Capable of living for up to an incredible 200 years old (7), the freshwater pearl mussel begins life as a tiny larva, measuring just 0.6 to 0.7 millimetres long, which is ejected into the water from an adult mussel in a mass of one to four million other larvae. This remarkable event takes place over just one to two days, sometime between July and September (2). The larvae, known as glochidia, resemble tiny mussels, but their minute shells are held open until they snap shut on a suitable host. The host of freshwater pearl mussel larvae are fish from the salmonid family, which includes the Atlantic salmon and sea trout (2). The chances of a larva encountering a suitable fish is very low (6), and thus nearly all are swept away and die; only a few are inhaled by an Atlantic salmon or sea trout, where they snap shut onto the fish's gills (2). Attached to the gills of a fish, the glochidia live and grow in this oxygen-rich environment until the following May or June, when they drop off. The juvenile must land on clean gravely or sandy substrates if it is to successfully grow (2). Attached to the substrate, juvenile freshwater pearl mussels typically burrow themselves completely into the sand or gravel, while adults are generally found with a third of their shell exposed (2). Should they become dislodged, freshwater pearl mussels can rebury themselves, and are also capable of moving slowly across sandy sediments, using their large, muscular foot (2). The freshwater pearl mussel grows extremely slowly (6), inhaling water through exposed siphons, and filtering out tiny organic particles on which it feeds (2). It is thought that in areas where this species was once abundant, this filter feeding acted to clarify the water, benefiting other species which inhabited the rivers and streams (2). Maturity is reached at an age of 10 to 15 years (2), followed by a reproductive period of over 75 years in which about 200 million larvae can be produced (6). In early summer each year, around June and July, male freshwater pearl mussels release sperm into the water, where it is inhaled by female mussels. Inside the female, the fertilised eggs develop in a pouch on the gills for several weeks, until temperature or other environmental cue triggers the female to release the larvae into the surrounding water (2).
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