Offering Tancho Swordtail (Xiphophorus hellerii). The image used above is for illustration purposes only.
Xiphophorus hellerii is a species of freshwater/brackish livebearing fish in the family Poeciliidae of order Cyprinodontiformes. The wild ancestors are native to an area of North and Central America stretching from Veracruz, Mexico, to northwestern Honduras.
The male grows to a maximum overall length of 14 cm and the female to 16 cm. The name «swordtail» is derived from the elongated lower lobe of the male’s caudal fin. Sexual dimorphism is moderate, with the female being larger than the male, but lacking the «sword». The wild form is olive green in color, with a red or brown lateral stripe and speckles on the dorsal and, sometimes, caudal fins. The male’s «sword» is yellow, edged in black below. Captive breeding has produced many color varieties, including black, red, and many patterns thereof, for the aquarium hobby.
The swordtail prefers swift-flowing, heavily vegetated rivers and streams, but is also found in warm springs and canals. Omnivorous, its diet includes both plants and small crustaceans, insects, and annelid worms. In the aquarium, it is unfussy and will accept just about anything offered. Offer a balanced diet comprising good quality dried products along with small live and frozen foods such as Daphnia, Artemia, Bloodworm, Tubifex, Whiteworm, Glassworm and so on.
An aquarium with surface dimensions of approximately 120 x 30 cm or equivalent should be the smallest considered. The choice of décor is not especially critical though it tends to look best in a heavily-planted set-up with a dark substrate. Beneficially the swordtails should also suit an aquarium arranged to resemble a flowing stream with water-worn rocks and small boulders. The addition of some floating plants and driftwood roots or branches to diffuse the light entering the tank also seems to be appreciated and adds a more natural feel. Filtration does not need to be particularly strong though it does seem to appreciate a degree of water movement. The recommended water parameters for Xiphophorus hellerii are the temperature 16°C – 28°C, pH 7.0 – 8.0, and hardness 179 – 447 ppm.
In confined spaces groups of males tend to form dominance hierarchies and may invest a significant proportion of time maintaining their respective positions. Adult males tend to be smaller than females and possess a prominent gonopodium, plus the characteristic extension of the lower caudal-fin lobe. In some ornamental strains, the upper caudal-fin lobe or other fins may also be extended, however, including in females.
Like other members of the viviparous family Poeciliidae, male Xiphophorus have a gonopodium, essentially a modified anal-fin, which is used for internal fertilisation of females. The reproductive behaviour of Xiphophorus is well-studied, and females are known to exhibit preferences for numerous physical and behavioural traits such as sword length (long, short, or without), colour pattern, body size, vertical barring on the body, chemical cues and specific nuptial behaviours. In some cases females show preferences for male traits of other species. Male behaviour is designed to both attract mates and drive away rivals. The females are capable of storing viable male sperm for months.
Selective breeding has led to the development of a bewildering array of ornamental forms which tend to display colours, and often fin morphology, that is not present in wild fish. Some of these forms are also the result of artificial hybridisation with other members of the genus, particularly Xiphophorus maculatus and Xiphophorus variatus.
Xiphophorus species are naturally distributed throughout the Atlantic slope of Mexico and adjacent portions of Central America, although a number of members have been introduced to and become established in other countries and territories. They are considered important in several fields of scientific research, including behavioural, genetic, and biomedical (including the cause and genetics of cancers) studies.