A scaleless dragonfish genus with a single species, the Smalltooth Dragonfish, Pachystomias microdon, found worldwide in oceanic midwaters - one of three stomiid genera with photophores that produce red light. Pachystomias microdon has evolved extremely specialized visual pigments capable of seeing this red light..
Pachystomias has a floor to its mouth with a membrane present between two halves of the lower jaw (mandibles); a short snout about equal to eye diameter; a simple chin barbel and jaws with many equally-spaced fang-like teeth. The dorsal fin is short-based and far back on the body, opposite the similars anal fin.
The crescent-shaped AOP photophore below the lower margin of the eye emits a red light.
Kenaley, C.P., DeVaney, S.C. & Fjeran, T.T. 2014. The complex evolutionary history of seeing red: molecular phylogeny and the evolution of an adaptive visual system in deep-sea dragonfishes (Stomiiformes: Stomiidae).Evolution 68: 996–1013. doi: 10.1111/evo.12322
Kenaley, C.P., Harold, A.S. & Gomon, M.F. 2008. Family Stomiidae. pp. 240-254 in Gomon. M.F., Bray, D.J. & Kuiter, R.H (eds). Fishes of Australia's Southern Coast. Sydney : Reed New Holland 928 pp.