Several fish sell under the name dragon fish, including Asian arowanas, dragon gobies and bichirs. They are not particularly related. The fish sold as dragon fish are somewhat difficult to sex reliably and are hard to breed in captivity. Still, on a good day, you can tell same-species male and female dragon fish apart.
Sexing Dragon Gobies
Gobioides broussonnetii has several common names, including violet goby, dragon goby and dragon fish. While they're sometimes sold in freshwater, most specimens need brackish water to thrive. The only way to tell males from females is to look closely at their genital papillae, protruding structures on the underside of the fish. In the males of the species, this structure is long and pointed. In females, it is shorter and has a blunter shape. The structure has a yellowish color in female dragon gobies.
Arowanas, specifically Asian arowanas, sometimes sell under the name dragon fish. These fish are notoriously difficult to sex. Generally, a female will not grow as large as a male of the same age and care. Additionally, females get a more rounded appearance when they're full of eggs, but only near spawning. Males have slightly wider and deeper mouths, but it usually takes considerable experience to reliably pick up on this trait.
Sexing Dinosaur Eels
A number of species of bichirs show up in the aquarium trade under a number of names, including dinosaur eels, dragon eels and dragon fish. Their multiple common names comes from the reptilian and snakelike appearance of these fish, as well as the fact that they have not changed much in millions of years. Of the various dragon fish, these probably have the most straightforward and obvious difference between the sexes: Males have thicker anal fins than females. The anal fin is the unpaired fin on the underside of the fish.
If the dragon fish in question is an arowana, you may have a bit of a problem. The arowana most often called dragon fish, the Asian arowana (Scleropages spp.), is illegal to own in the United States due to conservation concerns. This applies to all species of Asian arowanas. Arowana species get far too large for most home aquariums. It is highly irresponsible, and illegal in many areas, to release them into the wild if they get too big for an aquarium.
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