" Xenodexia ctenolepis, a little elongated Poeciliid living in a small mountainous area in the upper Usumacinta river drainage of southern Mexico and northern Guatemala, may be little heard about, but it is quite a special fish species, being the oldest evolutive lineage in the family Poeciliidae. X. ctenolepis is the only species not just in that family, but in the order Cyprinodontiformes to have ctenoid scales—which are scales that bear tiny tooth-like protuberances on the posterior margin, versus all the other species having cycloid scales—roundish scales that lack any protuberance. They also have the right pelvic fin modified into a clasper—a sort of hand—that serves the purpose of holding the females while copulating. The so–called Grijalva studfish is a rare species in aquaria, where it has proved difficult to breed. In this short article, I write about the distribution and habitat, natural history, aquarium keeping, and taxonomy of Xenodexia ctenolepis "
Classification: Species overview.