I've been observing the melanistic-spotted mutation in a wild population of mosquitofish in the Loxahatchee River District of Southeast Florida. From what I've read the mutation is documented as naturally-occurring in the phenotype of male G. holbrooki wild populations.
Wild females with the speckled phenotype are very rare? According to a decades-old article (I attached below) they believed they had found the first recorded incidence. This article claims that pollution was not a likely factor contributing to this female specimen's phenotype but a previous study suspected hypermasculinization (and therefore melanism) in wild mosquitofish populations could be caused by pollution.