Overview
The Western Pearlshell (Margaritifera falcata) is an ancient, venerable freshwater bivalve that ranks among the longest-lived animals on planet Earth, with individual mussels routinely living for over a century. Clustered in dense multi-generational beds along clean, gravel-bottomed salmon rivers of the Pacific Northwest, this living water filter cleanses entire river basins while maintaining an obligate symbiotic life cycle with native cutthroat trout and chinook salmon.
Biology and Physical Characteristics
Margaritifera falcata features a thick, heavy, elongated kidney-shaped shell (carapace) with a distinctly curved ventral margin. The outer shell surface is coated in a dark brown to pitch-black periostracum, while the interior is lined with a stunning, lustrous iridescent nacre that shimmers in pearly violet, rose-purple, and salmon-pink. It filters gallons of river water each day through its fringed incurrent siphons.
Behavior and Ecology
The Western Pearlshell has an extraordinary symbiotic relationship with native salmonid fishes: in spring, females release microscopic larvae (glochidia) that temporarily attach harmlessly to the gills of native cutthroat trout, steelhead, or salmon. The host fish transports the baby mussels upstream, after which the juvenile mussels drop off onto clean gravel beds, anchoring themselves for a century of sedentary filter-feeding.
Conservation and Human Impact
Classified as Vulnerable by the IUCN and designated as severely imperiled by the Xerces Society, the Western Pearlshell has suffered dramatic historical declines. River damming that blocks migratory salmon hosts, agricultural siltation that smothers gravel beds, and water warming have eliminated mussel beds from over 50% of their historical western range.
Interesting Factoid
Western Pearlshell mussels can live for over 100 years in the exact same spot in a riverbed; individual mussels alive in Oregon today were already filtering water when the Titanic sank in 1912.