Blue mussel (Mytilus edulis)

Description

The blue mussel (Mytilus edulis) has an oblong triangular shape with fine concentric lines on its dark blue to black shell valves. Blue mussel shells can be up to 7.5 centimeters in length, and are covered by a shiny periostracum (Weiss 1995). The inside of the shell is dark blue to violet with a white, pearly, nacreous layer. Blue mussels are semi-sessile, epibenthic bivalves that attach to substrates or other mussels via byssus threads. These threads are secreted and can be lengthened or shortened to slightly alter the location of an individual (Newell 1989).

Background

Distribution. Blue mussels have a large distribution, occurring in the Arctic, North Pacific, and North Atlantic, ranging from Labrador to Cape Hatteras. Blue mussels are commonly found in the littoral and sublittoral zones and in polyhaline portions of the Hudson River and New York/New Jersey Harbor (Ristich et al. 1977, Gosner 1971).

Adult blue mussels are most commonly observed in areas of rock and coarse gravel, although sand and mud can provide suitable habitat if there are surfaces for attachment. Large beds of blue mussels are found where wave and current energy is relatively low, but water flow must be high enough to facilitate feeding (Newell 1989). They are typically found in areas of fine sand in the New York/New Jersey Harbor (Franz and Harris 1988), but any area with a surface for attachment of the byssus threads can be suitable habitat (Newell 1989).

Feeding. All life stages are planktonic feeders, filtering particles from the water column. Their main food source is phytoplankton and possibly detrital bacteria. Different stages all use cilia to move particles along their oral groove and into their mouths by creating water flow. The labial palps in adult blue mussels are able to sort the particles, separating food from the indigestible material, which is then rejected through the gills. Blue mussels are able to change their filtration rate depending on current needs (Newell 1989).

Fishery. Interest in blue mussels as a U.S. fishery species has begun only in the last few decades, although mussels have been an important seafood in foreign markets for much longer. Recreational harvesting does occur, but little is known about the quantities, as blue mussels are not a regulated fishery species.

Life Cycle

An increase in water temperature, change in salinity or wave action, desiccation, or increases in phytoplankton concentrations can trigger spawning in blue mussels. Eggs and sperm are released through the excurrent siphon into the water column, at a ratio of 10,000:1 spermatozoa to egg, and sperm are released first, stimulating the release of eggs. The eggs are spherical with diameters of approximately 0.07 millimeters. The larval period ranges 15 to 35 days depending on environmental conditions, and is marked by development of the shell valves, umbo, photosensitive eyespots and elongated foot. The larvae then settle onto hard substrates, fix their locations via byssus threads, and metamorphose into plantigrades. They remain in this juvenile state until they reach 1 to 1.5 millimeters in length. Following this growth period, the plantigrades detach from the substrate and move with the currents into an adult blue mussel bed. Here they secrete new byssus threads and attach to the substrate or other mussels. Sexual maturity occurs in one to two years, and adult blue mussels grow to approximately 100 millimeters and live up to 20 years (Newell 1989). Blue mussel sexes are separate, but individuals can undergo periods of hermaphroditism.

Environmental Influences

Salinity. Blue mussels are euryhaline and capable of living in oceanic salinities (35 practical salinity units [psu]) to mesohaline estuaries (5 to 18 psu; Newell 1989). It can remain active even in areas that vary by 10 psu during the daily tidal cycles. During significant salinity fluctuations, the mussel will stop flow from its excurrent siphon and close its shell for up to four days. If the salinity change is permanent, the mussel will adjust itself osmotically to the new conditions (Newell 1989).

Predation. Blue mussel larvae are heavily preyed upon by species ranging from jellyfish to fishes. As mussels grow, they are preyed upon by fewer species, and adults are mostly at risk from large starfish, large crustaceans, and some birds (Newell 1989).

Pollution. Responses of blue mussels to pollutant exposure can include delay of maturation, inhibition of growth, and increased mortality, which make this species a useful indicator of ecosystem health (Newell 1989). As with many filter-feeding bivalves, blue mussels can filter and concentrate harmful bacteria from sewage, uptake metals from industrial waste, and concentrate petrochemicals from oil pollution. These forms of contamination not only compromise the health of blue mussel populations but they can result in fishery closure. Dredging activities can also adversely affect blue mussel populations. Dredging for channel maintenance or shellfish harvest can destroy mussel beds (Menzel 1979), and resuspension of silts and sediment-bound pollutants can hinder feeding and renew exposure to contaminants.

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