We know that most swordtails sport namesake sword carried by males but it seems that nature doesn’t always use only one script. One morning you look into your tank and see something unusual, a female with an unusually elongated tail. Something about it catches your eye; it’s contrary to the common description, and it makes you wonder right away: Is it a genetic mutation?
Is it an error in reproduction? It is another piece of the biological puzzle that most hobbyists never realize exist. It’s not as unusual as you might believe.
And it presents another example that throws a wrench into the binary understanding of physical differences between males and females we have for livebearers. A sword on a female are an expression of the interaction between environment, hormones, and genetics. Knowing this give you a greater appreciation for the variety you’re keeping and relieves some anxiety over wondering if something’s wrong with one of your fish.
Understanding Female Swordtail Sword Causes
Why Do Female Swordtails Have Swords
First off, it’s genetics. Swordtail fish is of the genus Xiphophorus. Several gene contribute to whether or not a sword will develop on an individual.
And these aren’t your typical “sex linked” genes as you’d think they would be. They are autosomal, or expressed equally in both sex. Both males and females may possesses and display the allele(s) for swords.
Same goes for mom and dad since both can carries and express the alleles that code for a sword. Where it differs for males versus females is in terms of the threshold. Males has less of a threshold, so it expresses fairly easy.
Females need more of a gene push.
The Role of Testosterone in Sword Growth
Another important factor is hormones. In fact, for most individuals it’s all about testosterone.
It’s the hormone that switch on the sword phenotype. Male fish has higher levels of testosterone than females and as a result they grows the caudal fin rays longer (swords). Under normal circumstances, female fish do not produces enough hormone to grow long sword-like fins, so their fins stays short or are non-existent.
But what if a female’s hormonal profile change? Perhaps she has some sort of disease, she’s stressed out, or maybe it’s just how her endocrine system works different from others. She have the genetic potential and the hormone turns on those genes… but just not as hard.
Environmental Stress and Hormonal Changes
This hormonal change can also be caused by environmental stress. Crowded tanks, poor water quality, and even aggressive tank mate cause elevated cortisol levels in your fish. Elevated stress causes disruptions in normal hormonal function.
Sometimes this mean higher testosterone or increased sensitivity to it. This doesn’t make the fish male, rather it’s a physiological response to the stress you’re placing on it. The fish may exhibit other signs of stress, like lethargy or faded colors.
Sword formation is one sign that tank needs improvement.
Selective Breeding and Genetic Traits
Genetics has also played a big role in this trait due to selective breeding. Swordtails are all about visual appeal and breeders pushes for long swords in both genders.
More and more the gene pool gets loaded with tendency to produce longer swords over time. Even females from such lines will tend to displays extensions. It’s not a flaw.
It’s an intended result of man’s tinkering. Expect more variation if you purchase from a specialty breeder. They’re producing them for drama, not strict matching with natural gender differences.
Identifying Male vs Female Swordtails
So how do you tell male vs. Female? Well, it’s not just their tail that gives it away. Males sports a modified anal fin called the gonopodium.
Regardless of its length, a female do not have one. Her anal fin will be soft and fan-shaped. So if you find yourself with a sword and no gonopodium then that’s a female.
Importance of Stock Ratios and Breeding
Why does this matter? It matters for stock ratios and for breeding purposes. You don’t want to unintentionally overstock your tank or expect fry from a wrong guess.
Seeing a sworded female reminds me that things aren’t fixed. They are an expression of a complex system, which means that biological flexibility exist. This plays out in your aquarium as well.
Look at your fish carefully. Pay attention to how healthy they are and what they do. A sword on a female isn’t typically a bad thing.
More times than not, it’s simply another interesting detail from the book you’re writing about your tank. Enjoy the variability. The hobby is all the better for it.
