Sympatric speciation through sensory isolation

Many people have claimed that new species have to be created by geographic isolation, that is, they do not believe that sympatric speciation (new species arising from a population in the same area) can happen. I personally think that is hogwash and it can certainly happen. A nice case in point is an article in the October 2 Nature by Seerhausen et al. called "Speciation through sensory drive in cichlid fish." Cichlids are amazingly diverse and many species occur alongside other species of cichlids. Seerhausen et al. studied several species that occur sympatrically when considering just two-dimensional geography, but occur at different depths. The species are different colors and their visual sensitivity matches the change in light absorption at the corresponding depth, meaning their eyes are optimized for lack of a better word for the depth at which they live and their coloration shows corresponding adaptation. it is fairly clear that these species all originated from the same ancestor that lived at all depths but then speciated by adapting to specific depths. The case could be argued that this depth separation counts as allopatric, or geographic separation, but I think that would be stretching the definition a bit thin as nothing prevents the fish from swimming throughout all depths other than their preferences.