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« on: August 06, 2004, 05:26:44 PM »

Stunning Swords
by Derek Lambert

Look in any general aquarium fish book and you will find swordtails. Indeed, look in any aquarium shop and you will find swordtails and, of course, look in almost any aquarist’s tank and you will find some swordtails. They are simply one of the most popular aquarium fish in the world - and rightly so. The only down side to this is the fact that nearly all of these swordtails are cultivated forms derived from the Common swordtail (Xiphophorus helleri). The two commonly kept platy species (Xiphophorus variatus and Xiphophorus maculatus have been hybridised with wild Common swordtails to produce the multitude of colour forms we see today.

What, however, of all the other species of swordtail? Currently there are 14 known wild swordtails and many of them are really beautiful fish well worth keeping. They range in size from 2.5 cm (1”) little gems to well over 6” (15cm) stunners with a sword another 6” (15cm) long. What! A foot long swordtail - sounds unbelievable I know but it is a fact. What makes it even more remarkable is these were not wild caught fish but captive bred in a small pond in someone’s garage.

Cultivated swordtails
More about these later but, for the moment, let us return to the cultivated fish we see in shops all over the world.These come in a multitude of colours and several different fin forms. Red is one of the commonest colours, but you can find black, green, yellow or any combination of these.

Apart from the normal fin form you may well come across Lyretail swords. These have the upper as well as lower rays of their tail fin extended. The first few rays of the dorsal and anal fins are also extended and most of the pectoral fin rays are elongated. Good specimens can be really beautiful but most of the fish we see in shops look somewhat ragged. This is because any nick or slight damage to these fish’s fins will grow out as a spike. Male Lyretail swordtails are unable to mate because the gonopodium is too long. Cutting this down to the right length does not work (you chop off the holdfasts which enable him to latch on to a female during mating) so the female has to be mated to a normal finned male. This produces broods of young with half normal finned youngsters and half Lyretail babies.

Another fancy finned swordtail you may be lucky enough to come across is Hi-fin or Simpson swordtails. These have their dorsal fin extended and widened into a large sail-like fin. A good specimen of these will really knock your socks off! Sadly, these don’t come in through the trade and what we usually see are fish with a long thin dorsal fin, a poor shadow of what they should be.

Aquarium care and their natural habitat
All cultivated swordtails require the same conditions. Since they are derived primarily from Common swordtails (Xiphophorus helleri) it is worth taking a quick look at this species natural habitat. Common swordtails are highly adaptable fish found throughout much of Mexico today. Originally, however, it was restricted to lowland river systems which drain into the Atlantic, from Rio Nautla in Veracruz, Mexico, southwards to the border with Belize and Guatemala.

These rivers are all warm, flowing, freshwater habitats usually containing some plant cover. Xiphophorus rarely occur in brackish water and don’t like, or need , the addition of salt to their aquarium. An exceptional habitat is Laguna de Catemaco, a large, freshwater lake surrounded by mountains. Here Common swordtails (X.helleri) occur in large numbers. Fry and youngsters can be found where-ever aquatic plants have become established (mostly in small feeder streams). The adults remain in deeper water during much of the day. This is because the shores are subject to strong wave motion which peaks during early evening. By midnight this has dropped to almost nothing and then you can catch some of the really large swordtails which inhabit this lake, as they search the sandy shoreline for insects and other food.

In the aquarium Common swordtails (X.helleri) will tolerate a wide range of conditions but a temperature of between 76 - 80 degrees F and hard, neutral to alkaline water suits them best. Good filtration combined with regular partial water changes are important to their well being. If the water quality starts to deteriorate the swordtails will soon show their distress. First indicators are shimmying (a slow weaving motion) and in bad cases a whitish sheen over the flanks of the fish. Don’t treat the fish with medication (particularly salt) - just perform a large water change, clean out the filters and check that you don’t have an ammonia, nitrite or even nitrate problem. High levels of nitrates will distress swordtails almost as much as nitrites do.

Other swordtails
I mentioned earlier the 14 species of wild swordtail. These can be divided up into 4 main groups. The first group are those species which are similar in many ways to Common swordtails (X.helleri). This includes the Upland swordtail (X.alvarezi), Yellow swordtails (X.clemenciae), Guatemalan swordtail (X. “PMH”) & Comma swordtails (X.signum). All tend to have well developed swords and be similar in body shape to the classic swordtails we all know and love. All occur in flowing water with some areas of plant growth and require the same conditions as Common swordtails.

Pygmy swordtails
There are 3 species in this group :- El Abra pygmy swordtail (Xiphophorus nigrensis), High-backed pygmy swordtail (X.multilineatus) and Slender pygmy swordtail (X.pygmaeus). These are characterised by having several different colour forms and size morphs. All three have a naturally occuring golden form with the golden form of Slender pygmy swordtail (X. pygmaeus) being well established in the specialist livebearer groups. The other two species are the most difficult swordtails to keep in captivity. They come in up to 4 different size morphs (males range in size from 1” to 2” with swords from almost zero to 2” long).

Xiphophorus pygmaeus Male


These occur in large flowing rivers with almost no plant cover and very hard alkaline water. Perfect water quality has to be maintained at all times for these fish to thrive in captivity. A good external power filter combined with regular partial water changes suits them best. In nature they feed on small insects and aufwuchs which they pick out of algae. In captivity they need plenty of live food (or a frozen alternative) in their diet combined with a good quality flake.

Montezuma swordtails and its relatives
This group of swordtails includes just three species:- Montezuma swordtail (X. montezumae), Northern mountain swordtail (X. nezahualcoyotol) and El Quince swordtail (X. Continens).The Montezuma swortail has to be the most dramatic of the wild swordtails. Males grow to about 3” (7.5cm) body length but can have a swordtail which is twice the body length - making a total length of 9” (22.5cm)! Another very attractive swordtail in this group is the Northern mountain swordtail which is deeper in the body and has a shorter curved sword.

The final species in this group the El Quince swordtail was initially thought to belong with the other pygmy swordtails because its males only grew to 1” (2.5cm). Females, however, can reach 2.5” (6cm) and look very similar to miniature Montezuma swordtail females.

All three species are easy to keep providing they have hard alkaline water conditions and a good diet. Coming from slightly higher altitudes than the previous species they tend to do better in slightly cooler conditions ( 72 - 76 degres F) with good water movement.

Cortes swordtail and its relatives
This group has some really beautiful creatures in it. All occur at higher altitudes than other swordtails and most of their habitats are fast flowing moutain streams and rivers. The three species which make up this group have several different colour forms (often within the same population) and have been difficult to separate scientifically. The original species was Cortes swordtail (X.cortezi) which has a moderate thin sword. The Highland swordtail (X.malinche) also has a moderately long sword but this is thicker than in the Cortes swordtail. The final species, the Sheepshead swordtail (X.birchmani) has almost no sword.

This group tends to be deeper in the body than other swordtails and require cool flowing water if they are to do well in captivity. A temperature of about 68 - 72 degrees F suits them fine, but all three species can cope with temperatures as low as 60 degrees F with Highland swordtails and Sheepshead swordtails being found in water as cold as 50 degrees F!

How do you sex swordtails?
When swordtails are born males and females look alike. Only when males become sexually mature do they develop a swordtail and at that time the anal fin changes into a gonopodium. This is used to transfer sperm from the male into a ripe female.
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