Breeding Xiphophorus species can be easy or extremely challenging. Most domestic platies and swordtails obtained from pet stores breed readily at home if they are healthy. Wild species can be more difficult to breed. Most Xiphophorus species readily adapt to new surroundings, but breeding success is most readily achieved by keeping breeders healthy, well fed, and by being mindful of the natural habitats and species-specific environmental cues that promote mating in the wild.
Males and Females: The Basics
If you've bred Xiphophorus before, the differences between males and females are quite obvious. Females, like the tank raised X. helleri shown on the left, have a fan-shaped anal fin (arrow). In contrast, the male on the right has a narrow, "stick-like" anal fin called the gonopodium. It contains a "hook" (see insert) that the male uses to anchor his body to the females genital opening (see image) and deliver sperm packets.
Identifying Fertile Females
Unfortunately, breeding Xiphophorus species, even pet store varieties, is not always as easy as breeding guppies, even when healthy, well fed fish are used. Part of the problem is that not all males and females are fertile. This can be especially problematic when breeders are of mixed-species ancestry or contain fancy finnage. Another problem is that even if the breeders are fertile, if the male and female do not exhibit normal mating behaviors, a successful mating will not occur.
There is no method that guarantees a female is capable of giving birth other than knowing that she has given birth previously. Even so, the female may not be fertile now. There are several indicators that I have found useful in identifying potentially fertile females.
Body Shape
These female siblings both contain fan-shaped anal fins, but the bottom fish has a "fuller" shape. This fish is not pregnant. It is well fed and is carrying eggs. Eggs are clearly visible in fertile females with light colored or transparent bodies (e.g. marigold). Fertile wild Xiphophorus species females also have a "fuller" shape, but it is generally not as pronounced as the domestic female shown here (see the X. birchmanni female below). The upper fish may contain eggs, but it is less likely. 
Anal Fin Shape
While most Xiphophorus females contain classic fan-shaped anal fins, many fancy finned livebearers have modified anal fin shapes. The female lyretail on the right has an extension of its anal fin (arrow), but based upon body shape and the clearly visible eggs, this fish is fertile.
This hifin female also has an extension of its anal fin. However, unlike the fertile lyretail female shown above, this extension is especially long and is thickened. Also note that the fan-shaped part of the anal fin is smaller than normal due to the thickening of the fin. As a male develops, the fan-shaped anal fin of immature young thickens to produce a functioning gonopodium. The presence of thickened anal fin rays in a "female" likely indicates that male-specific genes are being expressed, thereby preventing normal egg production and fertility. The female shown here is not fertile.
This fish has one of the best hifins that I have seen on a "female." Unfortunately, given the long, thickened extension of the anal fin, it is unlikely that this fish can ever reproduce.
Although the anal fin of this hifin has a thickened extension, it is not very long. This female is not pregnant, but she is carrying eggs.
In this section I will focus on issues and images related to breeding Xiphophorus species that are not readily available. Information related to breeding tank setups, feeding regimens, and breeding methods can be obtained from other web sites, aquarium forums, and hobbyist publications and books.

You may believe that a good way to identify fertile females is to watch which females the males court. The logic is that fertile females may release pheromones that the male can detect from a distance. It is well documented that certain wild Xiphophorus females identify males by olfactory cues, but less is known about pheromones produced by females and the role they may play in mating behavior. In my experience, males can attempt to mate with both fertile and non-fertile females and even other males, especially after a water change.


Occasionally, I find females that are definitely carrying eggs, but are unable to become pregnant either naturally or by repeated artificial inseminations. This may account for the fact that some presumably fertile females never reproduce.


In summary, when choosing a female to breed, it is best to pick from those that are likely to have eggs based upon body shape and that have fan-shaped anal fins without long extensions or thickened rays. For some females, a gravid spot should be visible, especially in wild species.

Gravid Spot
Wild Xiphophorus species and many domestic females contain a pigmented area called the gravid spot which co-localizes with the genital tract. The gravid spot is evident (arrow) in this wild caught X. birchmanni female. The presence of a gravid spot in wild species females generally indicates sexually maturity. However, body shape is a more reliable indicator of egg production and fertility. Note that many light colored varieties of domestic females (generally lacking the St gene) do not develop a gravid spot and that some males do contain "gravid spots."
Choosing a Male
Identifying a fertile male capable of courting and impregnating a female is not trivial. It is best to start with males that are seen actively courting females. Xiphophorus species males are generally aggressive and quickly establish a "pecking order." Less dominant males may be capable of courting females, but often they must wait until the dominant male is occupied elsewhere in the tank.
Male Maturation
As young males mature, the anal fin thickens and becomes the gonopodium. Only when the distal region of the gonopodium remodels to form the "hook" can a male successfully inseminate a female. In species containing a swordtail, the gonopodium generally develops before the swordtail is very long and these males are often producing sperm. The two fish on the left have thickened anal fins which will become the gonopodium.
Male Fertility

Clearly, a male is fertile when a virgin female placed alone with him gives birth. Similarly, if you have been breeding a line of fish for several generations then most of the males are likely to be fertile because you have been essentially selecting for males that are successful breeders. However, some of these males and others that have well formed gonopodia and actively court females may not be fertile all of the time, if at all.


The only way to tell if a male is producing viable sperm is to "milk" the male (see the Artificial Insemination page) and look for viable sperm under the microscope. Many males do not produce sperm, produce very small amounts, or produce inactive sperm. This is frequently the case for larger males, interspecies hybrids and some fancy finned fish. The fertility of Xiphophorus species males in the wild or wild caught fish reared in aquaria has not been adequately studied.

Sterile Males
Giving Birth
Following insemination of the female, the eggs are activated and the embryos develop until birth. The time of gestation for Xiphophorus species varies considerably. In general, domestic platy and swordtail females give birth after 25-30 days and 28-32 days, respectively. These are not precise numbers and can vary for individual fish. I usually transfer a female to a pregnancy tank 22-25 days after artificial insemination. Wild caught X. birchmanni and X. malinche have longer gestation times and their babies are generally larger at birth.
As the pregnancy progresses, the female gets "fatter" as the embryos grow. Within 24 hours of birth, the female noticeably increases in size in preparation for delivery.
A pregnant female within 24 hours of giving birth.

Female stops swimming in preparation for delivery.

A baby born in this group (red arrow) is expelled. A previously born albino baby (blue arrow) is visible.
The next baby in this group is protruding from the body.
The baby is expelled.
The mother moves away and the baby swims/sinks to the bottom of the tank.
One hour old babies on the bottom  of the tank.
A 12 hour old X. birchmanni- domestic swordtail hybrid born to a birchmanni mother. X. birchmanni have longer gestation times and the babies are larger at birth. The yolk sac has not yet been absorbed.
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Female behavior prior to birth is quite variable. In the days preceding delivery, some females become less active, more "skittish" and frequently hide. Others become very active, swimming up and down constantly. Delivery usually occurs in the early morning hours, but afternoon or early evening deliveries do occur. Babies can be born individually or in groups of 2-10. Deliveries generally last 1-3 hours.

There are several videos posted on YouTube of swordtails and platies giving birth. One of the best can be found in the second half of this video:    










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