Order Cyprinodontiformes
Suborder Cyprinodontoidei
Family Poeciliidae
Subfamily Poeciliinae
Genus Xiphophorus, or swordtails — Xiphophorus.
Swordtails are widely distributed in the waters of Central America, where they inhabit all kinds of densely vegetated water bodies with standing and slow-flowing water. The body is elongated, laterally compressed. Females are larger than males. The main feature: males have a sword-like appendage, thanks to which the fish got its name. This sword-like appendage is nothing more than greatly elongated lower rays of the caudal fin. The caudal fin is small and rounded. In the aquarium, these are calm, peaceful fish that get along normally with other inhabitants.
The aquarium should contain plants, including floating ones on the surface. The aquarium should be covered from above so that the fish do not jump out. They can be fed with live and dry food, plant supplements are desirable. These are livebearing fish. The eggs are fertilized inside the female’s body, and fully formed fry leave it, immediately beginning to feed. Swordtails do not need a spawning aquarium for reproduction, although it is better to isolate the fry, because adult fish eat their offspring. Gestation lasts 1-1.5 months.
Currently, many forms differing in appearance and coloration of the body and fins can be found in aquariums. The main species carrying the genetic material for most of these hybrids are X. helleri, X. maculatus, X. montezumae, and X. variatus. Most often, aquariums contain the Montezuma swordtail, less often Montezuma’s swordtail, Clementia’s, Cortez, dwarf and olive swordtails, lyretail (sometimes called twin-swordtail), and high-fin swordtail (also known as Simpson’s swordtail).
Swordtails are small or medium-sized fish: males of different species range from 3 to 10 cm in length, and females from 4 to 12 cm. By body shape and coloration, all wild, undomesticated swordtails can be divided into three groups of species:
1. Helleri group, uniting 2 species: X. helleri (uniting 4 subspecies) and X. clemenciae. In representatives of this group, the lower rays of the caudal fin are elongated, forming an outgrowth resembling a sword. The body is streamlined, relatively flat with red or brown stripes on the flanks. In amateur aquariums, only the green swordtail (X. helleri) has become widespread.
2. Maculatus group. Unites the following 4 species with subspecies: X. maculatus, X. variatus, X. xiphidium, X. couchianum. Only the first two species have become widespread in amateur aquariums. Unlike the first group, its representatives either lack a sword altogether or have only a small outgrowth. Longitudinal stripes on the body are absent. The first two species, of greatest interest to aquarists, were previously isolated into an independent genus Platypoecilus. Among aquarists, their most common names are: for X. maculatus – platy or simply platyfish, less often spotted platy, for X. variatus – variegated or variable platy. Due to the abundance of varieties bred by breeders that are similar in shape and color, difficulties sometimes arise in determining which of these two species a particular fish belongs to. The body of the variegated platy has a more streamlined shape than that of the spotted one. The dorsal fin in the variegated platy is longer and more closely resembles the fin of the green swordtail.
3. Montezumae group. Unites the following three species with their subspecies: X. milleri, X. montezumae, X. pygmaeus. In representatives of this group, the sword is short. The body is relatively elongated, marked with uneven longitudinal zigzag stripes consisting of spots. Representatives of this group are rare guests in amateur aquariums.
Representatives of all three groups easily crossbreed in aquarium conditions. Hybrids of green swordtails, spotted and variegated platies were used by breeders to develop many pleasing-to-the-eye color varieties of these three species. Practically almost all brightly colored breeds of green swordtails were bred by selecting their hybrids with the spotted platy. Crossing swordtails with the high-fin variegated platy was used to breed a swordtail with a huge dorsal fin (its length reached 11 cm and area 40 cm2).
Representatives of the genus are characterized by matrocliny, often observed in the formation of hybrid forms. Swordtails lack sex chromosomes, and the sex of the fry depends on growing conditions — favorable conditions produce more males, and vice versa. In addition, mature females often spontaneously transform into productive males. In this case, the male’s sperm serves only as a factor triggering the onset of development.
The resulting offspring at the first stage are represented only by female individuals and resemble the maternal organism. Having passed this stage, the fish acquire the ability for further transformations and realization of the genetic development program. External features characteristic of the female organism gradually disappear, replaced by male ones: the anal fin undergoes changes resulting in the formation of a gonopodium. In some species, a characteristic sword-like elongation appears on the caudal fin, the body becomes sleeker, and the spot characteristic of females disappears. The formation of males is also possible immediately, bypassing the stage described above.
Such “early” males are usually smaller, unlike full and large “late” males that have previously gone through the “female” stage of development. The sex determination of fry also depends on rearing conditions. At a water temperature of about 30°C, there are about 10 males per 1 female; at 20°C, the sex ratio is reversed. pH has a major influence on sex determination during the rearing of juveniles: at pH 6–6.5, the proportion of males is 80 to 100%, and at 7.5–8, it is 0 to 20%.
How to correctly determine the sex of a swordtail? Male or female? It is believed that the sex of swordtails is determined by the “sword” on the tail — yes, but not entirely. The above applies only to adult non-lyretail swordtails, as females can also have long outgrowths on their tails. Young male swordtails do not have time to grow a “sword” in the lower part of the caudal fin. Lyretail females have two or three such outgrowths on their tails. As with other livebearing fish, sex should be determined by the anal fin: in the male, the gonopodium looks like a thin stick, while in the female, the shape of the fin resembles a triangle with only a rounded corner. In lyretail females, the anal fin is triangular with a slight elongation. In lyretail males, the anal fin is not as wide and is very long. The gonopodium lengthens as the fish grows, meaning it is not as long in juveniles. In flag-fin males, the gonopodium is the same as in “simple” swordtails. The anal fin of flag-fin females is triangular.
Photo: Green swordtail — Xiphophorus hellerii.
Xiphophorus hellerii, Green swordtail.
Xiphophorus montezumae, nezahualcoyotl, pygmaeus, signum, xiphidium. 
