The Asian mussel, Musculista senhousia, is a small saltwater mussel, a marine bivalve mollusk species in the family Mytilidae. Its common names include: the Asian date mussel, the Japanese mussel, Senhouse's mussel, the green mussel and the green bagmussel. It is differentiated from other mollusks by its relatively small size and inflated shape, as well as the greenish colour of its outer periostracal layer. The mussel also has radiating reddish lines on its posterior, small internal teeth on the dorsal edge posterior to the ligament of the shell, and small ribs anterior to the umbones. It can grow up to 30 mm in length.  It is also known for its relatively fast growth and has a maximum lifetime of about two years.
Musculista senhousia is native to the Pacific Ocean, inhabiting coastal areas from Siberia and the Kuril Islands south to Singapore. The mussel is an invasive species in California, the Mediterranean, Australia, and New Zealand. It was introduced to the Western coast of the United States sometime in the early twentieth century with shipments of Japanese oysters. In 1983, large specimens of the species were collected in the Swan River estuary in Western Australia. The mussel has been present in Auckland harbour since 1980 and specimens have also been reported at Tamar estuary in Tasmania. Invasion of the aforementioned locales is thought to have been achieved in a variety of manners: the mussels were transported on the hulls of ships, in water-intake chambers and the ducts of ships, as planktonic larvae carried in the ballast water of bulk-cargo vessels, in association with intentionally introduced oysters, and by Lessepsian migration through the Suez Canal.
The Asian mussel is generally found in sheltered mud or other soft substrates up to twenty metres below the surface of the water. In China and Japan, the native mussel is found in intertidal zones with densities up to 2500 individuals per square metre. In Australia, the invasive species is found from 0.5 to 4 metres below the surface of the water in slightly higher densities. Along the Western coast of North America, however, the species generally reaches densities of 5000 to 10,000 individuals per square metre and has been recorded at densities of up to 150,000 individuals per square metre in Mission Bay in San Diego, California, USA. The mussel is most abundant in the summer and early autumn. In the Swan River estuary in Western Australia, the mussel population has a high mortality rate in the late autumn and early winter. This may be the result of a decrease in water salinity from increased rainfall.
Musculista senhousia uses byssal threads to form a sort of cocoon around itself. The cocoon may be necessary for protection due to the relatively thin shells of the species as well as to assist in the stabilization of the species in the sediment. The cocoons of each individual can intertwine to form a mat that tends to collect shells, sediment, algae, and detritus on its surface.
Predators of the Asian mussel vary depending on the native fauna of the area. In the Swan River estuary of Western Australia, for example, predation occurs mainly by boring carnivorous gastropods such as Nassarius burchardi or Bedeva paivae, which drill through the mussel's thin shell. In Mission Bay in San Diego, however, the mussel's main predators include the yellowfin croaker, spotfin croaker, sargo, willet, and the marbled godwit (three fish and two bird species respectively). Other less significant predators include carnivorous gastropods (especially Pteropurpura festiva), crustaceans, echinoderms, fish, and diving ducks. Recent research has emphasized the importance of the predation of Musculista senhousia by native fauna as a means of reducing the negative effects of the invasive species on ecosystems. Asian mussels also face predation by humans. The mussel is gathered as food in China, as food for domestic organisms in Japan, Thailand, and India, as well as bait for fishing throughout Asia. There are currently no commercial or recreational uses for the mussel in the invaded areas. It has been suggested that harvesting the species might help control their invasion, however there are a number of detrimental effects associated with benthic harvesting.
Impact on invaded ecosystems
Research has shown that the Asian mussel can have a variety of effects on various ecosystems. In the Swan River estuary in Western Australia, for example, the presence of mats of Musculista senhousia has caused an increase in the biomass of benthic macro-organisms in general. In Mission Bay in San Diego, California, USA, the increasing abundance of Asian mussels has caused a decrease in species richness and abundance of the native Solen rostriformis as well as the complete disappearance of the native Chione fluctifraga. The Asian mussel has also become one hundred times more abundant than any native bivalves. Two other native species have appeared in the bay, however: Macoma nasuta and Chione undatella. Laboratory experiments using these species show that the growth and survival of the surface-dwelling and suspension-feeding species of the Chione genus decrease dramatically in areas where the Asian mussel is abundant. Populations of the deeper-dwelling and deposit-feeding Macoma nasuta are not significantly affected, however, by the invasion of the mussel. Similar occurrences have been observed at Sacca di Goro, a brackish lagoon in the Po River Delta of the Northern Adriatic Sea in Italy, which the Asian mussel invaded in the early 1990s. Researchers there have found that growth and survival of suspension feeders has significantly decreased following the arrival of the Asian mussel. There has also been a concomitant increase in the abundance of detritivores and herbivores. Laboratory research has also shown that deeper-dwelling species are less affected by the invasion of the Asian mussel. Mussel mats have no significant effect on the mortality of deeper-dwelling clam species such as Tapes decussatus and Ruditapes philippinarum. Competition with native species is the primary cause of concern in areas of New Zealand where the Asian mussel has invaded. There are significantly fewer macrofaunal invertebrates and there has been an eightfold decrease in the abundance of infaunal bivalves. Scientists suggest, however, that the adverse environmental effects of the mussel in New Zealand will be localized and short-lived. Musculista senhousia shares its habitat with eelgrass (Zostera marina) in many of the areas in which it has invaded. The presence of the mussel has been shown to affect rhizome growth in the eelgrass. This decreases the ability of established patches to spread. The Asian mussel had the most detrimental affect on rhizome growth in areas where the eelgrass was sparse and patchy. This is a cause of concern for conservationists because beds of eelgrass are already degraded and sparse as a result of anthropogenic forces. The presence of Musculista senhousia can only worsen the situation.
- ^ a b c Edgar, Graham J. Australian Marine Life: The Plants and Animals of Temperate Waters. Victoria: Reed, 1997.
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