5-methylcytosine (5mC) is an important epigenetic mark in eukaryotes. Little information about its role exists for invertebrates. How 5mC contributes to phenotypic variation in invertebrates can be investigated by experimental alteration of methylation patterns. Here, we apply new non-nucleoside DNA methyltransferase inhibitors (DNMTi) to introduce global changes into the methylome of mollusk species. Flavanone inhibitor Flv1 was highly efficient in reducing 5mC in the freshwater snails Biomphalaria glabrata and Physa acuta, and to a lesser degree, probably due to lower stability in sea water, in the oyster Crassostrea gigas. Flv1 has no toxic effects and significantly decreased the 5mC level in the treated B. glabrata generation and in its untreated offspring. Drug treatment triggers significant variation in the morphometric traits in both generations. An epigenotyping by sequencing method corroborates hypomethylation effect of Flv1 in both B. glabrata generations and identifies one Differential Methylated Region (DMR) out of 8, found both in Flv1-exposed snails and its progeny, demonstrating a multigenerational effect of an induced epimutation. By targeted bisulfite sequencing, we confirmed hypomethylation in a locus associated with reduced gene expression.