Cryptic species are increasingly recognized as commonplace among marine gastropods, especially in taxa such as shell-less opisthobranchs that lack many discrete taxonomic characters. Most cases of poecilogony, the presence of variable larval development within a single species, have historically turned out to represent cryptic species, with each possessing a single canalized type of development. One well-characterized example of poecilogony was attributed to the sacoglossan opisthobranch Alderia modesta; in southern California, slugs resembling this member of a monotypic genus produce both long-lived, planktotrophic and short-lived, lecithotrophic larvae. Paradoxically, however, A. modesta is exclusively planktotrophic everywhere else in the northern Pacific and Atlantic Oceans. A recently completed molecular study found that slugs from poecilogonous populations south of Bodega Harbor, California, comprise an evolutionarily distinct lineage separate from northern, strictly planktotrophic slugs. We now describe the southern species as A. willowi n. sp., based on differences in morphology of the dorsum and radula, characteristics of the egg mass, larval development mode and nuclear and mitochondrial genetic markers. A DNA barcode is provided, based on 27 fixed differences in the cytochrome c oxidase subunit I gene that can reliably differentiate Pacific specimens of Alderia species. Genetic and morphological data are concordant with developmental evidence, confirming that A. willowi is a true case of poecilogony. An improved understanding of the ecological differences between these sister taxa may shed light on the selective pressures that drove the evolution of lecithotrophy in the southern species.