How to Tell If a Swordtail Fish is Male or Female?

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Swordtails are beautiful Poeciliidae, one of the most popular tank fish, along with mollies, platies, and guppies, which are part of the same family.

These are hardy fish species that require stable environmental conditions, a tropical-like setting, and a balanced diet, and they ask for nothing else.

Swordtails are ideal for novice fish keepers who look for easy-to-maintain fish. The problem is that swordtails display different behavior depending on gender, like any other fish.

They also have slightly different requirements based on gender differences, so knowing which is necessary to provide your swordtails with adequate care.

Let’s see the key differences between swordtail males and females, so you know what you’re dealing with.

Swordtail Male vs. Female – How to Tell the Gender?

All fish species will display physical and psychological gender differences, and swordtails are no different.

The main aspects that separate swordtail males and females include:

Body Size

Female swordtails always grow larger than the males, albeit not by that much.

Your typical female swordtail will revolve around 5.5 to 6 inches in length, while males will settle for about 4.5 to 5.

In many cases, the female can grow up to 6.5 inches but will rarely go beyond that, while males can go up to 5.5.

How much the fish will grow depends on their environmental conditions, genetic makeup, and food.

Receiving more nutrients and frequent meals as fry will also influence the fish’s adult size.

It also makes sense for females to grow larger since this relates to the males’ behavior.

Males tend to become quite pushy, almost aggressive during the mating phase, stalking females long after they’ve already mated.

Being larger and more powerful allows the females to repel the males and assert some physical dominance to scare them off.

Their larger size is also useful when fighting over food (which swordtails are sometimes known to do) and enforcing the female-specific pecking order.

Tail Fin Shape

This is the most glaring difference between male and female swordtails. The males will display an elongated tail fin which can often double their body size.

The caudal fin is believed to increase the male’s chances at mating; the longer the fin is, the greater the chances to mate with more females.

This elongated lobe is usually sharp and used to impress females and even assert dominance over other males.

The longer the lobe is, the larger the fish will appear, which immediately demands respect from other males.

As a footnote, not all males will develop their caudal fin at the same time. Some may lag behind others.

As a general rule, the dominant male will often mature first, while other, more submissive males will develop slower, causing many to confuse them for females.

Anal Fin Shape

The anal fin differs in both shape and biological role in swordtails between the genders. The female’s anal fin is normal, similar in shape to the ventral fins, just slightly larger.

It usually takes on a triangle shape, and you can see it waving in the water as the female swims.

The male’s anal fin is very different in shape, size, and purpose. In males, the anal fin is called a gonopodium since it has turned into a copulatory organ.

It no longer acts as a fin but as an organ for sperm delivery. At first, it may look like male swordtails don’t have an anal fin.

Only at a closer look will you be able to notice the sharp and thin gonopodium pressed against the male’s abdomen.

The gonopodium cannot open or move, unlike the female’s anal fin, and the male won’t use it for swimming. This organ’s role is strictly sexual.


Male swordtails tend to be more active and territorial within their environment. They are typically welcoming of other fish species but will display increased aggression towards their own kind, males in particular.

Males have a strict social hierarchy that dictates who eats first and more, gets access to females, and rules the territory.

As a result, you may often see males displaying more aggressive behavior towards other male swordfish.

In case of confrontation, the larger male will almost always gain the upper hand.

Female swordtails tend to be more docile in general since they don’t have all the testosterone to influence their behavior.

However, this will change at times. Female swordtails also have a pecking order to abide by.

Larger females will control the smaller ones, especially when it comes to feeding.

At the same time, female swordtails grow more aggressive when they’re pregnant.

A pregnant female may attack other females, males, and other tank inhabitants, especially towards the end of its pregnancy.

Gravid Spot

The gravid spot is typical to females only. This is a darker area situated in the lower portion of the abdomen, towards the cloaca (anus).

The gravid spot only appears in pregnant females and grows more visible as the female approaches labor.

The gravid spot effect is created by the uterus leaning on the stomach’s lining and will become more intense in color as the fry grow.

By the end of the pregnancy, you may even spot the fry moving inside the female’s body, where the gravid spot is.

If you notice a gravid spot in what you thought was a male, you got the fish’s gender wrong.

When Can You Tell the Gender of Swordtail Fry?

That’s kind of a loaded question. A more appropriate question would be, ‘When can you tell the gender of swordtail fish?’

That’s because you can almost never distinguish your swordtails’ gender during their fry phase; they all look the same. Most males won’t show any distinctive, male-specific traits until they are 3-months old at least.

This is generally the minimum time to expect, but most males will need far more time than that.

You should rather expect your males to develop their gonopodium and sword-like tail fin when they’re 6 months of age, even older.

At that point, they are no longer fry but adult fish. Until that happens, all you get is a lot of confusion since all fry will look and mostly behave the same.

Then there’s another aspect worth mentioning. Many male swordtails will develop even later than that.

This is generally due to the influence of a dominant male, taking over the group and inhibiting the development of the other males.

Their growth will resume if the dominant male dies or is removed from the environment.

Can Swordtail Fish Change Gender?

I have seen people posing the same question in relation to guppies and many other fish species as well.

The problem is that many people have noticed year-old females suddenly developing male-specific traits like a gonopodium and an elongated tail fin.

In other cases, the fish was 1.5 years old, practically nearing the middle of its average lifespan.

These are just instances of misgendering the swordtails and nothing more. Swordtails cannot change gender no matter the environmental conditions or other factors like missing one of the genders.

Some of the fish that can do that include:

  • Clownfish – The fish is born as a male and then turns into a female along the way. One of the main reasons for that is social hierarchy. The alpha of the pack is usually a female. When she dies, one of the males will turn into a female and take her place.
  • Broad-barred Goby – This fish can change from male to female and vice-versa depending on the environment and its needs. Alternating between the genders back and forth is a rare ability in the fish world.
  • Mangrove killifish – This fish species is hermaphrodite, allowing the killifish to self-fertilize without the risk of genetic malformations.

Swordtails, however, cannot change their gender. They appear to do so due to some males’ developmental delays, which may occur for a variety of reasons.

These include improper habitat conditions, too many males in the environment, improper dieting, etc.

If your female swordtail fish appears to turn into a male at some point, that’s simply a case of misgendering; nothing more.


It’s not difficult to differentiate between male and female swordtails for the most part.

Both sexes display different characteristics, which will become more obvious as the fish age.

Just keep in mind that swordtail fish develop at various speeds, which means there isn’t a set moment where the sex differences become obvious.

Author Image Fabian
I’m Fabian, aquarium fish breeder and founder of this website. I’ve been keeping fish, since I was a kid. On this blog, I share a lot of information about the aquarium hobby and various fish species that I like. Please leave a comment if you have any question.
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