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Thursday, Oct. 23, 2003


Freshwater pearl mussel

* Japanese name: Kawashinjukai
* Scientific name:Margaritifera laevis
* Description: Mussels are bivalves: mollusks that have their body contained between two shells. Young freshwater pearl mussels often have yellowish-brown shells, which become green-tinged and darker as they mature. This animal grows very slowly, and can take 100 years to reach 15 cm long.
* Where to find them: Mussels live partly buried in sand, gravel and small stones in clean, fast-flowing, unpolluted rivers and streams. The need for clean water is one of the reasons why pearl mussels are much harder to find today. The other is because they produce pearls, which has caused them to be heavily exploited by humans across their whole range, from Japan in the east to Ireland in the west.
* Food: This animal draws water through siphons that protrude from the shell. Tiny organic particles are filtered out and the mussel feeds on these. Some researchers believe this filtration helps to keep river water clean, which benefits other species, such as salmon and trout.
* Special features: In early summer (June to July), the males spawn. Sperm is released into the water and drawn in by the females as if it were food particles. But the females don't swallow it; they use it to fertilize their eggs, which develop in a pouch on the gills for several weeks. From July to September, the tiny larvae (0.7 mm long), called glochidia, are released. Each female releases between 1 and 4 million glochidia, almost simultaneously, over a few days; almost all die. A few, however, get filtered through the gills of salmon or trout, and some of these manage to attach themselves to the fish's gills. There they remain for the next six months, growing in the oxygen-rich environment, until they drop off when spring comes.

Pearl formation is rare. It occurs when something (usually a stray piece of food, but sometimes, as in legend, a grain of sand) irritates the mussel's mantle, the tissue that secretes the shell material. The mantle isolates the irritant in thin layers of shell material, like the human immune system would isolate a foreign body. Pearl mussels are classified as a vulnerable species.

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