How do mollusks receive nutrition?
Depends on the type of mollusk.
Clams and oysters and scallops generally filter nutrients such
as plankton out of the water. Some giant clams also have symbiotic
algae growing in their flesh (zoothanthellae) that also contribute
to their nutrition.
Some molluscs are fierce predators as well- octopi, squid,
cuttlefish, conch, cone shells, and others- they will eat pretty
much whatever they can catch. (And several are actually venomous! A
sting from a cone-shell can make you very very sick indeed, or even
be fatal!)
(Biology teacher/Professional Naturalist)