The northern swordtails live in and around the Panuco river basin and are separated from the southern swordtails by a geographical dividing line known as the Trans-Mexican volcanic belt. The northern swordtail group is made up of three clades and these are the Montezumae clade, the Cortezi clade, and the Pygmaeus clade.
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The Montezumae Clade
The Cortezi clade
The Cortezi clade consists of three species X. cortezi, X. birchmanni and X. malinche and are regarded by many as some of the most beautiful fish within the Xiphophorus Genus. The swordtails themselves are medium sized swordtails and grow about 50 – 60 mm in length. They are not too difficult to maintain and they lend themselves to planted community aquariums as well as species only set ups.
The Cortes sword has many populations with some of them having males that have striking bright yellow dorsal and caudal fins with a body sporting a series of thin vertical stripes. The Cortes sword has the largest distribution of the clade which ranges from just south of the Rio Tampaon – Santa Maria axis to parts of the Rio Claro and tributaries of the Rio Calabozo. Within this range they can be found at altitudes of between 100 metres to 1200 metres although they are reportedly more prevalent at the lower altitudes. The habitat they are found in is generally free of vegetation and fast flowing.
The birchmanni swordtail is also known by the common name of the ‘sheepshead’ swordtail. This reason for this common name is due to the large prominent bump that forms at the back of the head of mature males giving a similar profile to that of a sheepshead! The dorsal and caudal fins are a beautiful yellow colour which are scattered with a covering of small black spots. The caudal fin is most unusual as it doesn’t grow a sword or if it does it remains very small, but the dorsal fin (seemingly trying to compensate for the failing of the caudal fin) grows quite large in contrast and becomes almost sail like in appearance. The body shape is generally deeper than that of the others members of the clade and the flanks also show a series of vertical stripes similar to those of the Cortes sword apart from being distinctly much broader.
The distribution of the birchmanni swordtail is also fairly extensive, ranging from the Rio Sans Pedro down to the Rio Tuxpan drainage incorporating the Rio Vinazco and the Rio Beltran. This species of swordtail is generally found at higher altitudes than the Cortes sword and has been found between the altitudes of 500 and 1000 metres. This species is generally found in a habitat of fast flowing water over a rocky / sandy substrate with dense vegetation.
The malinche swordtail also grows a lump behind the head in the same fashion as the birchmanni swordtail, but this is not anywhere near as prominent. The colouring is similar to that of the birchmanni swordtail with a distinctive yellow caudal with a medium sized sword and yellow dorsal fin showing black spots. The body colour is also similar but does seem to have purple hue. Unlike the previous clade members the stripes on the flanks of the malinche swordtail are far more irregular, often at different angles to each other and sometimes broken in appearance.
The range of distribution for the malinche swordtail is at present relatively small due to the fact it has not yet been fully explored. Their current range includes the Rio Claro down to the tributaries of the Rio Calabozo.
The common name for the malinche swordtail is the Highland swordtail. This name derives from the altitude of the rivers and streams they inhabit. They are found at altitudes which range from 650 to 1300 metres. The habitat they can generally be found in is one of fast flowing water over a sandy substrate with dense vegetation.
The three species overlap within their respective ranges and hybrid populations do exist. Where the Cortes swordtail overlaps with either the birchmanni swordtail or the malinche swordtail it is at the higher end of the Cortes swordtail range and lower end of either the birchmanni or malinche swordtail range. Similarly when the birchmanni swordtail and the malinche swordtail ranges overlap it is at the higher end of the birchmanni swordtail range and lower end of the malinche swordtail range.
It is unfortunate that in recent times, the members of this clade apart from X. cortezi have been absent from the UK side of the hobby. Happily I am pleased to say that at recent events birchmanni swordtails have started to make a comeback and are already becoming more sought after. This is a trend that I hope will continue, and with luck (fingers crossed), malinche swordtails will also make a welcome return to the UK.
The Pygmaeus clade
The Pygmy swordtails are as the name suggests some of the smallest swordtails known within our hobby. The Pygmy swordtails form the Pygmaeus clade which is made up of three species (Xiphophorus pygmaeus, Xiphophorus nigrensis and Xiphophorus multilineatus), the three species live in the state of Sans Luis Potosi within the Rio Panuco river basin. The rivers these swordtails are predominately found in are the Rio Huichihuayan (Xiphophorus pygmaeus), the Rio Choy (Xiphophorus nigrensis) and the Rio Coy (Xiphophorus multilineatus). These rivers are formed in close proximity to each other by natural springs that surface in the Sierra Madre Mountains and although the rivers are formed and run closely to each other, there are no natural connections between any of them.
The pygmy swordtail is as the name suggests the smallest of the clade reaching a length of 30mm for males and 40mm for females. This species is well known within our hobby with many of us at one time or another keeping either the blue or yellow form, at the auctions this species is usually well represented and always popular.
The type locality for the pygmy swordtail is the Rio Axtla which was described as being a fast flowing river with little vegetation. This description was as you would expect with the information given became the model for how most aquarists kept this species for many years. Subsequent research has now shown that the pygmy swordtail is generally found in highly vegetative areas of slow moving water away from the main currents. These new findings have since altered the preferred method of maintenance to a well planted aquarium with moderate to little water movement.
The pygmy sword is found predominately in the Rio Huichihuayan, but they can also be found to a lesser extent in both the Rio Tancuilin and the Rio Axtla. The Rio Huichihuayan and the Rio Tancuilin merge together to form the Rio Axtla.
The El Abra swordtail (Xiphophorus nigrensis) is only found in the Rio Choy, a deeper bodied fish than the pygmy sword with the larger type males reaching a standard length of 40mm with a sword of up to 30mm long. This species not always available at the auctions these days, however if you can provide the correct conditions for these fish, they are definitely worth keeping.
The River Choy originates from a cave within the El Abra Mountain giving this swordtail its common name of the El Abra swordtail. The El Abra swordtail is found along most of the length of the river but is most prolific nearest to the source. The population density decreases the further downriver you go and is not found where the Rio Choy joins the Rio Tampoan on its northern side.
The body colouration is greyish blue with a single bold zigzag midlateral line, in fully mature males the colour becomes more of an intense metallic blue and the midlateral line fades out. The males for X nigrensis have three size variations with the larger type male being the one most commonly found in the hobby. There are two main reasons that I can see for this, the first reason from a selfish point of view being that they are the better looking fish with the best colours. The second reason is a lot more understandable, in the wild the presence of larger type males is maintained due to them being bigger than the other males, this allows them the more prominent areas with which to attract the females. Swordtail females (according to some of the studies I have read) for their part are more amenable to mate with the larger males. However in the aquarium the smaller males mature more quickly and are therefore able to mate at a much younger age, this if left unchecked would create an aquarium population with only smaller type males, the larger type would disappear from the hobby.
This species is renowned for being difficult to maintain, they are highly susceptible to changes in water conditions particularly P.H. and with this in mind water changes should be frequent and small, a well planted aquarium with hard water and good filtration is undoubtedly the best environment to maintain them.
Xiphophorus multilineatus is found in the Rio Coy and some parts of the Rio Tampoan. The Rio Coy is mainly formed from the waters of two springs, (the Arroyo Tambaque, and the Arroyo Oxitipa). Having a very similar description to the Rio Choy in being a fast flowing river with rocky habitats near the source and muddy/sandy substrates with dense vegetation in the slower areas. Xiphophorus multilineatus is found along the entire length of the river which is also a tributary to the Rio Tampoan but this time on the southern side.
Xiphophorus multilineatus is a similar looking species to Xiphophorus nigrensis and was often confused as such in early collections. In 1979 it was believed to be a separate population of X. nigrensis which I believe was responsible for the description being so much later than the initial collection. Xiphophorus multilineatus does have two major differences that separates it from X. nigrensis, the first being the stripes on the flanks of the males (this is where the name multilineatus comes from) and the second is the high back which forms on mature males somewhat similar to Xiphophorus birchmanni.
This species also has different size variations for males, the larger males develop a high back and vertical barring on the flanks. All males have a body colouring of pale metallic blue and a single bold zigzag midlateral line. As with the El Abra sword the larger type males are the ones most known within the hobby.
This is a shy species that requires a high level of vegetation in which to hide, although not as easily upset as X. nigrensis with water changes, the same routine is probably best followed. Water changes should therefore be frequent and small, the aquarium should be well planted aquarium with hard water and good filtration along with aeration.
Again this species is not as frequently seen at the auctions as it once was, these are lovely fish to keep and again also worth keeping.