Musculature Associated with the Water Canals in Freshwater Mussels and Response to Monoamines In Vitro.

The gills of freshwater mussels perform many functions that depend on water flow through the water canals and channels. Regulation of water flow depends in part on ciliary activity and in part on the contraction of musculature underlying the gill filament and water channel epithelium. Obliquely striated muscles control water canal openings (ostia) at the base of the filaments and also at the entry into the water channel (internal ostia, IO). The muscles adjacent to the ostia are oriented in an anterior-posterior direction (perpendicular to gill filaments), and those controlling the internal ostia are oriented in a dorso-ventral direction (parallel to gill filaments). Small bundles of fibers radiate from the major dorso-ventral IO muscle bands and appear to insert at the base of the water canal epithelial cells at the canal-channel junction. Both muscular bands are closely associated with the branchial nerves in the gill. When gills are exposed to 10-5 M serotonin in vitro, both ostial openings dilate and gill ciliary activity increases. The net result of serotonin treatment is an increase in ciliary activity, a maximal opening of the water canal ostia, and, presumably, an increase in water flow through the gill.