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Geukensia demissa, the ribbed mussel, is a mollusk belonging to the Class Bivalvia (or Pelecypoda), which includes invertebrates that have two-part shells and a connective hinge. (This species formerly was classified in the Genus Modiolus.) More than 15,000 species of bivalves are known to exist, the majority thriving in salt water. Some, like the Atlantic ribbed mussel, can tolerate an extreme range of salinities and water temperatures, but they are still sensitive to toxins and parasites. A bivalve's physical structure is well-adapted to burrowing into soft bottom mud or sand. Its wedge-shaped body ('pelecypoda' means 'hatchet foot' in Greek) is contained between two convex shells. A soft, muscular 'foot' extends between the valves to propel the animal forward-a relatively slow process in most species. Mussels are filibranchiates, referring to the branches or filaments of their gills, which are covered with cilia.
The Atlantic ribbed mussel is relatively large-it grows to an adult length of four inches, and 1½ inches high-and tolerates intense environmental fluctuations. Its shell is a glossy brownish-black, sometimes tinged with yellow, and the surface is grooved with pronounced radiating ribs. The mussel's foot extends from the mid-ventral portion of its body. Unlike most bivalves, which feed by drawing in water and nutrients through an incurrent siphon and then expelling wastes using an excurrent siphon, mussels feed while submerged at high tide, opening their shells and using the cilia attached to their gills to propel water and food particles into their mouths. At low tide, the shells close. Waste products that may be toxic to humans are retained in the mussel's soft tissues; gathering mussels at low tide is considered inadvisable. When the ribbed mussel extends its foot to the substrate, it secretes a plaque-like material; then the byssal gland, located in the foot, secretes individual threads that enable the animal to attach to the bottom. Sometimes as many as 600 threads hold the mussel in place. The ribbed mussel's range extends from the Gulf of St. Lawrence to Florida and Texas, and it has been introduced to Pacific waters through San Francisco Bay and Los Angeles Harbor. While this species prefers brackish estuaries or salt marshes, and can be found in great abundance between the high and low tide line, it also tolerates extraordinary water temperatures (up to 133 degrees Fahrenheit) and unusually high salinities. Ribbed mussels often can be found in eastern North American tidal marshes rich in Spartina alterniflora. They sometimes attach to one another or to hollow grass stems (or culms) using their byssal threads, in the low marshes and creek bottoms. Geukensia is thought to feed primarily on nannoplankton, microplankton and bacterioplankton in summer months. At the lower edge of the marsh, mussels also consume diatoms and other microorganisms that become resuspended in the water column on flood tides. A mussel's age can be calculated by counting the annual growth ribs on the shell. Shell growth appears to occur most intensively in late summer. Ribbed mussels are not hermaphroditic, and sex can be determined by the color of the mantle. Females tend to be a medium brown; males a yellowish-cream color. Gametogenesis-the formation of sex cells or gamates (ova or spermatozoa)-usually occurs in spring and peaks by midsummer. Mussels are not known to spawn more than once a season. In the Chesapeake Bay region, it's usually easier to find ribbed mussels than clams, because the tips of their shells protrude from the mud or sandy bottoms. It's recommended that you use a knife or trowel to cut their tough byssal threads is recommended, and you should take only mussels that are submerged and thus have been filtering out waste products. They should be cooked and consumed promptly. Like the Eastern oyster, ribbed mussels (in enough numbers) can filter high volumes of water in the tidal marshes during each cycle, are crucial to the cycling of energy and nutrients and are an important prey species of birds and the blue crab. Unfortunately, the ribbed mussel also is vulnerable to parasites and can no longer be found in great abundance in some areas of the Bay, including Maryland.
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