The shell
valves of species in this genus are fragile and subcylindrical in shape; there are no
hinge teeth. The shell has a persistent thin
periostracum which extends beyond the valve margins, hence the common name "awning clams". These clams have
chemosynthetic bacterial
symbionts that produce their food. The bacteria live within their gill cells, and produce energy by oxidizing
hydrogen sulfide, which they then use to fix carbon dioxide via the
Calvin cycle. This symbiosis has been best-studied in the Atlantic species
S. velum and the Pacific species
S. reidi. ==Species==