There are many differences between a male and female swordtail fish, but these differences are mostly recognizable only in the adult of the species.
A male swordtail fish is easily distinguishable from its female counterpart by its tail extension. Female swordtail fishes not only have shorter and rounder tail fins, they are also typically rounder and also bigger than the male.
Female swordtail fishes are approximately one and a half inches longer than the male but there have been instances where male swordtail fishes have been known to grow longer than females. That is not abnormal since the sexual dimorphisms of swordtail fishes are not very significant. Just as human males are typically bigger than females, there are women who are bigger than men.
Telling Males from Females
People are interested in differentiating male and female swordtail fishes because of breeding. Some owners don’t want to have fish fry appearing in their tanks suddenly when they are unprepared to take care of them, so they want to be able to separate the males and females.
Some aquarists, on the other hand, breed swordtail fish specifically for breeding. They want to have a mix of both and female swordtail fishes and in good ratio as well. There should be female swordtail fish to male swordtail fish in a ratio of three to one so that the females are not unduly stressed by the constant harassment by the males.
To do this, aquarists must be able to identify male and female swordtail fish. However, young swordtail fishes look nearly identical so it is difficult to tell. Male swordtail fish only develop their swords at maturity, so sexing is usually done when the fishes are older.
However, some males don’t develop a prominent sword even at maturity unless they are the alpha male of the group. This sometimes led to some people suddenly finding their “females” growing a tail and even having fish fry when they thought they had bought only female swordtail fishes.
There are some misgivings as to whether a swordtail fish can actually change their sex, the verdict is not out yet, but chances are that these “sex change” could be a result of the situation described above.
There have also been cases where people did really buy only female swordtail fishes but still ending up having fish fry in their tanks. A female swordtail fish can store enough sperm inside her for up to six fertilizations, so it is possible for female swordfishes to have mated with males while in the communal tank before they were bought.
There is no ideal solution to sexing swordtail fishes in their juvenile stages. Gender differentiation only becomes evident only at maturity and situations described above could potentially happen to you when buying mature swordtail fishes.
However, any inconvenience is likely to be small, and you never know – maybe you might actually take to breeding swordtail fish after an accidental breeding cycle.
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