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Marine species of gastropods and bivalves are more numerous than land and freshwater species, and the shells are often larger and more robust. The shells of marine species also often have more sculpture and more color, although this is by no means always the case.
In the tropical and sub-tropical areas of the planet, there are far more species of colorful, large, shallow water shelled marine mollusks than there are in the temperate zones and the regions closer to the poles.
Although there are a number of species of shelled mollusks that are quite large, there are vast numbers of extremely small species too, such as micromollusks.
Not all mollusks are marine however, there are numerous land and freshwater mollusks, for example snail and freshwater bivalves. And not all mollusks have an external shell: some mollusks such as some cephalopods (squid and octopuses) have an internal shell, and many mollusks have no shell, for example slug and nudibranch.