As previously noted, Gambusia affinis and Gambusia holbrooki are sufficiently similar in appearance and behavior to cause considerable taxonomic difficulties, and these may or may not be distinct species. In south Florida, Gambusia affinis co-occurs with the congeneric mangrove gambusia, G. rhizophorae. The latter species is not known from as far north as the IRL however (Ray et al. 1986).The characteristic net-like scale pattern and the poster origin of the dorsal fin relative to the anal fin are typically sufficient to distinguish Gambusia from the co-occurring poeciliid, the sailfin molly, Poecilia latipinna (Hoese and Moore 1977).Poeciliid fishes can generally be differentiated from the potentially similar looking cyprinodontids (the killifishes) by the presence of either a gravid spot (mature females) or an intromittent organ (occurring on mature males) on the former.
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