantial provisioning, and there is no interaction with offspring after gametes are released.

  • Parental Investment
  • no parental involvement


The age of ribbed mussels can be determined by back counting the annual growth ribs on the shell.

Mortality of plankton larvae is unknown. Mortality rates of juveniles in the year following settlement have been recorded to average about 55 % partially due to winter icing on the marsh.

Even though mussels are less abundant higher on shore, survivorship increases with increasing tidal height. Some reach 15 years or older. Mussels on the marsh edge tend to be around 6 or 7 years old. (Chesapeake Bay Program, 2004; Bertness and Grosholz, 1985; Brousseau, 1982; Cohen, 2005; Franz, 2001; Lutz and Castagna, 1980)

  • Average lifespan
    Status: wild
    15 years


Once settled, ribbed mussels are normally very sedentary. They can move slowly, but will do so only if forced to by changes in their environment.

These mussels are most active when submerged -- they open their shells and pump water through their body to feed, obtain oxygen, and dispose of wastes. When exposed by low tide, they close their shells for protection, and to retain moisture. However, unlike many aquatic bivalves, they sometimes "air-gape," opening their shell a bit while still exposed to air, perhaps for gas exchange, or to reduce their temperature by evaporation.

Mussels in an inter-connected clump will close their shells when neighboring mussels do, apparently detecting the movement through byssal threads.

Most ribbed mussels live in aggregations, which can reach densities of up to 2,000-3,000 per square meter in New England and 10,000 per square meter in Jamaican Bay in New York. (Bertness and Grosholz, 1985; Coen and Walters, 2005; Cohen, 2005; Franz, 1997; Franz, 2001; Kuenzler, 1961a; Lent, 1969; Lin, 1989; Stiven and Kuenzler, 1979)

Communication and Perception

Food Habits

When covered with water, the mussels opens and cilia on its gills draw water and food in. The ribbed mussel's primary diet consists of microscopic plankton and particles of detritus. (Chesapeake Bay Program, 2004; Cohen, 2005; Kemp, et al., 1990; Kreeger, et al., 1988; Newell and Krambeck, 1995; Wright, et al., 1982)


The primary defense of ribbed mussels is their shell. In their native range, their main predator is the blue crab (Callinectes sapidus). The mud crab Panopeus herbstii is also known to feed on them. Higher survivorship in mussels high in the intertidal zone suggest that marine predators are more important than terrestrial ones.

Shore birds, including clapper rails (Rallus longirostris), willets, and dunlins have been recorded feeding on them in San Francisco Bay (Cohen, 2005). One study found ribbed mussels to be more than half the prey (by volume) in stomachs of California clapper rails (Rallus longirostris obsoletus).

Some rails have been found to have ribbed mussels clamped to their toes, and others are found missing toes. (Bertness and Grosholz, 1985; Cohen, 2005; Laughlin, 1982; Lin, 1989; Lin, 1990; Bertness and Grosholz, 1985; Cohen, 2005; Laughlin, 1982; Lin, 1989; Lin, 1990)

Ecosystem Roles

Mussels are important in changing nutrient dynamics of marsh and estuary. They help cycle energy, phosphorous and nitrogen.

The ribbed mussel has a mutualistic relationship with marsh grass, Spartina alterniflora. Mussels attach to the base of the stem with their byssal threads. As a byproduct of their filter-feeding, they deposit fecal material on the surrounding sediment. This stimulates the grass to grow by increasing the soil nitrogen. Overall they increase marsh net primary production and stability.

Geukensia demissa is a host for the flatworm, Paravortex gemellipara. (Bertness, 1984; Jordan and Valiela, 1982; Kreeger, et al., 1988; Kuenzler, 1961a; Kuenzler, 1961b; Wardle, 1980)

Mutualist Species
  • marsh grass, Spartina alterniflora
Commensal/Parasitic Species
  • a flatworm, Paravortex gemellipara.

Economic Importance for Humans: Positive

These mussels help maintain saltmarshes, which are important nurseries for food fish and shellfish. Ribbed mussels can also be very useful bioindicators for pollution assessment studies.

The mussel is also an important prey species for desirable shellfish and bird species.

Ribbed mussels are edible, but are generally not considered to taste good. They can accumulate toxins from their environment, especially while exposed at low tide, and so should not be harvested then. (Chesapeake Bay Program, 2004; Coen and Walters, 2005)

  • Positive Impacts
  • research and education

Economic Importance for Humans: Negative

There should be no low tide collection of mussels. Mussels retain the pollution inside when their shells are closed. This can cause humans to become sick if eaten.

The high abundance and biomass of Geukensia demissa in Pacific coast wetlands (where it is not native) is a concern for conservation of these threatened habitats. (Cohen, 2005; Torchin, et al., 2005)

Conservation Status

This species is not rare, and not considered in need of special conservation effort.


Michelle Nestlerode (author), Environmental Concern, Inc., Sarah Toman (editor, instructor), Environmental Concern, Inc..


Atlantic Ocean

the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.

World Map

Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

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.

brackish water

areas with salty water, usually in coastal marshes and estuaries.


uses smells or other chemicals to communicate


the nearshore aquatic habitats near a coast, or shoreline.


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


an area where a freshwater river meets the ocean and tidal influences result in fluctuations in salinity.

external fertilization

fertilization takes place outside the female's body


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.


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.

intertidal or littoral

the area of shoreline influenced mainly by the tides, between the highest and lowest reaches of the tide. An aquatic habitat.


referring to animal species that have been transported to and established populations in regions outside of their natural range, usually through human action.


offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).


marshes are wetland areas often dominated by grasses and reeds.


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.


specialized for swimming

native range

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


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


an animal that mainly eats plankton


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


structure produced by the calcium carbonate skeletons of coral polyps (Class Anthozoa). Coral reefs are found in warm, shallow oceans with low nutrient availability. They form the basis for rich communities of other invertebrates, plants, fish, and protists. The polyps live only on the reef surface. Because they depend on symbiotic photosynthetic algae, zooxanthellae, they cannot live where light does not penetrate.

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

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


uses sight to communicate


animal constituent of plankton; mainly small crustaceans and fish larvae. (Compare to phytoplankton.)


Abbott, R. 1974. American Seashells. New York: Van Nostrand Reinhold.

Ansell, A. 1968. The rate of growth of the hard clam, Mercenaria mercenaria, (L.), throughout the geographic range. Journal du Conseil Permanent International pour l’Exploration de la Mer, 31: 364-409.

Bertness, M. 1980. Growth and mortality in the ribbed mussel Geukensia demissa (Bivalvia:Mytilidae). Veliger, 23: 62-69.

Bertness, M. 1984. Ribbed mussels and Spartina alterniflora production in a New England salt marsh. Ecology, 65/6: 1794-1807.

Bertness, M., E. Grosholz. 1985. Population dynamics of the ribbed mussel, Geukensia demissa: The costs and benefits of an aggregated distribution. Oecologia, 67: 192-204.

Blackwell, J., L. Gainey, Jr., J. Greenberg. 1977. Shell ultrastructure in two subspecies of the ribbed mussel, Geukensia demissa (Dillwyn, 1817). Biological Bulletin, 152: 1-11.

Borrero, F. 1987. Tidal height and gametogenesis: Reproductive variation among populations of Geukensia demissa . Biological Bulletin, 173: "160-168".

Borrero, F. 1987. Tidal height and gametogenesis: reproduction variation among population of Geukensia demissa. Biological Bulletin, 173: 160-168.

Borrero, F., T. Hilbish. 1988. Temporal variation in shell and soft tissue growth of the mussel Geukensia demissa. Marine Ecology Progress Series, 42: 9-15.

Brousseau, D. 1984. Age and growth rate determinations for the Atlantic ribbed mussel, Geukensia demissa Dillwyn (Bivalvia: Mytilidae). Estuaries, 7/3: 233-241.

Brousseau, D. 1982. Gametogenesis and spawning in a population of Geukensia demissa (Pelecypoda:Mytilidae) from Westport, Connecticut. Veliger, 24: 247-251.

Carlton, J. 1979. History, Biogeography, and Ecology of the Introduced Marine and Estuarine Invertebrates of the Pacific coast of North America. Davis, CA: Ph.D. Thesis, University of California.

Carlton, J. 1992. Introduced mairne and estuarine mollusks of North America: an end-of-the-20th-century perspective. Journal of Shellfish Research, 11: 489-505.

Chesapeake Bay Program, 2004. "Bay Field Guide" (On-line). Atlantic Ribbed Mussel. Accessed March 11, 2007 at

Coen, L., M. Luckenbach. 2000. Developing success criteria and goals for evaluating oyster reef restoration: ecological function or resource exploitation?. Ecological Engineering, 15: 323-343.

Coen, L., K. Walters. 2005. "Ribbed mussels" (On-line). South Carolina Department of Natural Resources, Comprehensive Wildlife Conservation Strategy. Accessed March 11, 2007 at

Cohen, A., J. Carlton. 1995. Nonindigenous Aquatic Species in a United States Estuary: A Case Study of the Biological Invasions of the San Francisco Bay and Delta. Washington, D.C.: U.S. Fish and Wildlife Service.

Cohen, A. 2005. "Geukensia demissa" (On-line). Guide to the Exotic Species of San Francisco Bay. Accessed March 11, 2007 at

Franz, D. 1993. Allometry of shell and body weight in relation to shore level in the intertidal bivalve Geukensia demissa (Bivalvia: Mytilidae). Journal Experimental Marine Biology Ecology, 174: 193-207.

Franz, D. 2001. Recruitment, survivorship, and age structure of a New York Ribbed Mussel population (Geukensia demissa) in relation to shore level - a nine year study. Estuaries, 24: 319-327.

Franz, D. 1997. Resource alloacation in the intertidal salt-marsh mussel Geukensia demissa in relation to shore level. Estuaries, 20: 134-148.

Franz, D. 2005. "Ribbed Mussels (Geukensia demissa)" (On-line). CUNY. Accessed March 11, 2007 at

Franz, D., J. Tanacredi. 1993. Variablility in growth and age structure amoung populations of ribbed mussels Geukensia demissa (Dillwyn)(Bivalivia; Mytilidae), in Jamaica Bay, New York (Gatewaya NRA). The Veliger, 36: 220-227.

Jordan, T., I. Valiela. 1982. A nitrogen budget of the ribbed mussel, Geukensia demissa, and its significance in nitrogen flow in a New England salt marsh. Limnology and Oceanography, 27: 75-90.

Kemp, P., S. Newall, C. Krambeck. 1990. Effects of filter-feeding by the ribbed mussel Geukensia demissa on the water-column microbiota of Spartina alterniflora saltmarsh. Marine Ecology Progress Series, 50: 119-131.

Kreeger, D., C. Langdon, R. Newell. 1988. Utilization of refractory cellulosic carbon derived from Spartina alterniflora by the ribbed mussel Geukensia demissa . Marine Ecology Progress Series, 42: 171-179.

Kuenzler, E. 1961. Phosphorus budget of a mussel population. Limnology and Oceanography, 6: 400-415.

Kuenzler, E. 1961. Structure and energy flow of a mussel population in a Georgia salt marsh. Limnology and Oceanography, 6: 191-204.

Laughlin, R. 1982. Feeding habits of the blue crab, Callinectes sapidus Rathbun, in the Apalachicola Estuary, Florida. Bulletin of Marine Science, 32: 807-822.

Lent, C. 1969. Adaptations of the ribbed mussel, Modiolis demissus (Dillwyn), to the intertidal habitat. Am. Zool, 9: 283-292.

Lin, J. 1989. Influence of location in a salt marsh on survivorship of ribbed mussels. Marine Ecology Progress Series, 56: 105-110.

Lin, J. 1990. Mud crab predation on ribbed mussels in salt marshes. Marine Biology, 107/1: 103-109.

Luckenbach, C., L. Coen, P. Ross, Jr., J. Stephen. 2005. Oyster reef habitat restoration: Relationships between oyster abundance and community development based on two studies in Virginia and South Carolina. Journal of Coastal Research Special Issue, 40: 64-78.

Lutz, R., M. Castagna. 1980. Age composition and growth rate of a mussel (Geukensia demissa) population in a Virginia salt marsh. Journal Molluscan Studies, 46: 106-115.

Newell, S., C. Krambeck. 1995. Responses of bacterioplankton to tidal inundations of a saltmarsh in a flume and adjacent mussel enclosures. J. Exp. Mar. Biol. Ecol., 190: 79-95.

Nielsen, K., D. Franz. 1995. The influence of adult conspecifics and shore level on recruitment of the ribbed mussel Geukensia demissa (Dillwyn) in Jamaica Bay, N.Y. Journal of Experimental Marine Biology and Ecology, 188: 89-98.

Stiven, A., E. Kuenzler. 1979. The response of two salt marsh molluscs, Littorina irrorata and Geukensia demissa, to field manipulations of density and Spartina litter. Ecological Monographs, 49: 151-171.

Torchin, M., R. Hechinger, T. Huspeni, K. Whitney, K. Lafferty. 2005. The introduced ribbed mussel (Geukensia demissa) in Estero de Punta Banda, Mexico: interactions with the native cord grass, Spartina foliosa . Biological Invasions, 7: 607-614.

Waite, J., D. Hansen, K. Little. 1989. The protein of ribbed mussels (Geukensia demissa): a natural adhesive with some features of collagen. Journal of Comparative Physiology, 159/5: 517-25.

Wardle, W. 1980. Occurrence of the symbiotic Rhabdocoele flatworm Paravortex gemellipara in Chesapeake Bay and Gulf of Mexico molluscs, with notes on its biology and geographic range. Estuaries, 3/2: 84-88.

Wright, R., R. Coffin, C. Ersing, D. Pearson. 1982. Field and laboratory measurements of bivalve filtration of natural marine bacterioplankton. Limnology and Oceanography, 27: 91-98., 2005. "Ribbed Mussel" (On-line). Accessed March 11, 2007 at