Hi Stephen,

If you are asking about specifically zoological symbionts, I can't think of
any true examples of symbiosis, though there are a number of examples of
commensal relationships, as well as parasitism.  But two examples of
symbiosis involving mollusks and other-than-animal symbionts are the
symbiotic algae found in Tridacna, Corculum, and some opisthobranchs (sea
slugs); and the symbiotic sulphur-fixing bacteria found in deep sea vent
clams.

Paul M.